Summary
In an E Ontario population of northern water snakes basking activity peaked at 09:00 then declined steadily until 14:00 before increasing again. In the field, temperatures of basking snakes averaged (±SE) 26.3 ± 0.7°C, while captive snakes in a thermal gradient showed a narrower selectivity, averaging 27.7 ± 0.4°C. The temperatures of basking snakes never exceeded 33°C, even though a model snake placed in the sun reached 48°C, suggesting that the snakes were thermoregulating to prevent overheating. Water snakes based more frequently as the temperature of the air increased relative to the water. Temperature influenced microhabitat selection independently of circadian patterns. When in water, snakes tended to frequent habitats where leopard frogs Rana pipiens, a common prey species, were most abundant, ie prey distribution may also be an important component of water snake habitat selection.
Methodology
We studied water snakes during the summers of 1988 and 1989 at the Queen's University Biological Station, near Chaffey's Locks in eastern Ontario. Field research was conducted in an isolated marsh 1 km south of Lake Opinicon. We captured 122 water snakes (76 male, 46 female) and brought them to the laboratory where we determined their sex, measured their snout-vent lengths (SVL), and weighed and heat-branded them with unique patterns on their ventral caudal scales (Weary 1969). Snakes used in experiments were housed unfed for up to 2 weeks before being returned to the marsh.
We distinguished three habitats within the marsh: shore, open water, and willow; we also distinguished two microhabitats: aquatic and arboreal. Both the shore and open-water habitats contained dense patches of submerged vegetation (primarily Utricularia spp.), but only the shore habitat contained emergent cattails (Typha spp.). Neither of these habitats provided aboreal perch sites for snakes. In contrast, snakes in the willow habitat frequently perched on the branches of willow trees (Salix spp.) that were 1-4 m in height and located in water near the centre of the marsh. Water depth in the marsh rarely exceeded 1.2 m, and was usually less than 0.6 m. From 1 June to 10 August 1988 and 1 June to 20 June 1989 we conducted 1349 twenty-pace transects randomly through all habitats in the marsh (354 shore, 238 open water, 757 willow). The frequency with which a habitat was sampled was roughly proportional to its area within the marsh. Because of the difficulty of traversing the marsh and seeing snakes at night, we conducted all transects during the day between 06:00 and 16:30. We counted all snakes sighted within 2 m of either side of the transect line, and after each transect we recorded water temperature at the surface and air temperature in the shade. Recognizing that snakes in water were more difficult to detect than basking snakes, we used these data to compare habitat use patterns within, but not between, microhabitats. We also captured basking snakes opportunistically and recorded their cloacal temperatures using a YSI series 400 flexible-tip cloacal probe and a Cole-Parmer digital thermometer. Air temperature was measured immediately afterwards in the shade close to where each snake was caught.