Could an animal with an open circulatory system survive in a near-zero gravity environment?

Could an animal with an open circulatory system survive in a near-zero gravity environment?

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An animal such as a crayfish relies on gravity to keep its circulatory system running. If it is turned upside down, the gravity works against the system, suffocating the animal. Now if, instead of placing the animal on it's back, we were to take take it to a space station, gravity would not be working against it, and random blood flow plus capillary action should get some blood flowing. Would the animal survive this?

I believe that fruit flies have been taken aboard the Space Shuttle for experiments. They survived and the adults have an open circulatory system.

173 Animal Form and Function

By the end of this section, you will be able to do the following:

  • Describe the various types of body plans that occur in animals
  • Describe limits on animal size and shape
  • Relate bioenergetics to body size, levels of activity, and the environment

Animals vary in form and function. From a sponge to a worm to a goat, an organism has a distinct body plan that limits its size and shape. Animals’ bodies are also designed to interact with their environments, whether in the deep sea, a rainforest canopy, or the desert. Therefore, a large amount of information about the structure of an organism’s body (anatomy) and the function of its cells, tissues and organs (physiology) can be learned by studying that organism’s environment.

Body Plans

Animal body plans follow set patterns related to symmetry. They are asymmetrical, radial, or bilateral in form as illustrated in (Figure). Asymmetrical animals are animals with no pattern or symmetry an example of an asymmetrical animal is a sponge. Radial symmetry, as illustrated in (Figure), describes when an animal has an up-and-down orientation: any plane cut along its longitudinal axis through the organism produces equal halves, but not a definite right or left side. This plan is found mostly in aquatic animals, especially organisms that attach themselves to a base, like a rock or a boat, and extract their food from the surrounding water as it flows around the organism. Bilateral symmetry is illustrated in the same figure by a goat. The goat also has an upper and lower component to it, but a plane cut from front to back separates the animal into definite right and left sides. Additional terms used when describing positions in the body are anterior (front), posterior (rear), dorsal (toward the back), and ventral (toward the stomach). Bilateral symmetry is found in both land-based and aquatic animals it enables a high level of mobility.

Limits on Animal Size and Shape

Animals with bilateral symmetry that live in water tend to have a fusiform shape: this is a tubular shaped body that is tapered at both ends. This shape decreases the drag on the body as it moves through water and allows the animal to swim at high speeds. (Figure) lists the maximum speed of various animals. Certain types of sharks can swim at fifty kilometers per hour and some dolphins at 32 to 40 kilometers per hour. Land animals frequently travel faster, although the tortoise and snail are significantly slower than cheetahs. Another difference in the adaptations of aquatic and land-dwelling organisms is that aquatic organisms are constrained in shape by the forces of drag in the water since water has higher viscosity than air. On the other hand, land-dwelling organisms are constrained mainly by gravity, and drag is relatively unimportant. For example, most adaptations in birds are for gravity not for drag.

Maximum Speed of Assorted Land & Marine Animals
Animal Speed (kmh) Speed (mph)
Cheetah 113 70
Quarter horse 77 48
Fox 68 42
Shortfin mako shark 50 31
Domestic house cat 48 30
Human 45 28
Dolphin 32–40 20–25
Mouse 13 8
Snail 0.05 0.03

Most animals have an exoskeleton, including insects, spiders, scorpions, horseshoe crabs, centipedes, and crustaceans. Scientists estimate that, of insects alone, there are over 30 million species on our planet. The exoskeleton is a hard covering or shell that provides benefits to the animal, such as protection against damage from predators and from water loss (for land animals) it also provides for the attachments of muscles.

As the tough and resistant outer cover of an arthropod, the exoskeleton may be constructed of a tough polymer such as chitin and is often biomineralized with materials such as calcium carbonate. This is fused to the animal’s epidermis. Ingrowths of the exoskeleton, called apodemes , function as attachment sites for muscles, similar to tendons in more advanced animals ((Figure)). In order to grow, the animal must first synthesize a new exoskeleton underneath the old one and then shed or molt the original covering. This limits the animal’s ability to grow continually, and may limit the individual’s ability to mature if molting does not occur at the proper time. The thickness of the exoskeleton must be increased significantly to accommodate any increase in weight. It is estimated that a doubling of body size increases body weight by a factor of eight. The increasing thickness of the chitin necessary to support this weight limits most animals with an exoskeleton to a relatively small size. The same principles apply to endoskeletons, but they are more efficient because muscles are attached on the outside, making it easier to compensate for increased mass.

An animal with an endoskeleton has its size determined by the amount of skeletal system it needs in order to support the other tissues and the amount of muscle it needs for movement. As the body size increases, both bone and muscle mass increase. The speed achievable by the animal is a balance between its overall size and the bone and muscle that provide support and movement.

Limiting Effects of Diffusion on Size and Development

The exchange of nutrients and wastes between a cell and its watery environment occurs through the process of diffusion. All living cells are bathed in liquid, whether they are in a single-celled organism or a multicellular one. Diffusion is effective over a specific distance and limits the size that an individual cell can attain. If a cell is a single-celled microorganism, such as an amoeba, it can satisfy all of its nutrient and waste needs through diffusion. If the cell is too large, then diffusion is ineffective and the center of the cell does not receive adequate nutrients nor is it able to effectively dispel its waste.

An important concept in understanding how efficient diffusion is as a means of transport is the surface to volume ratio. Recall that any three-dimensional object has a surface area and volume the ratio of these two quantities is the surface-to-volume ratio. Consider a cell shaped like a perfect sphere: it has a surface area of 4πr 2 , and a volume of (4/3)πr 3 . The surface-to-volume ratio of a sphere is 3/r as the cell gets bigger, its surface to volume ratio decreases, making diffusion less efficient. The larger the size of the sphere, or animal, the less surface area for diffusion it possesses.

The solution to producing larger organisms is for them to become multicellular. Specialization occurs in complex organisms, allowing cells to become more efficient at doing fewer tasks. For example, circulatory systems bring nutrients and remove waste, while respiratory systems provide oxygen for the cells and remove carbon dioxide from them. Other organ systems have developed further specialization of cells and tissues and efficiently control body functions. Moreover, surface-to-volume ratio applies to other areas of animal development, such as the relationship between muscle mass and cross-sectional surface area in supporting skeletons, and in the relationship between muscle mass and the generation of dissipation of heat.

Visit this interactive site to see an entire animal (a zebrafish embryo) at the cellular and sub-cellular level. Use the zoom and navigation functions for a virtual nanoscopy exploration.

Animal Bioenergetics

All animals must obtain their energy from food they ingest or absorb. These nutrients are converted to adenosine triphosphate (ATP) for short-term storage and use by all cells. Some animals store energy for slightly longer times as glycogen, and others store energy for much longer times in the form of triglycerides housed in specialized adipose tissues. No energy system is one hundred percent efficient, and an animal’s metabolism produces waste energy in the form of heat. If an animal can conserve that heat and maintain a relatively constant body temperature, it is classified as a warm-blooded animal and called an endotherm . The insulation used to conserve the body heat comes in the forms of fur, fat, or feathers. The absence of insulation in ectothermic animals increases their dependence on the environment for body heat.

The amount of energy expended by an animal over a specific time is called its metabolic rate. The rate is measured variously in joules, calories, or kilocalories (1000 calories). Carbohydrates and proteins contain about 4.5 to 5 kcal/g, and fat contains about 9 kcal/g. Metabolic rate is estimated as the basal metabolic rate (BMR) in endothermic animals at rest and as the standard metabolic rate (SMR) in ectotherms. Human males have a BMR of 1600 to 1800 kcal/day, and human females have a BMR of 1300 to 1500 kcal/day. Even with insulation, endothermal animals require extensive amounts of energy to maintain a constant body temperature. An ectotherm such as an alligator has an SMR of 60 kcal/day.

Energy Requirements Related to Body Size

Smaller endothermic animals have a greater surface area for their mass than larger ones ((Figure)). Therefore, smaller animals lose heat at a faster rate than larger animals and require more energy to maintain a constant internal temperature. This results in a smaller endothermic animal having a higher BMR, per body weight, than a larger endothermic animal.

Energy Requirements Related to Levels of Activity

The more active an animal is, the more energy is needed to maintain that activity, and the higher its BMR or SMR. The average daily rate of energy consumption is about two to four times an animal’s BMR or SMR. Humans are more sedentary than most animals and have an average daily rate of only 1.5 times the BMR. The diet of an endothermic animal is determined by its BMR. For example: the type of grasses, leaves, or shrubs that an herbivore eats affects the number of calories that it takes in. The relative caloric content of herbivore foods, in descending order, is tall grasses > legumes > short grasses > forbs (any broad-leaved plant, not a grass) > subshrubs > annuals/biennials.

Energy Requirements Related to Environment

Animals adapt to extremes of temperature or food availability through torpor. Torpor is a process that leads to a decrease in activity and metabolism and allows animals to survive adverse conditions. Torpor can be used by animals for long periods, such as entering a state of hibernation during the winter months, in which case it enables them to maintain a reduced body temperature. During hibernation, ground squirrels can achieve an abdominal temperature of 0° C (32° F), while a bear’s internal temperature is maintained higher at about 37° C (99° F).

If torpor occurs during the summer months with high temperatures and little water, it is called estivation . Some desert animals use this to survive the harshest months of the year. Torpor can occur on a daily basis this is seen in bats and hummingbirds. While endothermy is limited in smaller animals by surface to volume ratio, some organisms can be smaller and still be endotherms because they employ daily torpor during the part of the day that is coldest. This allows them to conserve energy during the colder parts of the day, when they consume more energy to maintain their body temperature.

Animal Body Planes and Cavities

A standing vertebrate animal can be divided by several planes. A sagittal plane divides the body into right and left portions. A midsagittal plane divides the body exactly in the middle, making two equal right and left halves. A frontal plane (also called a coronal plane) separates the front from the back. A transverse plane (or, horizontal plane) divides the animal into upper and lower portions. This is sometimes called a cross section, and, if the transverse cut is at an angle, it is called an oblique plane. (Figure) illustrates these planes on a goat (a four-legged animal) and a human being.

