What Are Amphibians?
Amphibians are small vertebrate animals that have four legs and glandular skin that lacks feather, scales or hair. Internally, the structure of living amphibians is intermediate between that of fishes and amniotes. They include as many species as there are species of mammals and exhibit more variation in reproduction than any group but fishes.
These creatures that lead double lives (amphi means “both” and bios “life”), typically inhabiting freshwater early on and then changing to forms that can live on land. The most pronounced changes, from gills to lungs, from fins to legs, and from a vegetarian diet to an animal one, occur among certain frogs and toads, but many salamanders undergo changes as well.
Amphibians have undergone a remarkable adaptation and great diversity. The term amphibian can be interpreted in 2 ways – either as an animal spending part of its life in water and then changing to an aquatic adult, or as an animal that alternates life in and out of water. Actually, both interpretations are valid in part, but neither applies to all amphibians, some of which are aquatic throughout of their lives, but others of which neither enter water nor have aquatic stages in their life histories.
The three major groups of amphibians are:
- Frogs & Toads (Salientia)
- Salamanders & Newts (Urodela/ Caudata)
- Caecilians (Gymnophiona)
Frogs and toads are specialized for jumping, with greatly enlarged hind legs, shortened bodies, no tail, and large heads and eyes. Salamanders and newts are much more elongated, with front and back legs of approximately equal size and a long tail.
Despite these differences, all amphibians share certain characteristics that set them apart from other terrestrial animals. One such feature is that their highly permeable skin allows for rapid passage of both water and respiratory gasses. This feature prevents them from living in saltwater or being active in very dry environments.
However, it allows them to make use of the skin for respiration to a much greater degree than any other vertebrate. Amphibians are incapable of generating their own body heat and consequently have much lower metabolic rates than birds and mammals.
The tailed amphibians (Urodela) make up the second of the 3 orders of Amphibia. There are about 450 species of salamanders and newts, most of which inhabit the temperate regions of the Northern Hemisphere. Many are terrestrial, retreating to crevices in hot, dry weather. Some live in trees, some never leave the water, and some live in the total darkness of deep caves.
North America has more kinds of salamanders (the group that includes the newts) than all the other continents. In the East they live throughout the Appalachians, especially in the Great Smoky Mountains. The West Coast has its own kinds, too. With several exceptions, including the Tiger Salamander, you won’t find any in the Rockies, the desert, or the Central Plains; glaciers kept them out of the Rockies, and the other places are just too dry.
Salamanders look much like lizards, but their skin is thin and moist (lizards have hard scales or plates), they have only 4 toes on their front feet (lizards have 5), and they have no claws. Being amphibians, salamanders are more closely tied to water than lizards, which are reptiles. Though most adults live on land, many species lay their eggs in water.
Mating behavior differs from that of amphibians. Salamanders and newts have no voice, so the mating display is all-important. Male alpine newts, for example, develop crests and striking coloration. Many species indulge in complex courtship rituals. During mating, the male deposits a spermatophore (sperm packet), which the female takes up in her cloaca. The sperms are then released as the female releases her eggs. When these hatch, the larvae resembles adults, although they have gills that disappear during metamorphosis.
Eggs laid in water hatch into larvae with tufted external gills. Eggs laid on land, such as those of woodland salamanders, bypass this stage.) Several salamanders, including the waterdogs and sirens, retain their gills and never leave the water; others, after periods ranging from several months (the Spotted Salamander) to several years (the Two-lined), lose their gills and transform into land-dwelling adults. The adults breath through their skin or with lungs. Salamanders are carnivorous as both larvae and adults, feeding on fish, insects, crustaceans, worms, and even small mice.
Finding salamanders takes a bit of searching; they are silent creatures, and most are active only at night. The best time of year to look for them are spring and fall; the best places, under stones in streams and logs and leaves in most forests. Some kinds, including the Spotted Salamander, spend much of their time underground, where their surroundings are damper and their enemies fewer than above the surface.
Frogs and Toads
The anurans, loosely called frogs and toads, form the largest of the three orders of Amphibia, with some 2,600 species that have adapted to a wider range of habitats than the others and live on every continent except Antarctica. Although most are terrestrial rather than aquatic, some, such as the tree frogs (Hylidae), are primarily arboreal.
The terms “frogs” and “toads” are based on appearance and do not relate to actual genetic distinctions. Anural classification is based on skeletal features, such as the presence of ribs, which distinguishes lower from higher anurans. Anurans show enormous variety.
More widespread than salamanders, these lively animals are also less secretive. But they too are mostly nocturnal (the Northern Cricket Frog and the Oak Toad are among the exceptions), resting during the day in trees, or under leaves, undetected unless they leap out from under your feet. They mainly depend on their eyes to catch prey; their vision system responds to small, irregular moving objects, and they will not react to even a preferred food source unless it moves.
An important feature distinguishing anurans from other amphibians is their voice. In the breeding season, the male anuran’s call serves to attract females and enables a female to recognize a male from the same species. At night during mating time (often in spring), however, groups of singing males loudly announce their presence. A flashlight may help you to locate some of the songsters; and with a little practice, you may be able to identify species by their calls. You can even buy a recording of frog sounds, as you can for birds.
Female frogs and toads tend to be larger than males. Males attract them with song, then cling to them (sometimes using special clasping pads that develop during the breeding season) and fertilize their eggs as they shed them into water. In most frogs and toads eggs and seminal fluids are emitted at the same time; fertilized eggs are then deposited singly or in clumps or strings.
The eggs hatch into “polliwogs”, round-bellied, long-tailed larvae that, like those of salamanders, have gills on the outside of their body. In frogs and toads, but not in salamanders, these external gills are soon covered with skin. So most of the tadpoles you catch will have no visible gills.
Eventually, in one of nature’s most dramatic transformations, the tadpole changes into a frog or a toad, a tailless terrestrial creature with long hind limbs and lungs instead of gills. How long tadpoles stay as tadpoles vary with both species and temperature. Desert-dwelling spadefoot toads may spend a mere two weeks at this stage, whereas North America’s biggest frog, the Bullfrog, may not metamorphose for several years in the colder parts of its range.
The third amphibian order, and the one about which least is known, is the Apoda. Caecilians have no legs and look like worms. Most live underground. Length vary from just over 2 inches to nearly 5 feet. They live in tropical areas of South America and Africa. These amphibians have small sense organs and no external ear openings. They rely on tactile feedback.
They usually burrow beneath the ground, seldom being seen in daylight above the ground. Eyes are of little use in such a habitat, but caecilians have developed a sensitive “feeler” or tentacle which probably helps them search for worms and insects that are the main constituents 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. Short tentacles between the eyes and nostrils may be used to feel their way around and finding food. Massive numbers of caecilians appear on the surface when their burrows are flooded after heavy rain, even in places where they seem rare and are seldom seen.
- William E. Duellman – Biology Of Amphibians
- Kentwood D. Wells – The Ecology And Behavior Of Amphibians
- Marshall Cavendish Corporation – Encyclopedia Of The Aquatic World: Newt And Salamander