Amphibians

Amphibians are surprisingly variable. There are three main types of Amphibians (Cochran 8). They are caecilians, salamanders, and anurans. Caecilians are legless, wormlike amphibians (Vogel 79). They normally live in burrows of tropical regions. All salamanders have scaleless skin, four legs and a tail. Anurans are either frogs or toads. They normally move around in hops.

Many amphibians metamorphose, or change, from a gill breathing aquatic larvae to an air breathing terrestrial adults. These amphibians have great abilities. A new born tadpole carries an aquatic life. After it metamorphoses, it becomes terrestrial. It is like being born again. The tadpole learns to travel by means of jumps, as well as to swim with limbs.

This variability of amphibians makes them one of the most interesting species to study and indeed amphibians have contributed a great deal to science. Studies on amphibian metamorphosis provide scientists with knowledge about the actions of the thyroid and pituitary hormones. Because of the ease of their breeding in laboratories and their relatively simple chromosome complements, important advances in studies of hybridization and speciation have been made. The study of vocals of frogs have provided advances in animal sound communication. The poison from poison-dart frogs shows great medical promise, especially for patients who do not respond well to pain killers made from opium seeds. The poison is far stronger and is two-hundred times more powerful than morphine, in killing pain.

Amphibians are the oldest known land vertebrates and accordingly have been ideal subjects to study the process of evolution. The earliest amphibians existed 360 million years ago (Radinsky 87). The early amphibians known as ichthyostegids, had a tail fin and canals on the skull. These two features suggest that they spent most of their time in the water.

Between 340 and 250 million years ago, the population of amphibians grew, and several groups emerged. One group of amphibians, known as labyrinthodonts, grew to extraordinary size. They could be up to four feet long. They were short-legged and large headed. Their skulls were deep and massive, and their jaws were lined with small, sharp, conical teeth. Also, there was a second row of teeth on the roof of the mouth. Many labyrinthodonts had a notch on the back of the skull which acted as a large eardrum and it transmitted vibrations to the inner ear. Their bulky skeleton and their short limbs suggest that the majority of the labyrinthodonts were slow, clumsy walkers on land, and they probably spent most of their time in shallow water. The large jaw and sharp teeth suggest that they were predators and they fed on large prey.

As some labrythodonts strengthened their limbs as adaptations for life on land, they reduced the size and strength of their limbs. The weaker ones developed long, flexible bodies with weak vertebrae columns. Scientist interpret this to mean that they had secondarily adapted to full aquatic life. Their heads flatted which suggested that they spent much time lying on the floor of a lake or pond. Some developed long and narrow snouts, which was a good characteristic to have for traveling through water.

Another grouping of early amphibians were the lepospondyls. They were mostly small, less than a foot, with long, slender bodies and weak legs. The lepospondyls lacked the notch on the back of the skull. They had small sharp teeth which suggested that they fed on small invertebrates.

A later group, known as the nectrideans evolved more long bodies. Their weaker limbs, and less stiffened vertebrae columns suggested that they had almost become completely aquatic. Like the labrinthodonts, the nectrideans had the notch on the back of their skull which acted as an ear drum.

A last group called the diplocaulids, developed large, flattened, triangular skulls, with the dorsal eyes and nostrils that are normally a characteristic of animals that live on the floor of a body of water. The study of these early amphibians provide convincing evidence of evolution.

Most amphibians go through annual periods of mating (Duellman 19). In anurans, rainfall can cause mating. Amphibians reproduce sexually. Some amphibians lay their eggs and let them develop by themselves. Others, lay eggs and guard them. The mother is usually the one who guards the eggs. A salamander will guard her eggs by wrapping herself around them. Some anurans carry their eggs imbedded in their backs.

All amphibian eggs are basically the same in that there are layers of semipermeable membranes surrounding the ovum. However, a lot of differences exist in individual eggs, such as size. Eggs laid in water form into large clumps, or they are scattered and deposited at different sites. Normally, the clumps are attached to sticks or vegetation in the water. This serves to maintain the position of the clutch. The terrestrial eggs are sometimes in strands connected by jelly between each of them. They can also be stacked on top of each other in a pile. Most anurans sort their eggs in piles and form nest around them.

Amphibians embryos contain all the nutrients for their development until hatching. Even if the mother bears live young, all of the nutrients for embryonic development are provided by the yolk, not the maternal tissues. Amphibian embryos normally get oxygen from external gills. In salamanders, three pairs of external gills provide for oxygen in take. Amphibian embryos generally remove waste in the form of ammonia.

The majority of amphibian larvae are aquatic. Unlike the embryos, amphibian larvae obtain nutrients from the environment for development and growth. Most larvae feed on small aquatic invertebrates. Some amphibians eat algae and continue to feed and grow until there is no more algae left. Larvae growth rates are dependent mostly on temperature and food availability. Some chemicals left by previous larvae can slow growth down. Anurans tadpoles are mostly short and develop length through growth, in salamanders the larvae are long.