Vertebrate animals have a number of defined body cavities, as illustrated in (Figure). Two of these are major cavities that contain smaller cavities within them. The dorsal cavity contains the cranial and the vertebral (or spinal) cavities. The ventral cavity contains the thoracic cavity, which in turn contains the pleural cavity around the lungs and the pericardial cavity, which surrounds the heart. The ventral cavity also contains the abdominopelvic cavity, which can be separated into the abdominal and the pelvic cavities.

Physical Anthropologist Physical anthropologists study the adaption, variability, and evolution of human beings, plus their living and fossil relatives. They can work in a variety of settings, although most will have an academic appointment at a university, usually in an anthropology department or a biology, genetics, or zoology department.

Nonacademic positions are available in the automotive and aerospace industries where the focus is on human size, shape, and anatomy. Research by these professionals might range from studies of how the human body reacts to car crashes to exploring how to make seats more comfortable. Other nonacademic positions can be obtained in museums of natural history, anthropology, archaeology, or science and technology. These positions involve educating students from grade school through graduate school. Physical anthropologists serve as education coordinators, collection managers, writers for museum publications, and as administrators. Zoos employ these professionals, especially if they have an expertise in primate biology they work in collection management and captive breeding programs for endangered species. Forensic science utilizes physical anthropology expertise in identifying human and animal remains, assisting in determining the cause of death, and for expert testimony in trials.

Section Summary

Animal bodies come in a variety of sizes and shapes. Limits on animal size and shape include impacts to their movement. Diffusion affects their size and development. Bioenergetics describes how animals use and obtain energy in relation to their body size, activity level, and environment.

Review Questions

Which type of animal maintains a constant internal body temperature?

The symmetry found in animals that move swiftly is ________.

What term describes the condition of a desert mouse that lowers its metabolic rate and “sleeps” during the hot day?

A plane that divides an animal into equal right and left portions is ________.

A plane that divides an animal into dorsal and ventral portions is ________.

The pleural cavity is a part of which cavity?

How could the increasing global temperature associated with climate change impact ectotherms?

  1. Ectotherm diversity will decrease in cool regions.
  2. Ectotherms will be able to be active all day in the tropics.
  3. Ectotherms will have to expend more energy to cool their body temperatures.
  4. Ectotherms will be able to expand into new habitats.

Although most animals are bilaterally symmetrical, a few exhibit radial symmetry. What is an advantage of radial symmetry?

  1. It confuses predators.
  2. It allows the animal to gather food from all sides.
  3. It allows the animal to undergo rapid, purposeful movement in any direction.
  4. It lets an animal use its dorsal surface to sense its environment.

Critical Thinking Questions

How does diffusion limit the size of an organism? How is this counteracted?

Diffusion is effective over a very short distance. If a cell exceeds this distance in its size, the center of the cell cannot get adequate nutrients nor can it expel enough waste to survive. To compensate for this, cells can loosely adhere to each other in a liquid medium, or develop into multi-celled organisms that use circulatory and respiratory systems to deliver nutrients and remove wastes.

What is the relationship between BMR and body size? Why?

Basal Metabolic Rate is an expression of the metabolic processes that occur to maintain an individual’s functioning and body temperature. Smaller bodied animals have a relatively large surface area compared to a much larger animal. The large animal’s large surface area leads to increased heat loss that the animal must compensate for, resulting in a higher BMR. A small animal, having less relative surface area, does not lose as much heat and has a correspondingly lower BMR.

Explain how using an open circulatory system constrains the size of animals.

In an open circulatory system, the heart(s) pump blood into an open cavity, bathing the tissues. As the blood diffuses through the tissue space, it delivers nutrients in exchange for receiving metabolic wastes. The blood then diffuses back to the heart to be pumped again. However, since this system relies on diffusion, the size of animals that use an open circulatory system is limited to fairly small volumes so that the blood can diffuse rapidly enough to efficiently exchange molecules with the tissues.

Describe one key environmental constraint for ectotherms and one for endotherms. Why are they limited by different factors?

Endotherms are constrained by the availability of food sources in the environment, while the temperature range in a geographic area limits ectotherms. The difference in how the two groups maintain their body temperature determines the key constraint for each group.


Such a creature is not really viable

The blood transports oxygen and nutrients because the diffusion of such molecules would take far too long to reach all parts of the body. For an organism that big to be viable it would need to actually breathe, drink and eat near everywhere throughout its body. With some suspension of disbelief one could imagine breathing and drinking, but eating is even more problematic. It would have to digest food outside its body and then bathe in it. Even then its body would be extremely porous and might be unable to mechanically support itself on land.

If the creature doesn't have blood to transport nutrients then it's going to need some other mechanism to to the trick. For example, some insects have open circulatory systems, where hemolymph instead of blood moves through interconnected sinuses or hemocoels spaces surrounding the organs.

Imagine if instead of eating through a mouth your creature spread its food all over it's body so it could ingest it through absorbtion. Mouths are access ports to the digestive system, so if it doesn't have blood to transport nutrients from the digestive tract then it doesn't necessarily need a mouth. You would have to re-imagine the physiology of your creature for this convention to work.

For a predator this could be particularly horrifying if it's routine was to literally bathe in the blood of it's prey disemboweling it's victims and draping their gore all over it's body sticking its legs deep inside it's smaller prey to rejuvenate it's aching limbs. Climbing inside larger prey (buffalo to elephant sized) and soaking in all the nutrients it needed. The creature may even have tiny appendages like dexterous villi that move the gore all over and through it's body to effectively distribute it.

The creature itself would probably need a high surface area to mass ratio, it couldn't practically have any one part of it's body be so dense that the nutrients couldn't soak in far enough, unless it used a convention such as insects with the interconnected sinuses or hemocoels spaces around the organs, but instead of flowing hemolymph through it's body, if flows the partially-liquefied tissues of it's prey all over it's organs using an esophageal action (Imagine the creature had a caustic mucus membrane, or sprayed it's prey with something to initiate the digestive process by partially breaking down the flesh into a usable ooze. ). Something else to consider is if it doesn't have blood, or a mouth, then it probably doesn't have lungs either, in which case it would have to stay in motion or in well ventilated areas in order to breath, or it would need to be at least some-what amphibious so it could breath while immersed in a pool of blood or bio-matter. Either way it would be hard to make it a strictly land-dwelling creature.

It wouldn't be a very efficient animal, there's a reason why large animals evolved with the physiology they have now, but in the right environment such a creature could thrive, but it would need a lot of large prey, or it would have to lay dormant for long periods of time like large spiders or snakes. However it turns out though, I think it's going to end up being a pretty terrifying creature.

Arthropods have an open circulatory system: instead of having arteries and veins to channel the blood, arthropods possess open sinus where blood bathes the organs directly. In which ways does this imply a constrain for a giant insect? While there is no active mechanism that pumps the blood throughout the body, it would be very difficult for a giant insect to oxygenate and nourish all its cells due to the gravity effect.

On the other side, most insects breath passively through their spiracles, which connect with an internal system of branched conducts called “trachea”. Thus, they don’t develop any active system to force air to enter inside their bodies, but it enters passively throughout these “trachea” and reaches the inner of arthropod’s body to oxygenate all cells.

Diffusion of gases is effective over small distances but not over larger ones. So, giant insects would face serious problems to oxygenate their tissues if they reach big sizes. In addition, current atmospheric concentration of oxygen (21%) wouldn’t be enough to oxygenate such a big organism with such a simple breathing mechanism.

It must be said that all these constrains are attenuated in aquatic ecosystems, where the cuticle’s weight and the diffusion of oxygen posed no problem for growth. That explains why the world’s biggest arthropods (and other invertebrates) are mainly located in aquatic ecosystems.

Maybe the animal has book lungs like a spider, or maybe the spiracles of an insect have branched inward, becoming an air-filled tracheal system intertwined with the fluid-filled cardiovascular system of blood. Each leg has its own “cardio-pulmonary complex” associated with it, plus a big one in the belly to feed the organs.

Rather than breathing in and out, these animals breath THROUGH, with air entering the system through spiracles near the head and exiting near the ail. Air is pumped by action of the muscular blood vessels that wrap around the tracheal tubes, or by muscular contraction of the whole abdomen (like a balloon inflating and deflating). Running also generates more flow-through.

Amphibians: Structure, Respiration and Sense Organs

In this article we will discuss about Amphibians:- 1. Origin of Amphibia 2. Factors that Caused Amphibian Evolution 3. Probable Ancestry 4. Structure of Amphibians 5. Digestive System of Amphibians 6. Respiratory System and Sound Production 7. Circulatory System 8. Nervous System 9. Urinogenital System 10. Reproduction and Development 11. 11. Reasons for Extinction.

  1. Origin of Amphibia
  2. Factors that Caused Amphibian Evolution
  3. Probable Ancestry
  4. Structure of Amphibians
  5. Digestive System of Amphibians
  6. Respiratory System and Sound Production in Amphibians
  7. Circulatory System of Amphibians
  8. Nervous System of Amphibians
  9. Urinogenital System of Amphibians
  10. Reproduction and Development of Amphibians
  11. Reasons for Extinction of Amphibians

1. Origin of Amphibia:

During the middle of Devonian time, the bony fishes had differentiated into the actinopterygians on one hand and into the dipnoans and crossopterygians on the other. The climax of evolution was reached when the descendants of some crossopterygians left water and invaded land. This event of transi­tion from water to land ushered a new phase in vertebrate evolution, the beginning of the land vertebrates.

The amphibians started the beginning of tetrapod history. By invading a new environment on land, the amphibians opened broad avenues for further evolution over a wide range of structural and functional adaptations. Before going into the discussion on the ancestry of the amphibians, the steps the first amphibians have taken to meet the basic requirements for life on land are described below.

The problem of terrestrial locomotion with additional gravitational complications was solved by profound changes in all the parts of the tetrapod body.

(a) The amphibian head attained power­ful musculature with corresponding changes of the articular processes of the skull and its adjacent endoskeleton.

(b) The lower jaw apparatus developed elaborate musculature for its operation and support.

(c) The vertebral column loses flexibility and gained strength and rigidity by ossification.

(d) The pectoral and pelvic girdles sup­port the limbs and also protect the important visceral organs from injury which may result from new upward thrusts. Walking on land resulted upward pressure. This upward thrust caused the diminution of dermal skeleton of the girdles. Powerful scapula and triradiate pelvic girdle with elaborate ilium are some of the important modifications in the girdles. These modifications are supplemented by the development of endo-skeletal processes for firm attachment of associated muscles.

