Physiological and Behavioral Variation in Estivation among Mud Turtles (Kinosternonspp.)

Kinosternid mud turtles, a primarily aquatic group, exhibit variable degrees of terrestrial activity in the Sonoran and Chihuahuan Deserts. We compared behavioral and physiological responses to dry conditions in four populations representing three species,Kinosternon sonoriense,Kinosternon flavescens, andKinosternon hirtipes. All four groups were subjected to simulated dry season conditions in the laboratory, during which activity was monitored and physiological responses (blood chemistry and rates of resting metabolism and evaporative water loss) were measured.Kinosternon flavescensandK. hirtipesrepresented extremes in apparent ability to estivate, based on activity and rate of increase of plasma osmolality. Two populations ofK. sonorienseexhibited intraspecific differences in behavioral and physiological measures that were related to extant environmental conditions. Large numbers ofK. sonoriensefrom Arizona andK. hirtipes, the poorest estivators, had to be rehydrated after only 30 d out of water.Kinosteron flavescenshad the lowest metabolic rates, but no evidence of metabolic depression during dehydration was found for any of the four populations. We conclude that the differences in capacity for estivation among populations are primarily linked to variable behavioral responses to dry conditions, though high rates of evaporative water loss inK. hirtipesrepresent a probable physiological constraint.