Thermal ecology of black rat snakes (Elaphe obsoleta) in a thermally challenging environment

A general model in thermal ecology predicts that ectotherms should stop the rmoregulating when the costs outweigh the benefits. Support for this model comes from studies of warm-temperate species, but the extent to which the m odel can be extrapolated to species living in climatic extremes is unknown, because of the lack of information regarding the thermoregulatory behavior of such species. We tested the applicability of this cost-benefit model us ing data for black rat snakes (Elaphe obsoleta obsoleta) studied at the nor thern extreme of their range in Ontario. During 1997-1999, we used automate d temperature-sensitive radiotelemetry to collect similar to 150000 body te mperatures from 53 free-ranging rat snakes. Simultaneously, we used physica l models of snakes to measure the environmental operative temperatures avai lable to black rat snakes, and we determined their preferred body temperatu re range in a laboratory thermal gradient. The mostly forested habitats inh abited by rat snakes in Ontario were more thermally challenging than the ha bitats of other species studied to date. The preferred body temperature ran ges of mate, nongravid female, and gravid female black rat snakes were not significantly different (preferred body temperature averaged across all ind ividuals, 28.1 degreesC). However, free-ranging gravid females tended to ma intain higher body temperatures in order to thermoregulate more effectively as well as exploit their thermal environment more than males and nongravid females. This difference was most pronounced during the day and prior to e gg laying, and constituted the first documentation of such a phenomenon in an oviparous snake. Black rat snakes had indices of thermoregulation effect iveness similar to other species but tended to exploit opportunities for th ermoregulation less. Overall, our data provided support that was at best am biguous for the current cost-benefit thermoregulation model, suggesting tha t this model may generally be less applicable to species inhabiting climati c extremes. We propose that, for species in extreme climates, the costs ass ociated with thermoconformity may be more important than previously recogni zed, We identified several problems associated with the index of thermoregu lation effectiveness used by previous researchers, and we propose a mathema tically simpler alternative that circumvents these problems. We also make r ecommendations regarding the future use of the various indices of thermoreg ulation developed in recent years.