Assessment of Seasonal Metabolic Acclimation in Gopher Tortoises (Gopherus polyphemus): Does Dormancy Reduce Metabolic Rate?

Environmental selective pressures, and associated constraints (such as sub-optimal thermal environments), imposed on organisms can drive important changes in energy usage in organisms exposed to the environmental constraints. Gopherus polyphemus, Gopher Tortoises, significantly reduce constitutive immunity during winter dormancy, a pattern that has been observed across ectothermic vertebrates. Herein, we assessed the metabolic consequences of winter acclimation to determine if the previously documented seasonal immune reduction confers measurable energy conservation. We measured oxygen (O₂) consumption, carbon dioxide (CO₂) production, and calculated respiratory quotient in G. polyphemus at two seasonally relevant temperatures (12.5°C and 32.5°C) and two seasonal acclimation states (winter and summer). We found no effect of season on either O₂ consumption or CO₂ production but found that RQ was significantly elevated in animals at the colder temperature (P < 0.001). Additionally, we found very high RQs (>1.0) in cold-acclimated animals during dormancy. High RQs in this species were likely a result of anaerobic digestive gases exiting the gastrointestinal tract, as we documented significant anaerobic carbon dioxide production in freshly collected fecal samples. Respiratory quotients were all below 1.0 and within normal ranges for vertebrates when a standardized mass-specific fecal CO₂ enrichment was removed from total CO₂ production. Oxygen consumption was greatest in animals at the warmer temperature, and the Q₁₀ for O₂ consumption in animals in this study was 2.34. While we failed to document a pattern of reduced temperature-independent metabolism during dormancy, results from this study further demonstrate important physiological modulation between dormancy and activity in this species of conservation concern.