Most species of salamander and anurans stay in schools as larvae. They stay in schools to avoid predators and to have bigger food supply. An organism that is slightly bigger than the larvae could not attack a school. Some species of tadpoles stir up the bottom, releasing mixtures of particles of food.

A metamorphosis is a series of complex physiological, biochemical, and behavioral transformations. Three major kinds of changes occur during an amphibian's metamorphosis (Vogel 83). The first one is the removal of structures and functions that are significant only to the larvae. The second kind is transformation of larval structures into a form suitable for adult use. The last kind is development of new structures and functions that are essential to the adult.

There are many fundamental as well as subtle changes throughout the changes of metamorphosis. For example, during metamorphosis, larger larval red blood cells are replaced with by smaller adult blood cells. Also, during metamorphosis, the amphibian gills are completely removed. In anurans, the intestine reduces in its size during metamorphosis. Even the vitamins in the eye change. In larvae, the eye contains vitamin A₂, while the adults have vitamin A₁ in the eye. Perhaps, the greatest changes occur in the respiratory mechanisms. Lungs are formed when the flow of amino acids and thymidine into the lung tissues is increased. In the gill tissues, the flow of amino acids and thymidine is decreased causing them to eventually disappear.

Some of the more obvious changes are the removal of the tail and development of eyelids. Initially, the fins reduce in size. Then the tail becomes smaller and smaller until it is removed. The formation of the mouth is very important to an amphibian. During the development of the mouth and jaw, the animal is unable to feed. In preparation for this phase, large quantities of foods are stored.

Metamorphosis is a fascinating transformation, providing scientists with a greater understanding of the biochemical and physiological processes.

Most amphibians avoid daytime temperatures and low humidity. During the day, they usually stay in areas with high moisture content, and they stay in insulated areas away from air currents. Inside a log and in mounds of soils serve as good places to spend the day. Amphibians may come out during the day, but only if there is a sufficient amount of moisture. However, they might risk water loss or even death to accomplish some goal such as feeding or mating. Amphibians can reduce water loss by reducing the amount of surface area exposed to evaporation. Some salamanders coil their bodies tightly to prevent evaporative water loss. Tree frogs reduce surface area by selecting a shaded site and tucking limbs close to the body. Some amphibians dig deep burrows and stay there for up to nine months at a time. Terrestrial amphibians are generally nocturnal, with the exception of some species of anurans.

The skin of amphibians is highly permeable. Most salamanders and all frogs that live in aquatic areas have smooth skin on the belly and sides. Most terrestrial anurans have rough skin on the belly and the thighs. The rough stomach surfaces provide a great surface for water absorption. Even here, the amphibians have proven to be capable of "double life". For example, sometimes, the amphibian may not want water to enter the body. For such occasions, some amphibians have developed ways of waterproofing the skin. They form a cocoon that encases the body. They make these cocoons during long periods of dormancy and during the day. An example would be the salamander Siren intermedia. Siren intermedia burrows into the mud at the bottom of drying ponds (Duellman 198). They make the cocoons to prevent too much moisture from entering the body. In frogs, the cocoon is made of a dry substance called statum corneum. It encases the entire frog leaving openings in the nostrils.

Amphibians generally have temperatures close to that of their immediate surroundings, and are therefore, categorized as cold blooded. Amphibians are not capable of internal heat-production so the body temperature and environment temperature are about the same. Therefore, amphibians are tolerant to a wide range of temperature. For example, some Central American salamanders can stand temperature between -2.0�C and 30�C. Some anurans can stand temperatures between 3.0�C and 35.7�C. Also, to raise body temperature, some anurans lay in the sun; however, this could create water loss problems, requiring them to balance their needs.

The feeding strategies of amphibians include their choice of prey and the ways they locate, capture, and eat the prey. All adult amphibians are carnivores. They feed mostly on insects and, but some eat a wide variety of invertebrates. Some large anurans, such as Ceratophrys ornata feed on large prey, such as birds, turtles, snakes, and other anurans (Vogel 83).

As amphibians grow larger, the kinds of prey they select may change. As larvae develop teeth, they capture larger prey. Hylid frogs eat increasingly larger preys, even during postmetamorphosis.

Seasonal difference in diets have been reported for various species of amphibians The diet of anurans living in West Africa vary greatly throughout the year. Among the thirteen species of anurans living in Amazonian Peru, the difference in food was greatest during the dry season. These differences are indications of the availability of prey. Some animals are dormant during the dry season. In some amphibians, the selection of larger prey is likely when moisture conditions are heavy.

Amphibians vary considerably in their hunting practices. The vast majority of anurans and salamanders use vision to hunt prey. For the species that have developed the sit-and-wait strategy, vision is important. Once a prey is sighted, it may be followed for a short distance any then captured. Sight is also important in identifying kinds of prey, such as those with a large energy content and or that may be distasteful or harmful. For example, toads learned to reject bumblebees by sight alone. Amphibians also use smell and hearing to hunt prey. Some species of toads can locate prey just by smell alone. Smell sense is of great value in tracking the prey, once it is located. Some amphibians can also detect insects by the sounds they make. A species of toad, Bufo marinus, is attracted to calling insects.