(e) The amphibians have developed two pairs of limbs. These are well-equipped with adequate muscles and strong girdles to lift the body away from the frictional contact of the ground. The carrying of the weight of body on the four limbs has caused great change in the vertebral column.

(f) The problems of breathing in air are solved by developing well-formed lungs for gaseous exchange in air. The moist skin in modern amphibians also acts as an accessory respiratory organ.

(g) All amphibians possess well-developed vascular system, a new scheme for the development of lungs, i.e., introduction of pul­monary circuit. This has caused tremendous change in the structure of the heart and the circulatory system as a whole.

(h) Development of a middle ear cavity with a bone to transmit the vibrations from tympanum to inner ear helps in the intensifi­cation of the sound waves of air.

(i) The skin becomes suited for terrestrial life to resist desiccation.

The modern teleosts have reached the peak of evolutionary success amongst the primary water-living vertebrates. They have undergone extensive adaptive radia­tion. The air-breathing fishes, the dipnoans and the crossopterygians exhibit a close relationship with the amphibians and it is an apparent bio­logical truth that a group of such lung-fishes gave rise to the new terrestrial population—the amphibians.

So the chance for dipnoans and crossopterygians to hold the significant position in amphibian ancestry needs consideration. How did the early amphibians meet the new requirements imposed upon them as a result of change from an aquatic to terrestrial life is to be solved first.

The early amphibians must have fulfilled the basic requirements for living on land by making the following modifications:

(a) Partial loss of armours although present in some earliest amphibia, e.g., Stegocephalians.

(c) Development of terrestrial appendages by transforming the paired fins into limbs.

(d) Loss of internal gills and acquisition of lungs.

Dipnoi—foreshadowed amphibian organisa­tion:

It was a general belief that the dipnoans stand in the direct line of tetrapod descend, because the dipnoans show many structural and functional resemblances with the amphib­ians. Although the pectoral and pelvic girdles in dipnoans cannot support the weight of the body on land, these girdles foreshadowed some amphibian features in several ways.

(a) The internal skeleton of the paired appendages is well-developed in dip­noans.

(b) Paired appendages articulate with the respective girdles by a single proximal bony piece, which can be compared with the humerus or femur.

(c) Outward extension of myomeric muscles into the paired fins is quite suggestive of the arrangement of muscu­lature of the paired appendages of Amphibia.

(d) Ability to breathe air by lungs (modified swim-bladder).

(e) Pectoral girdle of Necturus resembles closely that of dipnoans.

Although the dipnoans present some specializations towards a method of living out of water, the total evidences direct quite clearly to the fact that they are not on the direct line of emergence of amphibia from fishes. The dipnoans exhibit too many specialised features and such a specialised group cannot possibly hold the ancestry of another group of animals.

Striking similari­ties, especially in circulatory and respiratory systems, are possibly due to the physiological convergence for living in similar condition of life. The dipnoans, today, give an idea of the form that probably linked the fishes with the amphibians. The dipnoans are usually regarded as the collateral uncle of the amphibia but not the father of first tetrapod.

Recently considering the morphological and anatomical point of views, D. Rosen et al., (1981), and Duellman and Trueb (1986) opine that the nearest living relatives of recent amphibians are lung-fishes than the crossopterygians but this has been criticised by Jarvik (1980) and other scientists.

Crossopterygians — direct the channel of amphibian evolution:

To trace the direct line of amphibian origin from the fishes, the importance of the crossopterygians in holding the probable starting point needs consideration. The early crossopterygians, as exemplified by the Devonian genera, Osteolepis and Eusthenopteron furnish the strongest support. Because they possess many features which are cer­tainly amphibian or lead towards amphibia.

(a) Bony pattern of jaws and skull are comparable to that observed in early amphibians.

(b) Two large bones on the top of a skull can be homologized as the amphibian parietal bones.

The posterior skull table of Osteolepis and Eusthenopteron is more similar to that of early amphibians (e.g., Ichthyostega and Eryops) than that of dipnoans (Fig. 7.49).

(c) The jaws of Eusthenopteron possessed labyrinthodont teeth characterized by in-folding of a tooth wall around a central pulp cavity (Fig. 7.50).

(d) Pectoral girdle presents certain features which are prerequisites for amphibian fore- limbs.

(e) Skull of Eusthenopteron contains almost all the elements observed in the early amphibians.

(f) Pectoral fin of Eusthenop­teron can be compared to the forelimbs of amphibia. The single proximal piece of bone can be homologized with the humerus and the next two pieces can be compared to radius and ulna.

(f) Various wrist and ankle bones, the bony elements of hand and foot have evolved from the distal bony complex of the crosspterygian fins.

Thus, in many respects, the crossoptery­gians show close similarities with the amphi­bian and it is expected that these fishes are the direct progenitors of the early amphibians.

Cladistic analysis in favour of osteolepiforms as the ancestor of early tetrapods does not support fully.

At present, a late Devonian crossoptery­gians, Panderichthyidae (e.g., Elpistostege and Panderichthys) seem to be in the line of direct ancestor of amphibians. The members of Panderichthyidae were crocodile-like fishes with fins instead of limbs.

Their hands, bodies and the skull roof were flattened and they had elongated snout. Their eyes were on the top of head and they had no dorsal and anal fins. Those features suggest a closer link with the first tetrapods.

The frontal bones were present in both Panderichthys and tetrapods but were absent in Osteolepiforms. The ribs of Panderichthys project ventrally from the vertebral column whereas in osteolepiforms the ribs of the vertebral column project dorsally.

The most dramatic and widely accepted event to note in amphibian evolution is the transformation of the crossopterygian paddles into amphibian limbs. The sequences indica­ting how amphibian limbs arose from a fish fin is a controversial issue. It is probable that the lobed fins of crossopterygians specially seen in Eusthenopteron have become transformed into the tetrapod limbs.

As the fish came on the land, the paired appendages perhaps first carried only the weight of the body and the muscles of the appendages became capable of only forward and backward movements.

With the evolution of the tetrapods on land, the limbs become elongated and shifted under the body to raise the body further away from the ground. While on land the muscles became modified and arranged around the shoulder and hip joints for balanced movement on land.

For the attachment of the muscles the girdle became expanded into plates consisting of dif­ferent characteristic pieces. The shoulder girdle of an earliest amphibia, Eogyrius, inherited a shoulder girdle closely similar to the Osteolepis.

Fig. 7.51 gives an idea of the possible stages of transformation of the crossopterygian girdles and fins into the tetrapod girdles and limbs respectively. The actual and documentary tran­sitional limb is still unknown. But the most ancient footprint of Thin-opus throws much light on the process of transformation.

2. Factors that Caused Amphibian Evolution:

What were the factors that led Crossop­terygians to leave their primal aquatic home and to come on land? There are different views. A few of them are given below.

Barrell (1916) emphasised that the Devonian was a dry period whence many streams and ponds tended to dry up seasonal­ly. Certain crossopterygians were capable of movement from drying pools to places where water was available. The periodic escapes from drying pools possibly caused the deve­lopment of tetrapod limbs.

(ii) Desire for excessive water:

Romer (1958) rejected the view of droughtness in Devonian period. He sugges­ted that amphibians and early reptiles were inhabitants of water until the Pennsylvanian period. He also suggested that it was the desire for more water that caused the first excursion of crossopterygians from one place to other.

Berrill (1955) is inclined, to think that the enemies in water forced crossopterygians to leave for land. Other factors were the abundance of food in land, lure for atmospheric oxygen and recurrence of unfavorable environment.

It seems that the real cause is neither safety nor food nor the desire to breathe atmospheric air, but an adaptation which has been imposed repeatedly upon the crossopterygians by recurrence of hostile environment.

Most pro­bably, during late Devonian period, due to excessive periodic drought, crossopterygians were forced to search for new fresh-water streams and lakes, where they can live and thus escape the risk of survival. By this way they had to cross dry land to find suitable water. From such a start the amphibians evolved in the geological age and became adapted to the new terrestrial environment.

During the Devonian time, some of the crossopterygians came to land from aquatic home. This group who was successful to come to land from water was probably the rhizodonts being represented by Osteolepis and Eusthenopteron. It was a very significant step to come into a completely new environment.

On coming to land these advanced air-breathing fishes became trans­formed into primitive amphibians. Fig. 7.52 shows the evolution of amphibian vertebral structures from the crosspterygian fishes. The most primitive amphibians, known the Ichthyostegid whose components of vertebrae have the similarity to the crossopterygians.

Due to the similarity of the components of ver­tebrae, between Ichthyostega and crossoptery­gians, it is supposed that amphibians have evolved from crossopterygians and from this basic type a radiation of several types of verte­brae among other amphibians and also in other groups of vertebrates may have evolved.

Figure 7.53 shows the phylogenetic tree of the amphibians. Ichthyostega exhibited a transitional phase by possessing an admixture of piscine and amphibian characters. Ichthyo­stega, although retained many distinct piscine characters, has paved the path of amphibian evolution.

4. Structure of Amphibians:

The living amphibians exhibit diverse structural adaptation. The caecilians have a degenerated structural organisation and they furnish all the basic modifications for fossorial life. They are the most primitive forms amongst the living amphibians.

The modern urodeles include a large number of families and genera. Although built on a common fun­damental plan, each family is characterised by having peculiar anatomical structures.

The urodeles include many giant forms. The largest amphibia is the Megalobatrachus of Japan and China which may even reach a length of about 1.60 m. Most of the urodeles are aquatic. The terrestrial forms have limbs and are plantigrade.

Anatomically, the urode­les occupy an intermediate position between the caecilians and the anurans. The anurans constitute the highly specialised forms and show wide range of adaptive radiation. Hyla shows an adaptation for arboreal life and pos­sesses adhesive discs at the tip of the digits.

The plate-like adhesive discs are not suctorial in action, but have a moist, corrugated anti­skid surface which helps in adhesion to the tree. During climbing, a sticky secretion is expelled from the adhesive discs by the action of collagenous fibres which operate the glands.

