Winter Is Coming: Integrative Analysis of Cold Acclimation in a Freeze Tolerant Frog

Cope's gray treefrog Dryophytes chrysoscelis is a seasonally freeze tolerant anuran that undergoes a preparatory period of cold acclimation in order to survive repeated freezing and thawing each winter. The mechanisms that enable freeze tolerance in this species are not entirely understood and the ecophysiological cues that regulate cold acclimation are unstudied. In the present study, we describe aspects of behavior, morphology, and physiology of frogs progressing through cold acclimation, using a previously established protocol of changing light and temperature that successfully induces the capacity to survive whole body freezing and thawing. Wild-caught males were captured in July in southwest Ohio, and behavioral, morphological, and physiological variables were compared beginning in August among frogs maintained under constant ("warm") environmental conditions (22°C, 12:12 light: dark) and those undergoing cold acclimation (October through December) to 5°C, 8:16 light: dark. During cold acclimation frogs ceased eating, heart rate (HR) was reduced, body mass decreased, and the righting response and toe pinch reflexes were slower and less coordinated. Some of these variables also changed in animals maintained under constant, warm, and environmental conditions during the same period: feeding decreased, HR decreased, body mass increased, and dorsal skin color shifted from green to brown. However, these warm frogs began to reverse those changes and increased feeding and HR in late January. These data indicate that behavior, morphology, and physiology in D. chrysoscelis are subject to seasonal variations that are augmented by cold acclimation (i.e., a reduction in photoperiod and environmental temperature). Freeze competence derived from these events may be affected by volatile climates and seasonal warming.