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JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract. Leptodactylid frogs (Eleutherodactylus coqui Thomas) reduce rates of evaporative water loss threefold by adjusting their postures and activities in response to changing conditions of availability of water during their nocturnal activity periods. Frogs that do not make these adjustments experience a potentially lethal loss of body water on a rainless night. Dehydration of a frog's body tissues increases its resting metabolic rate and lowers its maximum rate of aerobic metabolism. Water is reabsorbed from urine in the bladder to maintain tissue water content on dry nights.Use of water-conserving postures precludes vocalization by male frogs and response to calling males by females. Frogs in water-conserving postures feed less readily than active frogs. Frogs in the forest canopy experience higher rates of evaporative water loss than those in the understory, but there are more arthropods in the canopy, and leaf surfaces are twice as likely to be wet by rain. Despite these potential benefits of activity in the forest canopy, most frogs remain in understory vegetation. In that microhabitat their behavioral and physiological adjustments permit them to occupy their normal perches despite wide fluctuations in hydric conditions.

The terrestrial eggs of the coqui of Puerto Rico are brooded almost continuously by the male parent from the time of oviposition until the fully metamorphosed hatchlings emerge from the eggs 15—20 d later. The gelatinous layer surrounding each egg offers no resistance to the exchange of water by the egg, and rates of exchange are determined by microclimatic conditions, structural characteristics of the nest, and the behavior of the male frog . During development in natural nests, the eggs experience a three— to fourfold increase in mass. Laboratory experiments coupled with field observations indicate that this increase is the result of the transfer of liquid water from the incubating male to his eggs. The transfer is driven by a difference in water potential between the eggs and the body fluids of the male frog that is nearly constant throughout incubation, despite an increase in egg mass. Eggs must take up water during incubation. Eggs that do not experience an increase in mass during development either die or produce small hatchlings. A water uptake that doubles the initial mass of the egg is necessary to produce a full—size hatchling with normal tissue water content.