The adjustment of the adhesive discs is facilitated by the development of inter­calary cartilage between the terminal and penultimate joints. Another tree-frog, Chiromantis, has opposable digits.

In most of the tree-frogs, the webs between the toes are absent or reduced, excepting in Rhacophorus of East Africa where the elongated digits are webbed. It has been recorded that Rhacophorus malabaricus can glide from a height of more than 9 m and Hyla venulosa can glide from a height of 42 m quite effec­tively. In Hyla venulosa, the digits are not webbed.

The skin of amphibians consists of epider­mis and dermis. The epidermis consists of sev­eral layers and is renewed by ecdysis. This process of renewal is controlled by the pitu­itary and thyroid glands. Localised thickening in epidermis is observed in the larvae, special­ly in the formation of the horny larval jaws and teeth. The warts of toad are also the instances of such thickenings.

The skin of modern amphibians is naked and remains moist due to the secretion of integumentary glands. The moist skin is necessary for respiration and also possibly for temperature regulation. There are two types of skin glands in amphibians. These are: mucous glands and poison glands. The mucus secreted by the mucous glands keeps the skin moist.

The poison glands are well- developed in toad and salamanders. The parotoid glands of toad are the best examples of the poison glands. Most of the warts on the dorsal surface of the toad open to the exterior by a minute opening which leads into poison gland.

The gland produces active toxins. The secretion is venomous and causes nausea, respiratory and cardiac dysfunctions. The tox­ins are isolated as the bufogin and bufotalin. The poison of Dendrobates acts on the ner­vous system. The secretion of the dorsal glands of a warty Newt (Triturus cristatus) is ven­omous. The poison glands are defensive organs.

The skin of the larval amphibians is ciliated. The colour of the skin of amphibia may vary from dull to brilliant. The urodela usually shows brilliant colouration which has a protective value. The green colouration of tree-frogs is a protective device, because it harmonizes with the surrounding green fliage.

The spotted salamander and some frogs exhibit warning colouration. In some tree- frogs, the brightness of the body colouration may vary with the change of intensity of light.

The colour change is caused by the physio­logical adjustment of the deep-seated melanophores, guanophores and overlying lipophores. The skin of Gymnophiona is thick and contains groups of granular dermal scales enclosed in sacs and large multicellular poison glands (Fig. 7.44).

Skeletal Structures:

The exoskeleton was present in fossil amphibians. But in modern forms it is restricted to majority of the caecilians and some anurans. In caecilians, clusters of small dermal scales lie in the skin in most of the cases (Fig. 7.44) In a few toads, bony plates remain embedded in the skin of the back and these are dermal in origin.

In Brachycephalus of Brazil, the dermal plates on the back become fused with the neural spines. Small and horny claws are present in the larval stage of an Asiatic urodele, Onychodactylus and in an African toad, Xenopus. In Xenopus, claws are present at the tips of first three digits of his hind limb. Claws have been recorded in some fossil amphibians too.

The claws present in these amphibians, although fore-shadowed the emergence of claws in higher classes of vertebrate, are not true claws. In Pelobates, the highly cornified areas on the feet can be com­pared with the epidermal scales.

The skull in the living amphibians varies greatly. There is a general tendency towards reduction in the thickness and number of dermal elements in the skull. The inter-pterygoid vacuities and the orbits are greatly enlarged. The rami of the lower jaw are short and the skull becomes much flattened. The skull of the anurans is highly specialised among the amphibians.

The inter-parietal foramen (present in fossil amphibians) is totally absent in modern amphibians. The skull of toads is devoid of teeth, but in Amphignathodon true teeth are present on the lower jaw. In frogs, teeth are present on the lower jaw. The skull of urode­les has certain peculiar features of its own. It is less specialised than that of anurans and differs in many important respects.

The chondrocranium is lower in organisation with many degenerative or paedomorphic features. The parietals and frontals are separate and in some forms both lacrimals and pre-frontals are persistent. The skull of urodeles differs from that of anurans by having a large prevomer.

In caecilians, the skull is peculiar. It has large investing bones and a small but complete orbit is present there. The skull is a rigid struc­ture. The compactness is correlated with the burrowing habit. The lower as well as the upper jaws bear teeth. Like that of urodeles, a tooth-bearing coronoid is present in the mandible.

The vertebral column in amphibia is large­ly bony and the vertebrae are articulated together. The flexibility of the vertebral col­umn is lost to give more strength. This modifi­cation is due to the lifting of the body on the limbs. In urodeles which spend much of the time in water, the vertebrae lack ossification and notochord persists to help in swimming.

The transition from aquatic to terrestrial life causes the shortening of the vertebral column. In anurans, the vertebral column is composed of nine vertebrae and an un-segmented urostyle behind. The vertebral column is com­posed of nine vertebrae and an un-segmented urostyle behind.

The vertebral column is differentiated into:

(a) The cervical region represented by the atlas,

(b) The thoracolumbar region with variable number of vertebrae,

(c) A sacral region containing a large vertebra and

(d) A caudal region comprised of the tail verte­brae. The urostyle represents the caudal region in anurans. The transverse processes and zygapophyses are well-developed for the attachment of muscles.

The number of verte­brae is variable. It may extend up to 250 in urodeles and caecilians. In anurans, the verte­brae are procoelous except the ninth vertebra of Rana which is peculiar. The anterior surface of centrum is convex while the posterior sur­face bears a double convexity. But in urodeles, two types of vertebrae are encountered.

The primitive urodele as exemplified by Ambystoma has amphicoelous vertebrae and the higher urodeles possess opisthocoelous vertebrae. Some primitive frogs exemplified by Ascaphus and Liopelma possess free ribs. The skeletal features of the girdles and limbs have changed considerably in amphibians but the basic plan of the limbs and girdles remains same throughout the group.

In caecilians, these are secondarily lost due to fossorial adaptation. In fishes, the girdles and paired fins are small and are largely carti­laginous, but in amphibians the girdles have become greatly enlarged and modified due to their weight-bearing function. The appendicular skeleton in urodeles is greatly simplified, but in anurans these are highly developed and are quite suitable for terrestrial mode of life.

5. Digestive System of Amphibians:

Adult amphibians feed mostly on the arthropods, but the larval forms are usually omnivorous. They may be cannibals. As a result of similar food habits, the digestive sys­tem shows little variation. In most amphibia excepting toads, the teeth are present. The teeth are borne on the premaxillae, maxillae and vomer.

The teeth are very small and poin­ted and are used only to catch the prey. Biting teeth are present in adult Ceratophrys ornata. Amphignathodon, a South American tree-frog, possesses teeth on the lower as well as on the upper jaws. The salivary glands are absent but some oral glands are present which produce mucus.

In terrestrial amphibians, cilia are pre­sent in the oral cavity which keep the oral fluid in movement. In case of many urodeles, tongue is immovably fixed. It may be movable as seen in most anurans but it is free behind and fixed anteriorly.

The tongue is used to cap­ture the prey. The adhesive power of the tongue is enhanced, particularly in the frogs, by the secretion from the lingual and inters- nasal glands. The tongue is altogether absent in Xenopus and Pipa.

The oesophagus is a simple tube and is not sharply distinguishable from the stomach. The stomach is simple with folded mucous layer. Simple tubular gastric glands open in the folds. These glands are composed only of one type of cells. The glands produce pepsin and hydrochloric acid.

The intestine is short in adult amphibians and is marked off from the stomach by having a well-developed pyloric sphincter. But the intestine in omnivorous lar­val forms is much coiled like the spring of a clock. Caecum is absent in amphibian alimen­tary canal. But in some anurans, especially in Hyla arborea the large intestine has a conspi­cuous anterior caecum.

The liver and the pancreas have typical histological picture and produce bile and pan­creatic juices respectively. The liver is basically a single massive gland with right and left lobes. The gall-bladder lies just right of the midline of the notch between the lobes.

The liver is attached to the duodenum and stomach by gastro-hepatic ligament. The pancreas is a thin and elongated structure along the duodenum on the side away from stomach. The amphibians can live for a considerable period of time without taking any food. Proteus, Typhlomolge are the typical examples of cave-swelling animals (troglodytes). Axolotl larva may remain alive for about 650 days in starvation.

6. Respiratory System and Sound Production in Amphibians:

Adult amphibians are lung-breathers. The skin acts as an accessory respiratory organ both in water and on land. The skin is highly vascular and specially so in the buccopharyn­geal cavity. The larval amphibians respire in water by the gills. Such gills are retained in many adult urodeles. Few urodeles retain external gills as the respiratory organs in adults.

Both external and internal gills are pre­sent in anuran larvae. Ascaphus, living in the mountain stream of U.S.A., has reduced lungs which help the animal to live in water. In per-ennibranchiate urodeles, the lungs are simple saccular organs and the hydrostatic function is predominant. In Salamandra atra and Desmognathus, the lungs are absent.

In Astylosternus, an African frog, the lungs are vestigial. In caecilians, the tracheal lung may be present but the left one is always rudi­mentary. In aquatic urodeles, the lungs act secondarily as hydrostatic organ. In all these above cases, respiration is exclusively pharyn­geal and/or cutaneous.

In almost all amphi­bians cutaneous respiration is a remarkable supplementary respiratory adaptation. In Cryptobranchus, there are vascular folds in the epidermis into which blood capillaries pene­trate. The amphibians are virtually the pioneers where true voice is produced by the vocal organ.

The production of sound is a protective response for fear and the males call the females during breeding season. The noise is produced by the vibration of the vocal cords in the laryn­gotracheal chamber. The vocal sacs in the males of some anurans, developed as the buc­cal outgrowths, serve as resonator.

7. Circulatory System of Amphibians:

The heart of amphibia consists of a sinus venosus, two auricles, an undivided ventricle and a conus arteriosus. The conus arteriosus is made up of two regions: pylangium and synangium. The portion of the conus next to ventricle is called pylangium while the distal part is designated as synangium. The pylangium is more muscular than synangium.

The distal end of truncus arteriosus becomes expanded as bulbus arteriosus in some urodeles. The left auricle is absent in the plethodontid urodeles where the lungs and the pulmonary veins are missing. The auricles are completely separated by a complete inter-auricular septum. It is per­forated in Salamandra or may be fenestrated with intervening spaces in lung less urodeles.

