Variation in size of cover used by watersnakes (Nerodia): Do smaller snakes use smaller rocks?

Snakes often seek shelter under rocks or other physical cover objects, which provide protection from both predators and unfavourable weather and also may be used actively in thermoregulatory behaviour. Presumably, snakes are limited, by their own body size, to some minimum rock size, so that we should expect a positive relationship between a snake's size and the size of the rock it uses. I tested this hypothesis for the watersnake, Nerodia sipedon, along a creek in a peri-urban setting in southern Ontario, Canada. Although larger snakes tended to be found under larger rocks, the relationship was weak. A composite measure of rock size from PCA was not markedly better than simple rock length. Sizes of rocks used by snakes did not change over the active season. Snakes tended to be found under the largest rock in their immediate vicinity, but distances from used rocks to equally big or bigger rocks rarely exceeded a few metres. Similarly, measurements of rocks on transects along the creek showed that, although small rocks predominated, all transects contained rocks large enough for snakes. Thus, it seems unlikely that cover is limited for watersnakes at this site. However, unmeasured factors such as temperature or other physical characters of rocks could change that conclusion. These factors would best be determined by manipulative experiments.