Amphibian (animal)

I

Introduction

Amphibian (animal), animal with moist, hairless skin through which water can pass in and out. Nearly all amphibians live the first part of their lives in water and the second part on land—a double life reflected in the name amphibian, which comes from the Greek words amphi, meaning “both,” and bios, meaning “life.” Amphibians were the first animals with backbones to adapt to life on land. They are the ancestors of reptiles, which in turn gave rise to mammals and birds.

Scientists recognize more than 5,000 species of amphibians, all of which are members of one of three main groups: frogs and toads, salamanders, or caecilians. Frogs and toads are the most abundant of all amphibians, numbering more than 4,000 species. Frogs have smooth skin and long limbs. Toads, in contrast, have warty skin and short limbs.

There are about 400 known species in the salamander group, which also includes newts and mud puppies. Members of this group have long, slender bodies ending in tails. Some salamanders live entirely on land, whereas others never leave the water, and still others spend some time in the water and some on land. Caecilians, with about 160 species, are the rarest of amphibians. They have no limbs and look much like earthworms. Most live underground and spend their time burrowing in the soil, but a few are aquatic.

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Range and Habitat

Amphibians inhabit every continent except Antarctica. The most widespread of all amphibians are frogs, which are found just about everywhere except on some islands, the polar regions, and the driest deserts. Salamanders are also widespread. In the western hemisphere, their range extends from North America to the northern part of South America. They also inhabit Europe, the Mediterranean area, Africa, and Asia. Caecilians have a more limited range; they are found in Central and South America, parts of Southeast Asia, and from India and Sri Lanka to the Philippines.

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Amphibians live in many environments, including grasslands, rain forests, conifer forests, alpine areas, and even deserts, although most species require freshwater habitats such as ponds, swamps, streams, or other wet environments for breeding. Remarkably, many amphibians are adept at finding moisture in seemingly dry environments. For example, a unique group of burrowing frogs survives in Australia’s dry outback by breeding and feeding only during the area’s infrequent rainstorms. Deep in tropical forests, some frog species rely on pools of water that collect in tree hollows or in the cup-shaped bases of epiphytes—plants that grow on the trunks and branches of trees.

III

Physical Characteristics

Amphibians range in size from the Japanese giant salamander, which can exceed 1.5 m (5 ft) in length, to tiny frogs, such as the gold frog, that reach only 1 cm (about 0.4 in). Most salamanders are between 5 and 20 cm (2 and 8 in) long, and most frogs measure between 2 and 8 cm (0.8 and 3 in). Caecilians are more variable in size—most species are between 10 and 50 cm (4 and 20 in), but some grow as long as 1.5 m (5 ft).

Most amphibians begin their lives as a larva, an immature form that has a significantly different body plan than that of an adult of the species. Amphibian larvae have many features in common with fish. Sometimes called pollywogs or tadpoles, these larvae live in water and have gills at the sides of their heads that enable them to breathe underwater. They also have a tail that they use in swimming. Most amphibian larvae have tiny teeth.

Adult amphibians typically have body structures that enable them to move about on land as well as in the water. Frogs and toads have hind legs that are longer and stronger than their fore legs for use in jumping, their primary mode of locomotion on land. Zoologists believe that adult frogs and toads do not have tails because they would be a hindrance in jumping. In contrast, most salamanders have four short legs and a long, strong tail, which they use for balance while walking on land and to propel them through the water when swimming. Caecilians have no limbs at all. They burrow in the soil by using their strong skulls as battering rams and swim by moving their muscular bodies back and forth like eels. Most adult amphibians retain their teeth, but in some species, teeth are reduced in size or not present at all.

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Internal Anatomy

Amphibian internal anatomy is similar to that of other vertebrates. Adult amphibians typically have lungs, rather than gills, for breathing oxygen, but some water-dwelling species have both lungs and gills, and others obtain all the oxygen they need to survive through their permeable skin. A three-chambered heart pumps blood throughout a complex circulatory system, delivering oxygen and nutrients to the cells and carrying away waste products. The amphibian digestive system consists of a mouth, esophagus, stomach, and intestine, which opens into a chamber at their posterior called the cloaca. Digestive wastes are processed by the kidneys, excreted into the cloaca, then expelled from the body through a muscular opening in the cloaca. This opening is also where eggs and sperm exit the body.

Amphibians are cold-blooded, or more correctly, ectotherms—that is, they are not able to generate their own body heat. Instead, their body temperature is determined by their surroundings. This means that they cannot control the speed at which their body systems work. In cold weather, they become sluggish, and some enter a state of reduced activity, or torpor, which is similar to hibernation.