The venous blood returns to the right auricle while the left auricle receives oxygenated blood. The spiral valve is present in anuran heart but it may be reduced in most urodeles or may be absent as in Necturus, Cryptobranchus and the caecilians. In all the amphibians where the conus arteriosus is present there are two sets of valves which prevent the back flow of blood.

The trabeculae carni (strands of muscle making up the muscular walls of heart) are observed in amphibians. These trabeculae are best deve­loped in the walls of auricles. In the urodeles where gills are retained in adults, the pattern of circulatory system is essentially fish-like. The venous system of the urodeles represents a tran­sitional stage between the fish and the anurans.

The RBC in amphibia are nucleated and oval. One of the notable features to record is the presence of largest RBC amongst the verte­brates. The RBC of Proteus measures about 58 pm in diameter. There are three types of leucocytes in amphibians.

These are lymphocytes, monocytes and polymorphs. The red blood cells are produced mainly in kidneys and are destroyed in the liver and spleen. The bone- marrow also serves as an important centre for formation of red blood cells in adults particu­larly in males during breeding season. The spleen is a source of lymphocytes.

8. Nervous System of Amphibians:

The brain of amphibia is basically built on the same fundamental plan in all forms. The prosencephalon is represented by two large evaginated cerebral hemispheres. The pineal organ is a simple sac in most cases, but in a few amphibians this forms a retina-like struc­ture. The optic lobes are well-developed. As the amphibians are sluggish animals, the cere­bellum is simple.

Sense Organs in Amphibians:

The sense organs are well-developed. Many aquatic adult amphibians and the larvae possess simple lateral line organs in the form of clusters of cells in an open pit. The skin contains tactile sense organs and chemoreceptors.

The olfactory organ Works both in water and on land. Organ of Jacobson is present in most amphibians. It is a special sensory sac developed as a diverticulum from the olfacto­ry chamber. It serves to test the scent of the food taken inside the mouth. It is absent in Proteus and Necturus.

The eyes of amphibia exhibit certain modifications due to transition from water to land. In water, the eyes were adapted for short­sighted vision but on land long-sighted vision becomes necessary. The eyes are extremely degenerated in caecilians and also in cave- dwelling urodeles. The eye ball is more or less spherical with a rounded cornea.

The lens is flattened in terrestrial amphibians, but in aquatic forms it is rounded. The eyelids are usually present in terrestrial forms excepting some primitive members. In tree-frogs, the eyelids may be transparent. The lacrimal glands are present in all the terrestrial amphi­bians. In aquatic amphibians, the lacrimal glands are absent but the lacrimal ducts are still retained in many cases.

In some caeci­lians, a single lacrimal gland occupies a posi­tion in the sightless eye-socket to lubricate the sensory tentacle. In all amphibians, the skin is also sensitive to light. This is highly developed in cave-dwelling urodeles.

The membranous labyrinth is com­posed of an utriculus with three semicircular canals, a sacculus with an outgrowth, called lagena. The middle ear consists of a funnel like cavity which communicates with the pharyngeal cavity by Eustachian tube. In this cavity, a rod (columella) is present which transmits the sound waves to the internal ear.

The columella fits into the fenestra ovalis by a broad foot (otostapes). The fenestra ovalis is partly occupied by a plate (operculum). So the transmitting rod is divided into an inner part, named as otostapes and operculum, a medium part called mediostapes and an outer part, designated as extra-columella.

In urodeles, caecilians and some anurans, the tympanic cavity and extra-columella may be absent. Cryptobranchus lacks the operculum. This is also absent in Xenopus and Pipa. In a terres­trial anuran, Bombinator, the tympanum and columella are greatly reduced.

9. Urinogenital System of Amphibians:

The kidneys in amphibia are of opisthonephric type and retain the characte­ristics of fishes. The shape of the kidneys cor­responds to the shape of the body. In urodeles and in a primitive frog, Ascaphus the kidneys are elongated.

Each kidney is divided into an anterior narrow non-renal part and a broad posterior renal part. In caecilians, the kidneys are extremely elongated and occupy the whole length of the body cavity. In case of anurans, the kidneys become condensed and divided into lobes. In amphibians, the genital organs develop from the genital ridges. Each such ridge is situated on the ventromedian aspect of the developing mesonephros.

The genital ridge is divided into three sectors:

(a) Anterior part (progonalis),

(b) Middle part (gonalis) and

The gonads proper develop from the gonalis. In anurans, the fat bodies develop from the progonalis while in the urodeles and caecilians, the fat bodies develop from the entire genital ridge. The testes in different amphibia assume various shapes.

The testes are smoothly rounded mass in anurans but in urodeles these may be elon­gated and lobed structures. In Desmognathus, each elongated testis is separated into a series of testicules which are budded off towards the anterior end, one for every year.

In caecilians, each testis is an elongated body and looks like a string of beads. The internal construction of testes is simple and consists of short seminiferous tubules. Each seminiferous tubule has a wide lumen and ends blindly.

The efferent ducts vary in number and extend up to the marginal canal of the kidney. Each marginal canal is developed from outgrowth of the cap­sule of primary mesonephric tubule of anterior portion of mesonephric kidney.

The meso­nephric tubules usually extend to epididymal duct. The kidney tubules serving as the carriers of sperms may retain their glomeruli in caeci­lians and a Salamander, Spelerpes, but in most amphibians the glomeruli are lost.

The testes discharge through the kidneys by the vasa efferentia. So the Wolffian or mesonephric duct serves as a urinogenital duct in males and as a ureter in females. In toads, a special Bidder’s organ is present. This organ is better developed in males.

The function of this organ is not known. It is assumed to be an endocrine organ, because it undergoes a cycle of size change. This organ is capable of developing into an ovary after castration in either sex. The Bidder’s organs develop from the gonalis sector just anterior to the gonad proper.

In females, the ovary is an irregular mass. The eggs are discharged into the body cavity. The Mullerian duct becomes swollen and con­voluted to become the gonoduct. It extends through the entire length of the body cavity. The eggs, after being discharged into the body cavi­ty, enter the ostium (opening of gonoduct) and traverse the duct.

In oviparous forms, the eggs get their jelly coating within the tube. In viviparous forms (Salamandra, Spelerpes fuscus, Typlonectes compressicauda, Dermophis thomensis), the eggs develop in the tube. The tube is divided into different parts in different amphibians.

(b) Infundibulum (wide lumen, thin wall and no glands),

(c) Tube (beset with glands in oviparous forms or without glands but mucous cells in viviparous forms),

(d) Uterus (wide lumen and much folded epithelium and

(e) Vagina (short section between uterus and cloaca).

The different parts of female gonoduct become modified in accor­dance with the modes of reproduction. In anu­rans, the uteri exhibit great modification. Bhaduri (1953) has classified the uterus of anu­rans into three broad categories (Fig. 7.45):

In Rana and Xenopus two uteri remain separate along their course and open into the cloaca by independent openings. This condition is called the uterus separatus. In Bufo and Rhino-derma a septum runs antero­posterior between two uteri, but posteriorly forms a common uterus by fusion at the terminal ends.

This condition is called uterus septatus. The two uteri have a common opening into the cloaca. The degree of fusion varies greatly and finally in Dendrobates, two uteri become con­fluent into an unpaired common uterus. The median partition wall between the uteri is absent.

This type of uterine condition is called the uterus communis. The uterus separatus is comparable with the duplex type, the uterus septatus with the uterus bipartite and uterus bicornis types and the uterus communis with the uterus simplex of the mammals.

Hermaphrodism, though occasional, is observed in adult amphibians. Hermaphro­dism occurs in adults as the consequence of failure of sex-directing mechanism to convert indifferent gonad into the specific sex. Because of dis-balance the anterior portion of the gonad remains as female while the posterior part becomes male. These two parts are usually separated by non-gonadal tissue bridge.

10. Reproduction and Development of Amphibians:

In majority of the amphibians, external fertilization is the rule. They are oviparous. But several instances of ovoviviparous condition are encountered. In Spelerpes fuscus, Typhlo­nectes compressicauda, Dermophis thomensis and Salamandra atra, the eggs are retained inside the oviduct where intra-uterine develop­ment occurs.

In most urodeles, the spermatozoa are transferred to the body of the female in the form of spermatophores. In almost all amphi­bians, the ontogenic development is indirect, i.e., accompanied by well-marked metamor­phosis.

Most of the amphibians undergo com­plete metamorphosis but some urodeles retain the larval features and become neotenic. The most remarkable instance of neotenous form is the Axolotl which breeds in larval state.

11. Reasons for Extinction of Amphians:

Amphibians are undoubtedly a neglected group as humans have a general tendency to dislike these creatures. They certainly deserve their share of attention as they perform a vital role in ecological balance and form an impor­tant link in the food chain. They also called “bio-indicator of pollution”.

The amphibian population throughout the world is decreasing alarmingly day by day. ‘The exact reasons for the declines are not known, though some local reasons are considered by her­petologists. We do not know the status of the Indian species as very little work has been carried out.

Habitat loss and alteration are the main threats to the amphibians. Breeding grounds are being altered at a fast rate, owing to the fill­ing up of the aquatic habitats for the construc­tion of modern complexes in the suburb areas of the cities and towns. Aquatic habitats are also being destroyed mainly by siltation and sewage contamination.

The forests are being rapidly converted to agricultural and cattle grazing purposes. Extensive use of insecticides and herbicides for agricultural purposes may be the reasons for amphibian declines.

The acid precipitation, and increased ultraviolet radiation are being linked to global declines. The reasons for amphibian vanishing are different in different countries. A brief discus­sion of the reasons of the declination in different countries is given here.

In Great Britain, the disappearance of some frog species with thermal pollution results from the hot effluents of nuclear power plant cooling systems. More frequently it is the chemical contamination that contributes to amphibian declines and disappearances. Progressive acidification of ponds has been responsible for the disappearance of numerous colonies of Bufo calamita in Great Britain.

General absence of amphibians of some regions of France and Belgium are extreme pollution by waste products, pesticides in agricultural zones, and heavy metals, etc. In Denmark Bombina bombina, Hyla arborea, Pelobates fuscus, Bufo viridis are declining fast than the previous years.

