Biology Reference
In-Depth Information
the first amphibians appeared. A mere 50 million years or so later, in the early Jurassic, evolution had
proceeded apace and produced what could reasonably be termed the first recognizable frog. To put this
development in perspective, the earliest true frog evolved even before most types of dinosaurs!
Ancient times were the heyday of amphibians, which comprised perhaps fifteen major groups at
their peak. However, the development of reptiles, followed by birds and mammals, provided serious
competition, and amphibians slowly declined. Today there are just three remaining orders of amphibi-
ans—frogs and toads, salamanders, and caecilians, a group of wormlike tropical creatures.
In a sense, amphibians represent a sort of arrested development. Unlike the reptiles—which, incid-
entally, evolved from an amphibian ancestor roughly 315 million years ago—amphibians never took
that final step to become completely terrestrial. Although many reptiles, such as turtles and crocodili-
ans, spend most of their time in the water, all reptiles (except for some sea snakes) bear their young on
land, and many are completely terrestrial.
Reptiles were able to accomplish this transition by developing eggs that wouldn't dry up on land and
skins that greatly reduced the loss of water. In contrast, the soft, gelatinous amphibian eggs are useless
out of water, and amphibians themselves can only withstand hot, dry conditions by protecting them-
selves in a moist environment. Perhaps the enormous early success of amphibians was a mixed bless-
ing, halting further development so that amphibians remained in a sort of evolutionary limbo, neither
fish nor fowl, so to speak.
The question sometimes arises, what is the difference between frogs and toads? From a strictly sci-
entific point of view, there's hardly any difference. Frogs have teeth in their upper jaw, whereas toads
lack them, but otherwise the distinction between toads and frogs is largely an artificial one for purposes
of convenience.
Most
toads have dry, rough skin, a plump body, and short legs best suited for hopping.
Most
frogs, on the other hand, have smooth, moist skin, a fairly slender body, and long legs well suited
for leaping. However, there are frogs with rough skin and toads with smooth skin, for example, so frogs
and toads can't be differentiated simply by appearances.
There has been a great deal of concern among scientists because of a recent worldwide decline in
some species of frogs and toads. Various theories have been propounded for this decline, ranging from
pollution to an increase in ultraviolet rays from sunlight, but no definite conclusions have been reached.
Fortunately, the common toad seems to be maintaining its numbers.
Life for toads (or frogs) begins in the water. After emerging from hibernation in March or April—or
even in May, depending on the climate—male toads make their way toward suitable breeding areas,
such as shallow marshes, ponds, and pools. There the males begin to sing, a loud, high trill that can
continue uninterrupted for as much as thirty seconds. This sound is produced when the male inflates his
vocal sac and vibrates it during the course of the call. The sound of trilling toads at a discreet distance
is a welcome sign of spring, but a number of males trilling simultaneously at very close range can be
almost deafening!
This trill evidently serves to attract female toads and perhaps gives them an indication which male
might make the best mate. Mating takes place when the smaller male toad climbs on the female's back