Amphibians also show major differences in ways of capturing and in taking the food; however, all terrestrial amphibians except caecilians use the tongue in capturing prey. Many caecilians and large anurans use fang like teeth to hold struggling prey. The tongues of amphibians have glands that produce a sticky substance, that immobilizes the prey.

Terrestrial caecilians feed primarily on long prey, such as earthworms located on the ground or in burrows. Prey capture involves a slow approach towards the prey until contact is almost made, then the prey is captured by a powerful bite.

In terrestrial salamanders, the tongue plays an important role in prey capture. The salamander's tongue has a sticky substance produced by a gland. The salamander sticks out its tongue and the prey gets stuck to it. Then the salamander pulls the tongue in the mouth. The tongue can reach up to eight percent of the length its body. The entire capture a prey lasts 0.10-0.15 seconds.

Anurans flip their tongues at their prey. Like salamander's tongue, it is has a sticky substance that is produced by a gland. The prey gets stuck to the tongue and then the anuran pulls the tongue back into the mouth. The process is almost exactly the same as the salamander. The completely aquatic frogs, known as pipids, do not have tongues. Therefore, they have an entirely different means catching their food. They suck in food and water, the water leaves the mouth before it closes completely.

Amphibians are no different than any other animal because they can be harmed by a wide variety of predators, parasites, and diseases. Most caecilians, some salamanders, and some anurans are known to be near the bottom of the food chain (Cochran 11). Amphibians are also subject to many diseases.

Amphibians can be harmed by many parasites. While, most of the time parasites live without harm. Massive infections over an amphibian population has been known to cause great disaster. For example, a parasite called Pleistophora which is normally a parasite of fish, caused a lethal epidemic of the toad Bufo bufo in southern England (Cochran 56). Infestations of the parasite, Carchesium have been known to clog the gills of tadpoles, causing retardation and death.

Amphibians are prey for a great variety of predators because they are small and they have soft skin. Because of their "double life", amphibians encounter predators both in water and on land. The predators include all classes of vertebrates and some arthropods. Some small anurans are even prey to the plant specie, Venus flytrap.

Aquatic eggs of amphibians are mostly the prey of fish and aquatic invertebrates. The leech is the most common invertebrate predator of the eggs. Some salamanders feed on each others' eggs. Larval and adult newts also feed on eggs of some species of anurans and salamanders.

Terrestrial eggs are eaten by a some groups of anurans and a variety of insects and vertebrates. They include the spider, cricket, crabs, and snakes. The snake from the genus Leptodeira can shape their jaw to part of a clutch of eggs and devour the rest of clutch because the eggs stick together.

Generally, amphibians have been classified as defenseless creatures (Duellman 244). However, they have evolved some features which will provide some protection. Amphibians have a variety of encounter behaviors. One example of a characteristic that has been evolved is escape behavior. Escape behavior is when a prey senses the presence of a predator and then attempt to leave the area. A terrestrial caecilian will dig into the soil quickly when it senses a predator. When an aquatic caecilian meets a predator, it will spit a small blob of water. Also, it will produce large quantities of mucus, which make it very difficult to hold. Some caecilians are capable of inflicting painful bites and some have poisonous secretions. A salamander will get into the position known as the Unken reflex. This is a immobile posture when the chin and tail are elevated. Toxic skin secretions come from the glands and cover the salamander. Salamanders also lash their tails at a predator. Toxic secretions also cover the tail. Some salamanders also head-butt their predators. They flex the head downward and lunge at the predator. Most anurans seem to rely on escape behavior to avoid predators, but some frogs are extremely poisonous. Many species of amphibians have color patterns that will match the environment they live in, thus providing a camouflage

One of the most interesting amphibian is from the anurans group. Phyllobates terribilis, a specie of poison-dart frog, produces one of the most toxic non-protein substances. The poison is so strong that it can be lethal to touch. Only 55 out of every 135 species of poison-dart frogs are known to be toxic. In addition, the frogs have neon colored skin which warns the predators to stay away. In the Amazon basin, the natives use the poison-dart frogs to poison their blow gun darts, which they use for their hunting for food. To poison their darts, they rub the dart against the frog's skin (Moffet 98). The poison is effective for more than a year. Poison-dart frogs range from a half inch to three inches, in size. They are found in a small area of lowland rain forest in western Columbia, where they contribute to human beings survival.

Amphibians have provided and continue to provide scientists with extraordinary opportunities to study and increase their knowledge. Specifically, amphibians have contributed to an understanding of the evolutionary process, particularly as to how physical features and behavior patterns can be altered to adapt to changing and diverse environments.

Works Cited:

Cochran, Doris. Living Amphibians of the World. New York: Doubleday & Company Inc., 1962

Duellman, William. Biology of Amphibians. Maryland: The John Hopkins University Press, 1994.

Moffett, Mark. "Poison-Dart Frogs." National Geographic May 1995: Vol. 187

Radinsky, Leonard. The Evolution of Vertebrate Design. Chicago: The University of Chicago Press, 1987

Vogel, Zdenek. Reptiles and Amphibians. New York: The Viking Press, 1964