Holm Anderson (1995) described that a team of pro­fessional biologists resurveyed 1300 localities from 1977-1986. This survey revealed that 19% of the breeding ponds had disappeared but a further 40% had been altered to some extent. However, amphibians disappeared to a much greater extent than did the ponds about 50% of populations disappeared from 1945 to 1980.

In Hungary, Miklos Puky (1995) has stud­ied urban amphibian populations around Budapest since 1988, and has come to conclu­sion that marked declines in several species are due to both human impact and drought.

The annual meetings of Societas Europaea Herpetologica that held in Bonn, Germany in 1995 included a symposium on declining amphibian populations. T. Hayes of Berkeley, U.S.A. reported work on the role of oestrogen mimics as possible endocrine disruptors.

In Russia, Vladmir Ischenko of Ekatermburg (1995) suggested on the basis of skeleto- chronological studies of a number of species that short lived species may be more vulnera­ble to local extinction than long lived species.

The third Latin American Congress of Herpetology was held at the University of Campinas, Sao Paolo, Brazil, in December in 1993 and the researchers analysed the decline of Latin American amphibian species. They reported that Atelopus, Melanophryniscus, Dendrobates, Hylodes, Telmatobius, Batrachophrynus and Centrolene are extremely depen­dent on water bodies.

One of the reasons for local declines is overexploitation. Human con­sumption of Telmatobius arequipensis, T. marmoratus and Batrachophrynus macrostomum in Peru and Caudiverbera caudiverbera in Chile is depleting fast as compared with former large populations. Other reason is extraction and exportation as reported for Chile. In 1985, 236 anurans were reported, while in 1992, there was an exportation of 1,00,000.

Another reason given for declines is the introduction of non-native fauna. Xenopus lae­vis, Rana catesbeiana and Triturus sp. in many places over the Brazil, Peru and Chile and rain­bow trout (Oncorhynchus mykiss) along the Andes are seen as non-native faunas.

Less than 40 years ago, thousands of Amargosa toad (Bufo nelsoni), inhabited the Oasis valley in Southern Nevada, U.S.A. In 1994, this species consists of fewer than 100 individuals. Some of the factors believed to adversely affect the toad and its habitat include grazing, off road vehicle use, grading for flood control and modification by heavy equipment for the development of commercial enterprises.

The introduction and existence of non-native predators such as cat-fish and crayfish pollu­tion, and diversion of spring water have also directly affected toad populations. Three species of U.S.A. are in most danger and need of listing.

They are the Amargosa toad, the Western boreal toad (Bufo boreas boreas (Southern Rocky mountains popula­tions), and the great basin population of the spotted frog (Rana pretiosa). In addition the Wyoming toad (Bufo hemiophrys baxteri) could become extinct in the next few years.

Boreal toads (Bufo boreas boreas) experi­enced a massive die off in the Rocky Mountains of Colorado in the late 1970’s and early 1980’s. Red leg syndrome, caused by a variety of bacteria or fungi, has been identified as the proximal cause of death. Chytrid fungi are killing amphi­bians in the wild.

Recently the deaths of endangered boreal toads in the southern Rocky Mountains have been linked to a chytrid fungus, as also being responsible for amphibian die offs in Central America and Australia. Chytrid fungus in amphibians was first identified in 1998 by Green and other researchers from the U.S., Great Britain and Australia.

The Post Metamorphic Death Syndrome (PDS) is considered for the mortality of all or post metamorphic individuals in a short period of time. This disease agent may be the primary cause of certain amphibian declines in Northern West America. The proximal causes of death are usually widespread pathogens such as Aero monas (Red leg disease pathogen).

In Canada, factors related to human over­population, environmental contamination and habitat destruction have clearly been shown to be detrimental to amphibians, though the severity of the effect varies from species to species. In the North Eastern Greenland region, UV-B is linked to the amphibian declines.

It is considered a global phenomenon. The increased amount of ultra­violet radiation has reached the earth’s surface, as a result of destruction of ozone layer in stratosphere by the chemical pollutants, such as CFC and greenhouse effect.

The effects were confined at the poles at first, gradually are spreading into lower lati­tudes in both hemispheres. Ultraviolet light, in between 290-320 nanometer UV-B band, kills amphibian eggs and embryos.

In India, no sufficient work has done on the decline of amphibians. Maximum work has emphasised on survey.

K. Vasudevan (1998) reported the reasons of declines in Kalakad Mundanthurai Tiger Reserve, Southern India that the possible threat to amphibian species may be removal of top soil in large quantity by brick industries, and use of chemical fertilisers and pesticides, and human interference in the reserve forest which ultimately results in habi­tat loss.

From Bangladesh 13 species have been reported of which the population of Microhyla ornata, Microhyla rubra, Rana cyanophlictis, Rana hexadactyla and Rana tigrina are declin­ing fast for habitat destruction and use of insec­ticides.

In Vietnam 112 amphibian species have been recorded of which many species are valu­able economically and scientifically, have become rare.

Some are in danger of extinction or serious decrease such as Ichthyophis glutinosus, Paramesotriton deloustali, Bombina maxima, Rana chaepensis, Rana fansipani, Rana cancrivora, Rana kokchange, Rana tomanoffi, Rhacophorus appendiculatus and Rhacophorus nigropalmatus. The main reasons of declines are habitat destruction, overhunting and inappropriate exploitation.

Since the late 1970s, at least 14 frog species have declined or disappeared from rainforest areas of Queensland, Australia. The causes of the declines in Queensland’s upland rain-forest are still unclear. Recent work by Berger et al., (1998), however, indicates the possible involvement of a fungal pathogen in the declines.

No definite reason is considered for the decline of amphibian population, though different local reasons are forwarded in diffe­rent countries for the declines of population. A few years ago, acid rain, UV-B and parasites were the focusing points. The evidence now suggests that chemical contaminants should now be considered the most likely than the UV-B and parasites.

The US National Science Foundation (NSF) organised a workshop on amphibian declines in Washington DC, 28th and 29th May, 1998. Several speakers reviewed the latest informa­tion on the geography of amphibian declines.

It is clear from the reports that amphibians are continuing to decline worldwide, for a variety of reasons. The potential causes for declines are UV-B radiation, deformities, toxins, viruses, chytrid fungi, diseases in salamanders, climate changes arid immunology.

Lastly we can say that herpetofauna rep­resent a major part of our natural heritage. If these animals are in trouble, we are also in trouble. Amphibians and reptiles are the bio-indicators of the environmental pollution. If they decline and ultimately disappear, we need to make amends. What happens to her­petofauna is a sign of happening to other wild life and may be even to us.

FROGLOG, the bimonthly newsletter of the Declining Amphibian populations at the Department of Biology of The Open University. This news-letter helps to receive any news on amphibian declines at free cost.

In 1995 the Declining Amphibian Populations Task Force established an amphibian conservation forum, Amphibian Decline, on the internet. Subscribers of the forum may receive all e-mail sent to Amphibian Decline and may also send infor­mation that will be automatically distributed to other subscribers.

In 1989, the First World Congress of Herpetology was held in England and in a week-long discussion, it was known that amphibian populations that were once abun­dant, has become rare. The events that were isolated instances, gradually spread as a global pattern. In February, 1990, the scientists those were concerned about the vanishing amphibians, met at the West Coast Centre of the National Academy of Sciences.

In this conference it was known that amphibian populations were disappearing in different countries and often there was no apparent rea­son. Following that meeting an international effort was initiated to find out the causes of declines of amphibian populations by the Declining Amphibian Populations Task Force (DAPTF) of the Species Survival Commission (SSC) under the World Conservation Union (IUCN).

Now this effort is being conducted by the voluntary efforts of concerned herpetolo­gists. Regional Working Groups of the Task Force are monitoring the status of the amphi­bian populations in their areas.

At first different methods were adopted to monitor the declining amphibian populations, so a book — Measuring and Monitoring Biological Diversity : Standard Methods for Amphibians, published by Smithsonian University Press, Washington D. C. which contained standard methods for surveying declining populations.

In India several organisations are doing a good job in creating awareness among biolo­gists and common people for conservation of amphibians.

Froglog — the Newsletter of the Declining Amphibian populations Task Force — South Asia, the regional network of the Declining Amphibian populations Task Force, SSC, IUCN, is being edited by Sanjay Molur and Sushil Dutta, and published by Zoo Outreach Organisation and Conservation Breeding Specialist Group, India from PB 1683, 79 Bharathi Colony, Peelamedu, Coimbatore, Tamil Nadu, India.

Three-Chambered Heart

Frogs have a three-chambered heart, consisting of two atria and a single ventricle. Blood leaving the ventricle passes into a forked aorta, where the blood has an equal opportunity to travel through a circuit of vessels leading to the lungs or a circuit leading to the other organs. Blood returning to the heart from the lungs passes into one atrium, while blood returning from the rest of the body passes into the other. Both atria empty into the single ventricle. While this makes sure that some blood always passes to the lungs and then back to the heart, the mixing of oxygenated and deoxygenated blood in the single ventricle means the organs are not getting blood saturated with oxygen. Still, for a cold-blooded creature like the frog, the system works well.

1 Answer 1

The first thing to go would be posture. This whole standing upright would have to be reconsidered. After all, putting the most oxygen hungry organs in the body up at the top is asking a lot of our earth-born heart and veins. Completely prone would become everyone's preferred pose which might cause a sharp decline in personal productivity and mobility.

We would also have to give up salt. After all, who needs high blood pressure when your blood already ways three times as much as normal. Limiting dietary iron to induce anemia would become good medical advice for putting a little spring back in your day.

At least dieting would get easier. Loosing multiple pounds per week is easy when even the lightest broth feels like thick pea soup on the tongue. Pigging out on ten pounds of ice cream would still leave the gallon container half full.

Okay, so we aren't going to get very far on this planet in our current condition. Time for the genetic scientists to step in.

The name of the game is decentralization. Instead of relying on a single centralized pump, smaller fluid flow assistance organs should be spread out around the body. No major vein or artery should be run for more than a foot before reaching the next of these smaller hearts. In blood pathways which defy gravity, the hearts should be even closer together.

In later generations of the genetic manipulation, the entire idea of hearts and veins will be abandoned. Instead the blood passage ways would themselves be pumps. Every inch would be segmented with valves and lined with muscles such that no portion of the system simply holds blood. Every part contributes to the circulation of the body's blood supply.

