Life History And Behavior
Like other amphibians, such as salamanders (order Caudata) and caecilians (order Gymnophiona), most frogs hatch from a shell-less egg into a gilled, water-dwelling, larval stage (a tadpole). After a period of growth they metamorphose into the adult form. Most species of tadpoles are vegetarians, feeding upon algae and other plant material. All adult frogs, however, are carnivores, most of them feeding upon insects and other invertebrates.
In the temperate zones of the world the breeding season begins in the spring, but the precise time depends upon the species of frog. In much of temperate North America, for example, the beginning of springtime is proclaimed by the breeding calls of chorus frogs (Pseudacris spp.). Their high trills are soon followed by the calls of the spring peeper (Hyla crucifer). These may be followed by the rasping calls of the wood frog (Rana sylvatica), the leopard frog (Rana pipiens), and the green frog (Rana clamitans). Then the American toad (Bufo americanus) trills in, and when the larger ponds eventually warm up, the bullfrog (Rana catesbeiana) begins its booming jug-of-rum calls. As many as 16 different species of frogs have been found calling at various times at a single pond in Florida.
The males of each frog species have their own distinctive call. It has recently been found that the ear of the female is "tuned" to the call of her own species, so that not only is she not attracted to the calls other species, she may not even hear them! A female carrying eggs will typically approach a calling male of her choice (and of her species), and nudge him. He immediately ceases calling and grasps her around the waist. They enter the water (if they are not already in it), and as she expels eggs from her cloaca, the male sprays sperm over them. Depending upon the species, the eggs may appear in strings, in clusters, or as individual ova.
The eggs are enclosed in a protective jelly coating, and will develop over several days to a week into a tadpole. The tadpole will grow over a period of time (weeks, months, or years, depending upon the species), and ultimately sprouts legs, changes other elements of its external and internal morphology, and emerges as a small replica of the adult.
This sequence is typical of frogs living in temperate regions. In the tropics breeding is often initiated by a change in weather (such as dry to wet), the calling males may be on the moist forest floor or in a tree, the eggs may be laid on foliage or beneath a rock or in a pond, and the tadpole stage may be completed inside the egg capsule, so that froglets appear directly from the egg. In other words, there is enormous variation in breeding habits, particularly in the tropics.
Science EncyclopediaScience & Philosophy: Formate to GastropodaFrogs - History And Fossil Record, Adult Morphology, Ecology, Life History And Behavior, Classification, Frogs And Humans - Morphology, Larval morphology