According to paleontologists, the oldest fossils of amphibians date back to the end of the Devonian Period—more than 350 million years ago. The Devonian Period was a time of great ecological change, which resulted from massive disruptions of the surface features of the earth. For the evolution of amphibians, the most significant of these were changes that altered the level of the seas.
As water levels rose and then receded, the sea left behind organic materials in which plants could thrive. This tended to encourage the development of areas of lush vegetation in swamplike coastal regions. These conditions favored creatures that could get their oxygen from air as well as from water, and it is probable that it was in such an environment that the first amphibians evolved.
In prehistoric times, there were at least six orders of amphibians (class Amphibia), but now only three orders remain: Urodela, or newts and salamanders; Anura, or frogs and toads; and Apoda, or caecilians. Although the majority of amphibians inhabit the tropics, representatives of the class are found throughout the world, except in places where there is no water at all. In cold climates, amphibians hibernate during winter. Like most animals except birds and mammals, they are poikilother-mal, which means that their body temperature changes with fluctuations in the temperature of their environment.
Amphibians vary greatly in size, from the African sedge, or reed, frogs of the Hyperolius genus, which measure an inch or less in length, to the Japanese giant salamander and a caecilian from Colombia at more than 5 feet (1.5 meters) long.
Amphibians have a wide flat skull attached to the spinal column, which may be short, as in frogs and toads, which usually have eight vertebrae in the trunk, or extremely long, with as many as 250 vertebrae in some salamanders and caecilians. The urodeles and anurans have four limbs, with four fingers on each forelimb and five toes on each hindlimb. The apods have lost their limbs. Unlike the urodeles, adult anurans have no tails.
The skin of all amphibians is toughened (cornified) on the animal’s upper surface, and smooth on the lower surface. All shed their skin regularly. The skin produces slimy or poisonous secretions that make these animals unpalatable to most predators and so afford protection. In many species, the skin color can change, usually for camouflage or mating purposes. The skin also plays a part in respiration.
In evolutionary terms, one of the most profound modifications needed for animals to survive the emergence from an aquatic environment was the ability to absorb oxygen from air rather than from water. In amphibians, perhaps reflecting stages in this transition, respiration takes place in gills, lungs, the lining of the mouth, or the external skin, either alone or in combination, depending on the species and the stage of development. The role of the skin in respiration varies. In some species it is not particularly important, while for others the skin is the sole means of absorbing oxygen. For example, the skin of some lungless salamanders, such as the European alpine salamander, absorbs oxygen directly from the aerated water of the mountain streams in which the salamanders live.
Amphibians exhibit a variety of sensory needs. In almost all species the sense of touch is well developed, but the development of organs for sensing things at distance—for sight, hearing and smell—varies greatly. Cave dwellers, such as the olm, a type of European salamander, have little need for sight and their eyes are undeveloped. The eyes of burrowing caecilians are either very small or completely absent, although caecilians have another organ similar to a small feeler, which is associated with eye muscles and seems to have a sensory function. In other species, particularly frogs, the eyes are well developed. Underground and cavedwelling amphibians tend to have a good sense of smell, and this is also important for aquatic species, unless they live in clear water, where good eyesight is more beneficial.
Lateral line organs are present in aquatic amphibians, although they are less sophisticated than in fishes. They appear as lines of small pits along the head and body, distributed in relation to the lateral line nerves. They respond to vibrations or changes of pressure in the water, enabling the animal to control its equilibrium and posture and to detect predators or prey.
The existence of a sense of hearing in amphibians seems to depend on whether or not the animal has a voice. Urodeles and apods have neither voice nor visible ears, but appear to be able to detect vibrations. Frogs and toads, in contrast, have remarkably loud, even strident voices, and have an eardrum just behind each eye.
In water, amphibians can both walk and swim. During courtship, for example, newts often walk along the bottom of a pond. Frogs and toads use their hindlimbs for swimming, whereas newts propel themselves with their tails, keeping their hind legs pressed together. Caecilians swim like eels. Although urodeles can walk on land, their tails and short legs restrict their mobility. Frogs and toads crawl on land, but also use their powerful hind legs for leaping—several species can jump 20 times their body length. The limbless caecilians usually burrow below the ground. When observed aboveground, however, their movements resemble those of a snake.
Apart from anuran larvae (tadpoles), which eat mostly plant material, amphibians feed chiefly on insects and small invertebrates. Caecilians have a more varied diet, which includes other amphibians, fishes, and even some reptiles, small mammals, and birds.
Amphibian teeth have no roots and grow all the time as they are worn down. Some urodeles have fixed, immobile tongues, but others, such as the cave salamander, have tongues that can protrude to catch prey. This ability is particularly marked in frogs and toads, in which the tongue is attached to the front of the lower jaw. As the tongue flicks forward, it scrapes the roof of the mouth, collecting a sticky covering that adheres to the prey, so that it can be drawn back to the amphibian’s mouth.
