Energy and water flux during terrestrial estivation and overland movement in a freshwater turtle

The doubly labeled water (DLW) method for studying energy and water balance in field-active animals is not feasible for freshwater animals during aquatic activities, but several species of nominally aquatic reptiles leave wetlands for several critical and extended behaviors, where they face challenges to their energy and water balance. Using DLW, we studied energy and water relations during terrestrial estivation and movements in the eastern long-necked turtle (Chelodina longicollis), a species that inhabits temporary wetlands in southeastern Australia. Water efflux rates of 14.3-19.3 mL (kg d)(-1 ) during estivation were nearly offset by influx, indicating that turtles did not maintain water balance while terrestrial, though dehydration was slow. Estivation energy expenditure declined over time to 20.0-24.6 kJ (kg d)(-1) but did not indicate substantial physiological specializations. Energy reserves are predicted to limit survival in estivation to an estimated 49-261 d (depending on body fat), which is in close agreement with observed bouts of natural estivation in this population. The energy cost and water flux rates associated with overland movement behavior ranged from 46 to 99 kJ (kg d)(-1 ) and from 21.6 to 40.6 mL (kg d)(-1), respectively, for turtles moving 23-34 m d(-1). When a wetland dries, a turtle that forgoes movement to other wetlands can save sufficient energy to fuel up to 134 d in estivation. The increasing time in estivation with travel distance gained in this energy "trade-off" fits our previous observations that more turtles estivate when longer distances must be traveled to the nearest permanent lake, whereas emigration is nearly universal when only short distances must be traversed. The DLW method shows promise for addressing questions regarding the behavioral ecology and physiology of freshwater turtles in terrestrial situations, though validation studies are needed.