Combining Captive and Field Studies to Investigate the Energetics and Water Balance of Ectotherms

Obtaining sufficient amounts of water is essential for all living organisms. Maintaining water balance can be particularly challenging in xeric habitats, cold habitats where water remains mainly in a frozen state, and marine habitats with saline water. While the potential pathways by which animals obtain water are similar across species, the contribution of each source to the total body water pool varies considerably between species. Little research has been done to investigate how herpetofauna adjust specific pathways of water gain and loss to maintain water balance. To address this gap of knowledge, I examined the impact of metabolic rate, diet, and water availability on the oxygen isotopes in the body water pool of two reptile species, utilizing the novel method of interpreting Δ17O of body water, or Δ17OBW. I experimentally manipulated diet and temperature for captive collared lizards, Crotaphytus collaris, to evaluate how oxidase water and food water contribute to the total water balance. Temperature phases had a strong influence on water consumption and Δ17OBW values; however, Δ17OBW did not decline with increased metabolism or reduced food and water intake, contrary to expectations. An oxygen flux model could accurately predict Δ17OBW only under high-temperature conditions, suggesting that current models do not capture key aspects of water turnover in ectotherms under low- or moderate-temperature. I applied Δ17OBW to wild cottonmouth snake, Agkistrodon conanti, populations inhabiting both freshwater and marine island habitats. Field samples from all sites exhibited Δ17OBW values remarkably like those of meteoric water, indicating efficient water retention and potentially slow water turnover rates. However, once A. conanti is brought into captivity, Δ17OBW declined significantly under experimental hydration and dehydration, consistent with increased reliance on oxidase water. Captive A. conanti, regardless of hydration status, exhibited more negative Δ17OBW values than when sampled in the wild, possibly due to stress-induced metabolic shifts. Together, these findings highlight Δ17OBW as a powerful tool for inferring water sources and physiological states in reptiles, demonstrating that reptilian water balance may be more dynamic than previously recognized.