Frogs and toads are the most familiar amphibians. Their squat, tailless bodies; broad, flat heads with huge mouths, and long, muscular hind limbs are unique and unmistakable.
The Diversity and Distribution of Anurans
Arrayed into 21 families, the order Anura contains over 5,200 species making it by far the largest group of extant amphibians. In fact, about 88% of all amphibian species are frogs.
Frogs are distributed from the tropics to subarctic regions worldwide, but they exist in the greatest abundance and variety in tropical rainforests. Some frogs inhabit arid regions such as deserts, where water may not be easily accessible. However, they are not found on Antarctica and many oceanic islands.
The Australian genus Cyclorana and the American genus Pternohyla will bury themselves underground during dry periods, create a water-proof cocoon, and estivate. Once it rains, they emerge, find a temporary pool and breed. Racing against the desert heat, the tadpoles develop at a rapid speed to become adults and burrow in before their pool dries up.
Some frog types are adapted to cold, dry regions. The wood frog (Lithobates sylvaticus) which ranges as far north as the Arctic Circle, buries itself in the ground during winter where much, but not all, of its body freezes solid. Frog species are often patchy in their distribution, being restricted to certain localities (specific streams or ponds) and absent or scarce in similar habitats elsewhere.
The Body Plan of Anurans
The use of the common names “frog” and “toad” is more a vernacular designation than a scientific distinction. From a classification standpoint, all anurans are frogs, but only members of the family Bufonidae are considered true toads. The use of the term “frog” in common names usually refers to species that are aquatic or semi-aquatic with smooth, moist skins, while the term “toad” generally refers to species that tend to be terrestrial with dry, warty skin.
Compared with the other orders of amphibians, frogs are unusual because they lack tails as adults and their hind legs are more suited to jumping and swimming than walking.
Frogs range in size from the Brazilian gold frog (Brachycephalus didactylus) at 10 mm (0.4 inch) long to the 30 cm (1 foot) goliath frog (Conraua goliath) of West Africa. This giant eats animals as big as rats and ducks. (As you contemplate these measurements, keep in mind that frogs are measured from snout to anus, not from snout to the ends of out-stretched hind legs.)
The skin texture of frogs varies from smooth to warty or folded with the skin hanging loosely on the body because of the lack of connective tissue. Frogs have three eyelid membranes: a transparent one to protect the eyes underwater, and two that vary from translucent to opaque.
The mouth of frogs is large and basketlike and most frogs do have teeth of a sort. They have a ridge of very small cone teeth known as maxillary teeth around the edge of the upper jaw and several vomerine teeth on the roof of their mouth. Frogs lack teeth on their lower jaw and toads lack teeth altogether. Teeth are used to grasp prey and prevent its escape, not for biting or chewing. Thus, all food is swallowed whole.
Anurans are Both Predator and Prey
Frogs are carnivores with huge appetites that will eat anything live they can get in their mouth. They prey mainly on insects, worms, small crustaceans, small molluscs, small fish, reptiles, birds, and even mammals as well as tadpoles and other adult amphibians.
Most frogs are ambush hunters sitting camouflaged and concealed waiting for prey to approach. When prey is in range, the frog fires out its sticky tongue and pulls the prey back into the mouth. The frog then contracts its eye muscles to help force the food down into the stomach.
Adult frogs have many enemies, such as snakes, birds, turtles, raccoons, large fish, and humans. While they appear defenseless, many frogs have survival strategies they employ against potential aggressors. Some feign death, while others inflate themselves with air and push themselves up off the ground as far as possible to present a large and threatening an appearance all the while emitting screeching sounds.
A frog’s best defense is its ability to leap as anyone who has been startled when walking along the edge of a pond by a frog leaping from underfoot to land kerplop in the water just out of reach can attest. Some types are brightly colored to warn potential predators of the deadly poisonous nature of their skin, and bullfrogs in captivity do not hesitate to snap at tormentors inflicting painful bites.
Unfortunately, the populations of many species of frogs are in serious decline and as many as one-third of all species are in danger of extinction.
Additional Articles
- The Characteristics of Caecilians investigates the diversity, distribution, and body plan of the legless amphibians – the caecilians.
- The Characteristics of Salamanders investigates the diversity, distribution, and body plan of the tailed amphibians – the salamanders.
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