Thermal Consequences of Reptilian Viviparity: Thermoregulation in Gravid and Nongravid Garter Snakes (Thamnophis)

Reptilian viviparity is assumed to have evolved in response to cool climates, because of the developmental benefits of maternal thermoregulation. However, very few data are available to test this assumption and there is currently little evidence of a consistent pattern of thermoregulation associated with viviparity. I used temperature-sensitive radiotelemetry to measure the differences in thermoregu- lation between gravid and nongravid garter snakes (Thamnophis sirtalis and T. elegans). Gravid females had higher mean body temperatures and thermoregulated more precisely than did nongravid females. These differences in thermoregulation persisted throughout gestation, but disappeared relatively quickly following parturition. Gravid females thermoregulated with high precision despite voluntarily selecting a wide range of mean Tbs indicating that precision may be a critical, although largely overlooked, com- ponent of the thermal relations of viviparous reptiles. The differences in thermoregulation between gravid and nongravid Thamnophis suggest that viviparity imposes an energetic cost, but determining the mag- nitude of the cost will require more information on the relationship between temperature and metabolic rate.