Frogs, toads, and other amphibians usually need water in which to spawn—for
the first stage of their lives tadpoles are aquatic gill-breathers. Nevertheless,
surprisingly, a few amphibians have managed to survive in the desert
environment.
In North America various species of spadefoot toads (genus Scaphiopus),
such as the western spadefoot (S. hammondi), are well adapted to desert life.
They are mostly mottled pale green and brown in color, dig burrows with their
club-shaped back feet for shelter by day, and emerge only at dusk to hunt for
insects and spiders. As the dry season takes grip, they extend the burrows to 1
meter (39 in.) or deeper and stay there all the time—in rare cases for as long as
six months. During this time they live off nutrient reserves in the body and may
lose up to half their body weight. They also store virtually pure water rather than
urine in the bladder, which may swell like a balloon to take up half of the toad’s
volume, while the waste substance urea generally found in urine is instead
retained in concentrated form in the blood and other tissue fluids.
Surrounded by damp soil in its deep burrow, the toad acts like a sponge and
draws water into itself from the soil by a natural process of osmosis (whereby
Creatures of the desert 159
dissolved substances exchange solutes or solvents to even out their
concentrations). When the brief rains arrive, the toads emerge, mate, and lay
eggs quickly in the puddles. The whole life cycle from fresh spawn to dry-land
toadlet lasts just six weeks.