Wiping Behavior and Its Ecophysiological Significance in the Indian Tree Frog Polypedates maculatus

Abstract

We studied complex wiping behavior of the Indian tree frog, Polypedates maculatus (Rhacophoridae) and here report that wiping expels both mucus and lipid secretions from common cutaneous glands homologous with typical anuran mucous glands. Discharge of skin secretions occurred synchronously at multiple gland openings on dorsal and lateral body surfaces when the skin was touched. Secretion also could be stimulated by epinephrine or isoproterenol and was inhibited by the P-adrenergic antagonists propranolol and timolol. These and other findings indicated that tactile stimulation of the skin elicits a P-adrenergic secretomotory reflex. The self-wiping behavior characteristic of Polypedates evolved independently but is virtually identical to that of certain hylid tree frogs which secrete lipids and then wipe themselves to become transiently waterproof. However, cutaneous secretions of P. maculatus were shown to provide comparatively low resistance (0.14.2 sec * cm-') to evaporative water loss that averaged about half that of a comparably sized ranid frog or a free water surface. Therefore, wiping behavior is not restricted to highly waterproof species and conceivably evolved before skin secretions provided a significant barrier to evaporation. Species of anurans that have been shown to wipe lipid secretions over the body all have geographic ranges that include arid or semiarid habitats. Dehydration stress thus appears to be the principal selective force which has coupled the evolution of lipid secretion with wiping behavior.