G. Blouin-demers et Pj. Weatherhead, Thermal ecology of black rat snakes (Elaphe obsoleta) in a thermally challenging environment, ECOLOGY, 82(11), 2001, pp. 3025-3043
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.