Along with the enhanced plumbing, it would probably be wise to rearrange some of the systems. Eyes and ears don't weigh much so they can stay up at the top, but the brain could migrate downwards. Men have been accused for centuries that we keep our brains in our pants. Now in a very literal sense, that would be true of women as well.

We would want to keep the overall humanoid shape unchanged for aesthetics as well as to assist with dealing with other humans from other worlds. But we would not want to put anything heavy up in the head section with the ears and the eyes. So maybe just a cartilage shell containing helium. We are the airheads and proud of it!

Which leaves the bones and muscles. that is the tough part. It is sort of like building a muscle car. A bigger engine produces more power but it weighs more meaning it takes more power to move. The geneticists would have to work closely with material scientists. The age of all carbon and calcium would have to end. Light weight metals and silicon cording might replace the bones and tendons, allowing our current muscle mass or maybe even a little less to carry the load. If not, then we would have to plunder the animal kingdom for a muscle upgrade. Human muscle to strength ratios are abysmal compared to many animals.

Culture of a Hive Insect Population

The defining characteristic of the hive mind is an unusually strong correlation between the actions of individuals, apparently centered around the mind (historically depicted as the "queen" of the hive). In order to do this, there must be communication -- constant communication. Much of this communication is built at the genetic level (especially given the short lifespan of bees, giving few opportunities for learning). This is where your question gets interesting. Going to the moon is a HUGE communications gap, which limits this communication. This is going to drastically decrease the culture's desire to travel to the moon. It's just not the nature of a hive mind's mentality.

Two directions which could help are RF technology and genetics. Consider a world colonized by one or more of these hives (you did not specify if the entire species was a hive, or if there were multiple competing hives, like ants). They would eventually reach a balance, and live in the best harmony the planet can afford. The discovery of RF communication could dramatically increase the rate of their communication, and allow it to occur through the vacuum of space. This would, in a flash provide a new surface to colonize and trigger a space race.

However, a hive mind would use communication as simple as ours, so it wouldn't be as simple as a phone conversation. Hive minds require coherency, and the easiest way to do this is analog communication such as sounds or dances (digital brings up all sorts of timing woes that, in my opinion, prevent a hive from functioning adequately). Just because they had a new technology like RF wouldn't cause them to immediately shoot for the moon. They would need to adapt the technology to their communications.

Thinking about how such a race would become so powerful, their individuals would have to be well programmed. Such a race would probably need a way to genetically wire their workers to listen to the right commands and act accordingly. They would probably conquer genetics and biology long before RF. Accordingly, they could design workers that would be custom engineered to respond to RF communication just like their primitive songs and dances.

I would also expect them to split the difference between biology and technology: I would expect the technology to be physically adapted to be more similar to their primitive songs and dances. How much of this balance is genetic, and how much its technological is pretty much up to you.

36.7 Behavioral Biology: Proximate and Ultimate Causes of Behavior

In this section, you will explore the following questions:

  • What is the difference between innate and learned behavior?
  • How are movement and migration behaviors a result of natural selection?
  • What are different ways members of a population communicate with each other?
  • What are examples of how species use energy for mating displays and other courtship behaviors?
  • What are examples of various mating systems?
  • What are different ways that species learn?

Connection for AP ® Courses

Behavior is the change in activity of an organism in response to a stimulus. Innate behaviors have a strong genetic component and are largely independent of environmental influences. In other words, these instinctive behaviors are “hard wired.” Examples of innate behaviors include a human baby grabbing her mother’s finger and the stork using its long beak to forage. Learned behaviors result from environmental conditioning and are modified by learning. For example, you likely have learned by now that reading these AP ® Connections help you digest the information and that studying for a test improves your grade.

Information presented and the examples highlighted in the section support concepts outlined in Big Idea 2 and Big Idea 3 of the AP ® Biology Curriculum Framework. The AP ® Learning Objective listed in the Curriculum Framework provide a transparent foundation for the AP ® Biology course, an inquiry-based laboratory experience, instructional activities, and AP ® exam questions. A learning objective merges required content with one or more of the seven science practices.

Big Idea 2 Biological systems utilize free energy and molecular building blocks to grow, to reproduce, and to maintain dynamic homeostasis.
Enduring Understanding 2.C Organisms use feedback mechanisms to regulate growth and reproduction, and to maintain dynamic homeostasis.
Essential Knowledge 2.C.2 Organisms respond to changes in their external environments.
Science Practice 6.4 The student can make claims and predictions about natural phenomena based on scientific theories and models.
Learning Objective 4.13 The student is able to predict the effects of a change in the community’s populations on the community.
Enduring Understanding 2.E Many biological processes involved in growth, reproduction and dynamic homeostasis include temporal regulation and coordination.
Essential Knowledge 2.E.3 Timing and coordination of behavior are regulated by various mechanisms and are important in natural selection.
Science Practice 4.1 The student can justify the selection of the kind of data needed to answer a particular scientific question.
Learning Objective 2.21 The student is able to justify the selection of the kind of data needed to answer scientific questions about the relevant mechanism that organisms use to respond to changes in their external environment.
Essential Knowledge 2.E.3 Timing and coordination of behavior are regulated by various mechanisms and are important in natural selection.
Science Practice 5.1 The student can analyze data to identify patterns or relationships.
Learning Objective 2.38 The student is able to analyze data to support the claim that responses to information and communication of information affect natural selection.
Essential Knowledge 2.E.3 Timing and coordination of behavior are regulated by various mechanisms and are important in natural selection.
Science Practice 6.1 The student can justify claims with evidence.
Learning Objective 2.39 The student is able to justify scientific claims, using evidence, to describe how timing and coordination of behavioral events in organisms are regulated by several mechanisms.
Essential Knowledge 2.E.3 Timing and coordination of behavior are regulated by various mechanisms and are important in natural selection.
Science Practice 7.2 The student can connect concepts in and across domain(s) to generalize or extrapolate in and/or across enduring understandings and/or big ideas.
Learning Objective 2.40 The student is able to connect concepts in and across domain(s) to predict how environmental factors affect response to information and change behavior.
Big Idea 3 Living systems store, retrieve, transmit and respond to information essential to life processes.
Enduring Understanding 3.E Transmission of information results in changes within and between biological systems.
Essential Knowledge 3.E.1 Individuals can act on information and communicate it to others.
Science Practice 5.1 The student can analyze data to identify patterns or relationships.
Learning Objective 3.40 The student is able to analyze data that indicate how organisms exchange information in response to internal changes and external cues, and which can change behavior.
Essential Knowledge 3.E.1 Individuals can act on information and communicate it to others.
Science Practice 1.1 The student can create representations and models of natural or man-made phenomena and systems in the domain.
Learning Objective 3.41 The student is able to create a representation that describes how organisms exchange information in response to internal changes and external cues, and which can result in changes in behavior.
Essential Knowledge 3.E.1 Individuals can act on information and communicate it to others.
Science Practice 7.1 The student can connect phenomena and models across spatial and temporal scales.
Learning Objective 3.42 The student is able to describe how organisms exchange information in response to internal changes or environmental cues.

Behavioral biology is the study of the biological and evolutionary bases for such changes. The idea that behaviors evolved as a result of the pressures of natural selection is not new. Animal behavior has been studied for decades, by biologists in the science of ethology, by psychologists in the science of comparative psychology, and by scientists of many disciplines in the study of neurobiology. Although there is overlap between these disciplines, scientists in these behavioral fields take different approaches. Comparative psychology is an extension of work done in human and behavioral psychology. Ethology is an extension of genetics, evolution, anatomy, physiology, and other biological disciplines. Still, one cannot study behavioral biology without touching on both comparative psychology and ethology.

One goal of behavioral biology is to dissect out the innate behaviors, which have a strong genetic component and are largely independent of environmental influences, from the learned behaviors, which result from environmental conditioning. Innate behavior, or instinct, is important because there is no risk of an incorrect behavior being learned. They are “hard wired” into the system. On the other hand, learned behaviors, although riskier, are flexible, dynamic, and can be altered according to changes in the environment.

Innate Behaviors: Movement and Migration

Innate or instinctual behaviors rely on response to stimuli. The simplest example of this is a reflex action, an involuntary and rapid response to stimulus. To test the “knee-jerk” reflex, a doctor taps the patellar tendon below the kneecap with a rubber hammer. The stimulation of the nerves there leads to the reflex of extending the leg at the knee. This is similar to the reaction of someone who touches a hot stove and instinctually pulls his or her hand away. Even humans, with our great capacity to learn, still exhibit a variety of innate behaviors.

Kinesis and Taxis

Another activity or movement of innate behavior is kinesis, or the undirected movement in response to a stimulus. Orthokinesis is the increased or decreased speed of movement of an organism in response to a stimulus. Woodlice, for example, increase their speed of movement when exposed to high or low temperatures. This movement, although random, increases the probability that the insect spends less time in the unfavorable environment. Another example is klinokinesis, an increase in turning behaviors. It is exhibited by bacteria such as E. coli which, in association with orthokinesis, helps the organisms randomly find a more hospitable environment.

A similar, but more directed version of kinesis is taxis: the directed movement towards or away from a stimulus. This movement can be in response to light (phototaxis), chemical signals (chemotaxis), or gravity (geotaxis) and can be directed toward (positive) or away (negative) from the source of the stimulus. An example of a positive chemotaxis is exhibited by the unicellular protozoan Tetrahymena thermophila. This organism swims using its cilia, at times moving in a straight line, and at other times making turns. The attracting chemotactic agent alters the frequency of turning as the organism moves directly toward the source, following the increasing concentration gradient.

Fixed Action Patterns

A fixed action pattern is a series of movements elicited by a stimulus such that even when the stimulus is removed, the pattern goes on to completion. An example of such a behavior occurs in the three-spined stickleback, a small freshwater fish (Figure 36.35). Males of this species develop a red belly during breeding season and show instinctual aggressiveness to other males during this time. In laboratory experiments, researchers exposed such fish to objects that in no way resemble a fish in their shape, but which were painted red on their lower halves. The male sticklebacks responded aggressively to the objects just as if they were real male sticklebacks.


