Energy use and management of energy reserves in hatchling turtles (Chrysemys picta) exposed to variable winter conditions

• We measured energy use in hatchling turtles under different winter thermal regimes.
• Cold-winter turtles were sustained predominantly by hepatic energy stores.
• Mild- and warm-winter turtles were sustained predominantly by carcass-derived energy.
• Metabolic rate was extremely sensitive to temperature exhibiting a Q10 of 9.1.
• Warm winters may constrain post-emergence fitness via greater energetic costs.

The painted turtle (Chrysemys picta) is an especially useful organism in the study of metabolic regulation during dormancy because it is sustained by finite energy reserves from hatching until emerging from its nest, about nine months later. In this study we subjected overwintering C. picta hatchlings to 4, 10, or 15 °C, temperatures simulating cold, mild, and warm winters, respectively, to investigate how various energy reserves are impacted by differential metabolic demands. An energy budget based on seasonal changes in caloric content showed that these turtles consumed an average of 0.39, 0.75, or 1.21 kJ g−1, respectively, during the 6-month period of simulated hibernation. These estimates of energy use agreed reasonably well with estimates based solely on respirometric data. Unexpectedly, turtles in autumn contained little residual yolk, none of which was consumed by turtles in the cold- and mild-winter groups, this finding contradicting the widely held belief that residual yolk plays an important, direct role in the survival of turtles that overwinter inside their natal nest. By contrast, a marked reduction in dry mass of both liver and carcass attested to their importance in fueling metabolism and, indeed, catabolism of substrates from these components accounted for 31–52 and 35–63%, respectively, of the energetic cost of overwintering. The greater dependence on carcass reserves and relatively poor physiological condition of turtles in the mild- and warm-winter groups implies that metabolic demands imposed by high environmental temperatures would likely constrain post-emergence fitness.