CONTRERAS, H.*; TALBOT, C.; Calif. State. Univ. San Bernardino: Dehydration Threshold of California and Pacific Tree Frogs, Pseudacris cadaverina and P. regilla

Evaporative water loss occurs rapidly across amphibian integument, thus highly terrestrial amphibians, such as the California (Pseudacris cadaverina) and Pacific (P. regilla) Tree Frogs, are susceptible to high degrees of dehydration. To counteract this many species of amphibians have developed behavioral and physiological adaptations that allow rapid rehydration. One of these behavioral adaptations, the water absorption response (WR), is observed when a dehydrated animal comes into contact with water and begins to �drink� by absorbing water across its ventral abdominal skin. P. cadaverina and P. regilla were studied to determine the minimal level of dehydration (threshold) befor this behavior is induced. For each trial frogs were dehydrated to a specific percentage of their standard body weight (SBW) then placed in an observation tank containing a tissue saturated with 1 mM NaCl. Fifteen-minute observations were conducted and time on the wet tissue, time in WR and weight changes were recorded. The threshold for each species was determined as the minimal level of dehydration where at least 50% of the frogs first showed WR. Although there seemed to be no difference in the rate at which these two species dehydrated (0.078 g/hr) there was a marked difference in the threshold dehydration. P. regilla consistently showed WR at dehydration levels at 95% of the animals� SBW whereas P. cadaverina did not begin to consistently show WR until they were dehydrated below 90% of their SBW. Rehydration rates for both species also did not differ (0.018 g/min) and both species of frogs tended to show WR for the entire 15 minute observation period or not at all, independent of dehydration level. Supported by CSUSB ASI Student Research Grant.