Migration is the long-range seasonal movement of animals. It is an evolved, adapted response to variation in resource availability, and it is a common phenomenon found in all major groups of animals. Birds fly south for the winter to get to warmer climates with sufficient food, and salmon migrate to their spawning grounds. The popular 2005 documentary March of the Penguins followed the 62-mile migration of emperor penguins through Antarctica to bring food back to their breeding site and to their young. Wildebeests (Figure 36.36) migrate over 1800 miles each year in search of new grasslands.

Although migration is thought of as innate behavior, only some migrating species always migrate (obligate migration). Animals that exhibit facultative migration can choose to migrate or not. Additionally, in some animals, only a portion of the population migrates, whereas the rest does not migrate (incomplete migration). For example, owls that live in the tundra may migrate in years when their food source, small rodents, is relatively scarce, but not migrate during the years when rodents are plentiful.


Foraging is the act of searching for and exploiting food resources. Feeding behaviors that maximize energy gain and minimize energy expenditure are called optimal foraging behaviors, and these are favored by natural section. The painted stork, for example, uses its long beak to search the bottom of a freshwater marshland for crabs and other food (Figure 36.37).

Innate Behaviors: Living in Groups

Not all animals live in groups, but even those that live relatively solitary lives, with the exception of those that can reproduce asexually, must mate. Mating usually involves one animal signaling another so as to communicate the desire to mate. There are several types of energy-intensive behaviors or displays associated with mating, called mating rituals. Other behaviors found in populations that live in groups are described in terms of which animal benefits from the behavior. In selfish behavior, only the animal in question benefits in altruistic behavior, one animal’s actions benefit another animal cooperative behavior describes when both animals benefit. All of these behaviors involve some sort of communication between population members.

Communication within a Species

Animals communicate with each other using stimuli known as signals. An example of this is seen in the three-spined stickleback, where the visual signal of a red region in the lower half of a fish signals males to become aggressive and signals females to mate. Other signals are chemical (pheromones), aural (sound), visual (courtship and aggressive displays), or tactile (touch). These types of communication may be instinctual or learned or a combination of both. These are not the same as the communication we associate with language, which has been observed only in humans and perhaps in some species of primates and cetaceans.

A pheromone is a secreted chemical signal used to obtain a response from another individual of the same species. The purpose of pheromones is to elicit a specific behavior from the receiving individual. Pheromones are especially common among social insects, but they are used by many species to attract the opposite sex, to sound alarms, to mark food trails, and to elicit other, more complex behaviors. Even humans are thought to respond to certain pheromones called axillary steroids. These chemicals influence human perception of other people, and in one study were responsible for a group of women synchronizing their menstrual cycles. The role of pheromones in human-to-human communication is still somewhat controversial and continues to be researched.

Songs are an example of an aural signal, one that needs to be heard by the recipient. Perhaps the best known of these are songs of birds, which identify the species and are used to attract mates. Other well-known songs are those of whales, which are of such low frequency that they can travel long distances underwater. Dolphins communicate with each other using a wide variety of vocalizations. Male crickets make chirping sounds using a specialized organ to attract a mate, repel other males, and to announce a successful mating.

Courtship displays are a series of ritualized visual behaviors (signals) designed to attract and convince a member of the opposite sex to mate. These displays are ubiquitous in the animal kingdom. Often these displays involve a series of steps, including an initial display by one member followed by a response from the other. If at any point, the display is performed incorrectly or a proper response is not given, the mating ritual is abandoned and the mating attempt will be unsuccessful. The mating display of the common stork is shown in Figure 36.38.

Aggressive displays are also common in the animal kingdom. An example is when a dog bares its teeth when it wants another dog to back down. Presumably, these displays communicate not only the willingness of the animal to fight, but also its fighting ability. Although these displays do signal aggression on the part of the sender, it is thought that these displays are actually a mechanism to reduce the amount of actual fighting that occurs between members of the same species: they allow individuals to assess the fighting ability of their opponent and thus decide whether it is “worth the fight.” The testing of certain hypotheses using game theory has led to the conclusion that some of these displays may overstate an animal’s actual fighting ability and are used to “bluff” the opponent. This type of interaction, even if “dishonest,” would be favored by natural selection if it is successful more times than not.

Distraction displays are seen in birds and some fish. They are designed to attract a predator away from the nest that contains their young. This is an example of an altruistic behavior: it benefits the young more than the individual performing the display, which is putting itself at risk by doing so.

Many animals, especially primates, communicate with other members in the group through touch. Activities such as grooming, touching the shoulder or root of the tail, embracing, lip contact, and greeting ceremonies have all been observed in the Indian langur, an Old World monkey. Similar behaviors are found in other primates, especially in the great apes.

Link to Learning

The killdeer bird distracts predators from its eggs by faking a broken wing display in this video taken in Boise, Idaho.

  1. The parent creates a distraction to attract the predator away from young fledgling by pretending to have a broken wing. It is an altruistic behavior as the parent runs the risk of getting killed or harmed by predator.
  2. The parent creates a distraction by being more aggressive and showing its willingness to fight. Altruistic behavior is seen as the parent runs the risk of getting attacked, killed, or harmed by the predator.
  3. Parent creates distraction to attract the predator away from young fledgling by pretending to have a broken wing. It is exhibiting an altruistic behavior as in saving its fledgling it is increasing its own fitness along with the fitness of the young bird.
  4. Parent creates distraction by being more aggressive and showing its willingness to fight. It is exhibiting an altruistic behavior by saving its fledgling it is decreasing its own fitness along with the fitness of the young bird.

Altruistic Behaviors

Behaviors that lower the fitness of the individual but increase the fitness of another individual are termed altruistic. Examples of such behaviors are seen widely across the animal kingdom. Social insects such as worker bees have no ability to reproduce, yet they maintain the queen so she can populate the hive with her offspring. Meerkats keep a sentry standing guard to warn the rest of the colony about intruders, even though the sentry is putting itself at risk. Wolves and wild dogs bring meat to pack members not present during a hunt. Lemurs take care of infants unrelated to them. Although on the surface, these behaviors appear to be altruistic, it may not be so simple.

There has been much discussion over why altruistic behaviors exist. Do these behaviors lead to overall evolutionary advantages for their species? Do they help the altruistic individual pass on its own genes? And what about such activities between unrelated individuals? One explanation for altruistic-type behaviors is found in the genetics of natural selection. In the 1976 book, The Selfish Gene, scientist Richard Dawkins attempted to explain many seemingly altruistic behaviors from the viewpoint of the gene itself. Although a gene obviously cannot be selfish in the human sense, it may appear that way if the sacrifice of an individual benefits related individuals that share genes that are identical by descent (present in relatives because of common lineage). Mammal parents make this sacrifice to take care of their offspring. Emperor penguins migrate miles in harsh conditions to bring food back for their young. Selfish gene theory has been controversial over the years and is still discussed among scientists in related fields.

Even less-related individuals, those with less genetic identity than that shared by parent and offspring, benefit from seemingly altruistic behavior. The activities of social insects such as bees, wasps, ants, and termites are good examples. Sterile workers in these societies take care of the queen because they are closely related to it, and as the queen has offspring, she is passing on genes from the workers indirectly. Thus, it is of fitness benefit for the worker to maintain the queen without having any direct chance of passing on its genes due to its sterility. The lowering of individual fitness to enhance the reproductive fitness of a relative and thus one’s inclusive fitness evolves through kin selection. This phenomenon can explain many superficially altruistic behaviors seen in animals. However, these behaviors may not be truly defined as altruism in these cases because the actor is actually increasing its own fitness either directly (through its own offspring) or indirectly (through the inclusive fitness it gains through relatives that share genes with it).

Unrelated individuals may also act altruistically to each other, and this seems to defy the “selfish gene” explanation. An example of this observed in many monkey species where a monkey will present its back to an unrelated monkey to have that individual pick the parasites from its fur. After a certain amount of time, the roles are reversed and the first monkey now grooms the second monkey. Thus, there is reciprocity in the behavior. Both benefit from the interaction and their fitness is raised more than if neither cooperated nor if one cooperated and the other did not cooperate. This behavior is still not necessarily altruism, as the “giving” behavior of the actor is based on the expectation that it will be the “receiver” of the behavior in the future, termed reciprocal altruism. Reciprocal altruism requires that individuals repeatedly encounter each other, often the result of living in the same social group, and that cheaters (those that never “give back”) are punished.

Evolutionary game theory, a modification of classical game theory in mathematics, has shown that many of these so-called “altruistic behaviors” are not altruistic at all. The definition of “pure” altruism, based on human behavior, is an action that benefits another without any direct benefit to oneself. Most of the behaviors previously described do not seem to satisfy this definition, and game theorists are good at finding “selfish” components in them. Others have argued that the terms “selfish” and “altruistic” should be dropped completely when discussing animal behavior, as they describe human behavior and may not be directly applicable to instinctual animal activity. What is clear, though, is that heritable behaviors that improve the chances of passing on one’s genes or a portion of one’s genes are favored by natural selection and will be retained in future generations as long as those behaviors convey a fitness advantage. These instinctual behaviors may then be applied, in special circumstances, to other species, as long as it doesn’t lower the animal’s fitness.

Finding Sex Partners

Not all animals reproduce sexually, but many that do have the same challenge: they need to find a suitable mate and often have to compete with other individuals to obtain one. Significant energy is spent in the process of locating, attracting, and mating with the sex partner. Two types of selection occur during this process and can lead to traits that are important to reproduction called secondary sexual characteristics: intersexual selection, the choosing of a mate where individuals of one sex choose mates of the other sex, and intrasexual selection, the competition for mates between species members of the same sex. Intersexual selection is often complex because choosing a mate may be based on a variety of visual, aural, tactile, and chemical cues. An example of intersexual selection is when female peacocks choose to mate with the male with the brightest plumage. This type of selection often leads to traits in the chosen sex that do not enhance survival, but are those traits most attractive to the opposite sex (often at the expense of survival). Intrasexual selection involves mating displays and aggressive mating rituals such as rams butting heads—the winner of these battles is the one that is able to mate. Many of these rituals use up considerable energy but result in the selection of the healthiest, strongest, and/or most dominant individuals for mating. Three general mating systems, all involving innate as opposed to learned behaviors, are seen in animal populations: monogamous, polygynous, and polyandrous.

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