Endotherm Energetics Under Environmental and Physiological Constraints

Metabolic rate governs how animals allocate energy toward maintenance, activity, growth, and reproduction, linking physiology to environmental context. This dissertation investigates how two endothermic mammals regulate metabolic rate in response to environmental variability, focusing on thermoregulatory flexibility and physiological constraints. Using flow-through respirometry, I measured resting metabolic rates (RMRs) in two complementary systems: tropical bears housed in semi-natural outdoor enclosures and marsh rice rats (Oryzomys palustris) exposed to controlled thermal treatments. Asiatic black bears (Ursus thibetanus) exhibited unexpectedly low mass-specific metabolic rates even under high ambient temperatures, yet their RMR increased with temperature, suggesting proximity to their upper thermal limits. Sun bears (Helarctos malayanus), by contrast, remained active throughout trials, precluding resting measurements; nonetheless, the data still represent the first oxygen consumption measurements reported for the species. To evaluate plasticity in metabolism and thermal tolerance, I compared wild-caught and lab-raised marsh rice rats subjected to prolonged cold and warm conditions. Wild-caught rats initially displayed clear thermoneutral zones, but after acclimation, their metabolic rates followed a more continuous response to temperature. Lab-raised individuals showed a similarly graded pattern, suggesting that while their absolute rate of energy use is plastic, the overall structure of their thermal response curves may remain stable. Finally, I tested whether the increased energetic demands of cold exposure were supported by changes in digestive organ morphology, as predicted by the central limitation hypothesis, which posits that sustained metabolic output is constrained by digestive capacity. Cold-acclimated rats exhibited elevated RMRs alongside larger large intestines and ceca, consistent with increased nutrient assimilation demands. These findings suggest that cold induced energetic challenges may be met in part through gut hypertrophy, linking metabolic performance to digestive investment. Together, these studies reveal that metabolic rate reflects both intrinsic species traits and environmentally induced flexibility. By combining ecologically realistic measurements in large mammals with experimental manipulations in a tractable rodent model, this dissertation advances a mechanistic understanding of how animals meet energetic challenges in variable environments and provides insight into the capacity of species to cope with a warming world.