PENICK, D. N.; University of Connecticut, Storrs & Bucknell University, Lewisburg PA: Endothermy of Ambystomid Salamanders: New Perspectives on the Evolution of Endothermy in Vertebrates

Several Ambystomid species migrate from hibernacula to breeding pools during late winter rains when temperatures can quickly drop below freezing. In these studies, I describe a novel thermoregulatory strategy found within Ambystomid salamanders. Adults of three species (A. maculatum, A. jeffersonianum, & A. tigrinim) are facultative endotherms. Under limited conditions, salamanders are homeothermic using heat production within viscera and secondarily, primitive, shivering-like contractions to maintain body temperatures up to 8&degC above air temperature (+9.5hr). One congener (A. opacum) breeds in early fall and freeze tolerant larvae (to -3&degC) over-winter in vernal pools. I assessed thermocompetence of the endothermic Ambystomids using down-up metabolic trials. Salamander metabolic rates (VO₂ & VCO₂) increased with decreasing temperatures below 2&degC. Secondary metabolic peaks following recovery suggest anaerobic debt. Under stringent cold challenge, salamander body temperatures drop to a crystallization freezing point and exhibit a freezing exotherm similar to freeze tolerant frogs. In this new thermal strategy, �cryothermy�, latent heat is harvested from ice formation and added to both aerobic and anaerobic core heat production to essentially �surf� the freezing exotherm establishing critical visceral temperatures above zero. Ice forms in a thin layer encasing viscera, halting convective heat loss, increasing insulation, and reflecting thermal radiation back to the core. Respiratory quotients of cold challenged animals soared above 2 approaching 1&degC demonstrating an anaerobic component of heat production and strongly suggesting heat generation by an undocumented anaerobic decarboxylation pathway. Based on these discoveries, I propose the Facultative Endothermy Model for the evolution of endothermy in vertebrates.