To wipe and wax: in dry times, some frogs employ a curious method for saving water

Natural History, Dec, 2001 by Harvey B. Lillywhite

The skin is a large and prominent organ, but we often take it for granted. While we think of our other important organs--heart, liver, kidneys, and the like--as securely placed inside us, doing complex and hidden things, we regard our skin as little more than a covering. Indeed, the principal function of skin is protective, but in various animals this role has given rise to myriad devices. Although the skin of all vertebrates has common properties, evolution has produced great variation in its shape, size, and functions and in its scales, claws, feathers, fur, spines, horns, and other appendages. Not all skin is created equal. The varied functions performed by the skin--including respiration, sensation, control of body temperature, transport of ions and water--reflect both phylogeny, or lineage, and evolutionary adaptation to a wide spectrum of different habitats.

Because amphibians we're the first vertebrates to venture onto dry land, their skin has long been a subject of fascination to scientists. Amphibian skin is very different from--and, in some respects, simpler than--that of other vertebrates. It generally lacks appendages such as scales or hair and is comparatively thin and quite permeable to moisture. As a result, amphibians face an increased risk of drying out and have evolved behavioral specializations for maintaining water balance on land. In dry weather they retire to burrows, dense vegetation, or other protective places. Some amphibian species have adopted a nocturnal lifestyle to avoid the drying rays of the sun; others seldom stray far from sources of moisture with which they can replenish body water after bouts of activity. Surprisingly, some of the world's driest deserts harbor one or more species of frogs or toads, though they are seldom seen except during the rainy season, when they form breeding aggregations at temporary pools. During the remainder of the year, these animals burrow into the soil and remain there in a state of dormancy, often surrounded by a protective cocoon: multiple layers of shed epidermis (the outermost skin) interspersed with layers of mucus or other secretions.

Some amphibians living above ground in arid regions are especially challenged by drying conditions and have developed some novel adaptations to conserve water. One well-known example is the South American painted belly monkey frog, Phyllomedusa sauvagii, which was investigated during the 1970s by Rodolfo Ruibal, Vaughan Shoemaker, and their students at the University of California, Riverside. Monkey frogs secrete waxes from specialized glands in their skin, then wipe themselves in a stereotypic manner to spread the waxes over the body. A typical wax layer produced by wiping is fifty to a hundred molecules thick--comparable to the thickness of protective waxes secreted onto the cuticle of beetles and spiders.

Reports of wiping behavior in several species of phyllomedusine frogs (members of the family Hylidae) were preceded by reports that certain African tree frogs of the genus Chiromantis (family Rhacophoridae) were highly resistant to drying out. However, wiping itself was first noted by Charles. McCann. In a 1940 issue of the Journal of the Bombay Natural History Society, he described the wiping behavior of Polypedates maculatus, a rhacophorid species known as the common Indian tree frog. During dry weather, some of these frogs took refuge in McCann's bungalow in Andheri, near Mumbai (formerly Bombay), providing him with a perfect opportunity to observe their curious mannerisms. At night they emerged from their hiding places, moved to a wet spot where they sat to absorb moisture for a while, and then ventured out in search of food. When the frogs returned in the morning, McCann witnessed each frog go through what he called "a process of `self-massage,' a somewhat ludicrous performance." His description continues: "The head is first rubbed down from the top over the snout, by the forelegs, and then down the throat and thorax as far as the limbs will reach. This is followed by a `massage' of the back, flanks, and abdomen, by the hind legs. The hindlimbs then massage one another. When all this `toilet' is over, the limbs are collected under the body and the frog `retires.'"

During the 1970s, when scientists began publishing reports on the self-waterproofing of South American phyllomedusine frogs, McCann's observations went unnoticed, as did his speculations on the purpose of wiping. "I have tried to figure out the reason for this performance," he had written, "and can only ascribe it to the possibility that the animal tries to clear its body of foreign matter adhering to its skin, perhaps also, an even coating of slime, when dry, acts as a sort of film, to prevent further evaporation of moisture." The latter interpretation turned out to be correct.

I learned about McCann's publication and became intrigued with Indian tree frogs during a visit to India roughly ten years ago, and I later returned there for a formal investigation of their wiping behavior. My collaborator, Ajay Kumar Mittal, a professor of biology at Banaras Hindu University in Varanasi, and I initially found that, like the South American Phyllomedusa, common Indian tree frogs could be stimulated to wipe themselves if we handled them. We next conducted some experiments in which frogs were subjected to repeated cycles of moderate but harmless dehydration before being handled. Under these conditions, the incidence of wiping increased markedly. It appeared that wiping was an adaptive response to water deprivation, but we needed to establish just how it worked.