From the mouth, food passes to the stomach, where secretions from stomach glands start the digestive process. This process continues through the intestine, aided by enzymes from the pancreas and bile from the gall bladder, to the rectum and cloaca. Urine also empties from the urinary bladder into the cloaca, from which waste products, in the form of urine and feces, are expelled.
Reproduction and life expectancy
Most amphibians lay eggs, and fertilization usually takes place outside the body. The young pass through a larval stage, although in tailed amphibians and caecilians the distinction between the larval and the adult form is less obvious than it is in anurans. Some species (most caecilians, for instance) bear live young. Others lay eggs that contain partially developed young, and still others (such as the common European frog) develop from embryo through the larval stage in the egg.
In captivity, amphibians have been known to live for 15 years or more, buttheir life expectancy in the wild is usually much shorter. Among the longest-lived are some slender salamanders and cave salamanders, which live for as long as 10 years in their natural habitat.
Frogs and toads
The anurans, loosely called frogs and toads, form the largest of the three orders of Amphibia, with some 2,700 species. Anurans have adapted to a wider range of habitats than the other orders and live on every continent except Antarctica. Most are terrestrial rather than aquatic, and some, such as the tree frogs (Hylidae), dwell primarily if not exclusively in trees. In some of these species, eggs are laid in water trapped among leaves, but some build nests and some spawn in ponds and similar locations, then return to their arboreal habitat.
The terms frog and toad are based on appearance and do not relate to actual evolutionary distinctions. Anuran classification is based primarily on skeletal features, such as the presence of ribs, which distinguishes lower from higher anurans, or the structure of the pectoral and pelvic girdles. Beyond such relatively simple criteria anurans show enormous variety. Frogs and toads vary according to the extent to which hands and feet are webbed, in the shape of and function of fingers and toes, and in the distribution or arrangement of teeth. Some species have no teeth, others have teeth in the upper jaw only, and one genus, Amphignathodon, has teeth in both jaws.
Most anurans are nocturnal, although newly metamorphosed frogs take several days to assume their nocturnal habits. They are mainly dependent on their eyes for catching prey— their visual system responds to small, irregularly moving objects, and they will not react to even preferred prey unless it moves.
An important feature distinguishing anurans from other amphibians is their voice. Almost all anuran males can increase the volume of the sounds they make by using vocal sacs in the mouth. The mating call of the natterjack toad, for example, can be heard more than half a mile away. Females, which are generally larger than males, have a quieter call.
In the breeding season the male anurans call serves to attract females and enables a female to recognize a male of the same species. When a male anuran is ready to mate he will jump upon anything of the appropriate size, and will often clasp a male or some other inappropriate object (even a piece of mud or earth) before managing to find a female.
In most frogs and toads, eggs and seminal fluid are emitted at the same time. The female then deposits fertilized eggs singly or in clumps or strings. The majority of European and North American anurans lay clusters of hundreds of eggs, each in its own gelatinous envelope. One exception is the North American tailed frog, which lays eggs that have been fertilized internally. Internal fertilization also occurs in one genus of toads, Nec-tophrynoides, found in Africa, which do not lay eggs at all, but give birth to live young.
Salamanders and newts
The tailed amphibians (Urodela) make up the second largest of the three orders of Amphibia. There are about 330 species of salamanders and newts, most of which inhabit the temperate regions of the Northern Hemisphere. Urodeles live in a wide variety of habitats. Many are terrestrial, retreating to moist crevices in hot, dry weather. Some live in trees, some never leave the water, and some live in the total darkness of deep caves. Like frogs and toads, the tailed amphibians need moisture; insects, worms, or even fishes for food; and some species need fresh water in which to lay eggs.
The mating behavior of salamanders and newts differs from that of anurans. Salamanders and newts have no voice, so mating display is important. A male salamander’s particular pattern of courtship behavior and coloring elicits a response only from a female of the same species. Male alpine newts, for instance, develop crests and striking coloration—blue on the back and flanks, orange on the belly—in the breeding season. Many species indulge in complex courtship rituals, ranging from lashing the tail in order to waft fragrant secretions toward the female to swimming back and forth in front of her.
During mating the male deposits a spermatophore, or sperm packet, which the female takes up into her cloaca. The sperm are then released as the female releases her eggs. When they hatch the larvae resemble adults, except they have gills that disappear during metamorphosis.
The third amphibian order, and the one about which least is known, is the Apoda. Apods, or caecilians, have no limbs and look like large worms or smooth snakes. Lengths vary from just over 2 inches (6 centimeters) to nearly 5 feet (1.5 meters). Nearly all species live in warm temperate and tropical regions. They usually burrow beneath the ground, and are seldom seen above ground in daylight, although it is believed that most come to the surface at night Eyes are of little use in such a habitat, but caecilians have developed a sensitive “feeler” or tentacle that probably helps them search for the worms and insects that form most of their diet. Reproduction is by internal fertilization, and it is believed that caecilians either lay eggs or retain the eggs until the young hatch.