It's not easy being green: some frogs employ special tactics for desert survival

DID YOU KNOW?

* As a wiping frog uses its feet to massage its body, the stimulation produces a nerve reflex that causes muscle cells to contract around glands in the skin. This triggers special glands to produce a protective wax.

* Once thought to be rare, wiping frogs have been reported on five continents.

CRITICAL THINKING:

* Thanks to lipid structures within the epidermis, humans don't lose as much water through their skin as amphibians do. Still, can you think of any "wiping behaviors" that people perform to lock in moisture? (Answer:. Many people use body lotion.)

CROSS-CURRICULAR CONNECTIONS:

LANGUAGE ARTS: In some of the earliest accounts of frogs wiping themselves, scientists described the act as a "ludicrous performance." Write a journal entry as if you were one of the fast researchers to record this strange behavior; be sure to include a drawing.

RESOURCES

* To learn about other frogs that keep their cool in the heat, check out "To Wipe and Wax" by Harvey B. Lillywhite, Natural History, December 2001-January 2002.

In the Chaco desert region of Argentina, the Chacoan monkey frog sits on a branch. Using its feel the frog kneads its entire body.

It even rubs its legs together, making sure no part of its body surface is missed. Why is this tree frog, or Phyllomedusa sauvagii, giving itself a rubdown?

Most frogs can't last more than a few hours away from water, so this desert-dweller needs special tactics to survive. Its so-called wiping movements cover its skin with a waxy barrier that locks in moisture. Read on to find out how wiping behaviors help several species of tree frogs conserve water in ways that would turn ordinary frogs green with envy.

THIN SKIN

Frogs are amphibians, or animals that spend part of their life in water and part on land. Like mammals, birds, and reptiles, amphibians have a protective outer skin called an epidermis. When cells in the outermost layer of the epidermis die, they harden into a type of protein called keratin. In some animals, many layers of keratin build up to form a tough, waterproof layer. But amphibians have only one or two cell layers of keratin. That's because when a new layer forms beneath the old one, frogs peel off the topmost layer and eat it for a protein-rich snack. This munching makes for some very thin skin.

Because an amphibian's skin is so thin, gases and water pass right through it. Frogs breathe and drink through their skin. But if water can get in, it can get out just as easily. Zoologist Tamatha Barbeau of Francis Marion University in South Carolina compares a frog's skin to a sponge. "If you were to put a wet sponge on a counter and come back 5 hours later, it would be dry. Likewise, most amphibians are going to dry out if you keep them away from water for 5 to 10 hours."

Most frogs protect themselves from dehydration (water loss) by sticking close to water or staying out of the sun's moistre-sapping heat. But the Chacoan monkey frog seems to laugh in the face of danger by climbing to the tops of trees and basking in the sun. How does it survive in conditions that would make most frogs croak?

NOT YOUR AVERAGE AMPHIBIAN

Frog skin contains glands that produce various substances. Mucous glands release a fluid that keeps the skin moist and healthy. Poison glands (also called granular glands) release toxic substances that keep predators away. It turns out that the Chacoan monkey frog has another type of gland that most frogs don't have: a specialized gland that produces lipids, or waxes. The frog wipes these waxes over every part of its body, and when the lipids dry, the frog looks like it's made of plastic. "That lipid barrier is almost like a wax coat you'd put on your car," Barbeau says. "You wax your car to keep water out. These frogs are waxing their bodies to keep water in."

This frog's hydrophobic (water-repellent) coating makes it one extreme amphibian. "Following wiping, the lipids on [the Chacoan monkey frog] actually reduce the rate of water loss to very close to that of a desert lizard," says Harvey B. Lillywhite, a zoologist at the University of Florida.

Because frogs drink through their skin, a water-resistant coating could be deadly. But the Chacoan monkey frog overcomes the problem by doing something no other frog is known to do: It swallows water. The little croaker tilts up its head and lets raindrops roll into its mouth. It also takes in air through its nostrils, filling up its lungs, to make up for the air that can't pass through its sealed skin.

MORE TRICKS

The Chacoan monkey frog may be the frog prince of water conservation, but a few other tree frogs can wax themselves without a specialized lipid-producing gland. Two species of Australian tree frogs and seven species of Florida tree frogs secrete waxes from their poison glands; but these frogs' coverings aren't as waterproof as the Chacoan monkey frog's wax coat. One possible reason: The waxes in their coverings are mixed with other substances produced in the glands, diluting the waxes. And unlike the Chacoan monkey frog, which wipes its entire body, the Florida frogs never wipe their bellies. Instead, these frogs protect their stomachs from water loss by shielding them from the desert air. "When they settle down, they tuck their legs and arms in tight against their bodies, and press their bellies as close as possible to the surface they're on," Barbeau explains.