Summary
The ecology of ectotherms should be particularly affected by latitude because so much of their biology is temperature dependent. Current latitudinal patterns should also be informative about how ectotherms will have to modify their behavior in response to climate change. We used data from a total of 175 adult black ratsnakes (Elaphe obsoleta) radio‐tracked in Ontario, Illinois, and Texas, a latitudinal distance of >1500 km, to test predictions about how seasonal patterns of activity and mortality should vary with latitude. Despite pronounced differences in temperatures among study locations, and despite ratsnakes in Texas not hibernating and switching from diurnal to nocturnal activity in the summer, seasonal patterns of snake activity were remarkably similar during the months that snakes in all populations were active. Rather than being a function of temperature, activity may be driven by the timing of reproduction, which appears similar among populations. Contrary to the prediction that mortality should be highest in the most active population, overall mortality did not follow a clinal pattern. Winter mortality did increase with latitude, however, consistent with temperature limiting the northern distribution of ratsnakes. This result was opposite that found in the only previous study of latitudinal variation in winter mortality in reptiles, which may be a consequence of whether or not the animals exhibit true hibernation. Collectively, these results suggest that, at least in the northern part of their range, ratsnakes should be able to adjust easily to, and may benefit from, a warmer climate, although climate‐based changes to the snakes' prey or habitat, for example, could alter that prediction.
Methodology
Data were collected from 1996 to 2004 at Queen’s University Biological Station in eastern Ontario, Canada (44°34' N, 76°19' W), from 2002 to 2004 at Cache River State Natural Area in southern Illinois, USA (37°23' N, 88°54' W), and from 2004 to 2007 at Fort Hood in central Texas, USA (30°10' N, 97°45' W). The Ontario and Texas sites are near the northern and southern limits of ratsnake distribution, respectively, and the Illinois site is in the middle of the latitudinal range. The coldest month in Texas is warmer than all but five months in Ontario and seven months in Illinois based on daily maximum temperatures, and four and five months, respectively, based on minimum temperatures (see Supplement). Snakes in Ontario emerge from hibernation between mid-April and late May and return in late September (Weatherhead and Hoysak 1989, Blouin-Demers et al. 2000). In Illinois snakes emerge from hibernation from early April to early May and return in October (Carfagno and Weatherhead 2008). Ratsnakes in Texas are active on warm days throughout the year (see Results).
Although we refer to ratsnakes at our three study sites as populations, the taxonomy of ratsnakes is unclear (see Burbrink et al. 2000, Burbrink 2001, Gibbs et al. 2006). What is important for our purposes, however, is that our study ‘‘populations’’ are closely related and ecologically similar. Two potentially important differences among the populations are that during winter ratsnakes in Texas do not hibernate, although they alter habitat use (Sperry and Weatherhead 2009b), whereas ratsnakes in both Illinois and Ontario hibernate, and that ratsnakes in Ontario and Illinois are exclusively diurnally active, but in Texas switch from being diurnally to nocturnally active during summer (J. Sperry, unpublished data).
Data on activity and mortality were collected using radiotelemetry. For Ontario, we used activity data from snakes tracked from 1996 to 1999, using the same methods used at the other two sites. We expanded our survival data by including snakes tracked in 2001–2004 that provided mortality but not activity data. In Ontario and Illinois, snakes were caught as they emerged from communal hibernacula in spring and opportunistically throughout the season. In Texas, ratsnakes were caught only opportunistically. At all sites, transmitters weighed <3% of snake body mass, transmitters were surgically implanted (Reinert and Cundall 1982, Blouin-Demers and Weatherhead 2001), and snakes were released at their capture locations. Transmitters weighed 9 or 13 g with batteries lasting 12 and 24 months, respectively (Model SI-2T, Holohil Systems, Carp, Ontario, Canada). Snakes were relocated approximately every 48 h and date and location (UTM coordinates) were recorded. Temperature data were obtained from weather stations at Queen’s University Biological Station, Ontario; Carbondale, Illinois; and Fort Hood, Texas. All weather stations were <40 km from snake study areas.