We investigated the link between thermal quality and the effectiveness of thermoregulation in milk snakes in a thermally challenging environment. We defined thermoregulatory effectiveness as the extent to which an individual maintains its body temperature (T b) closer to the preferred range (T set) than allowed by the thermal quality of its environment. We defined thermal quality as the magnitude of the difference between operative environmental temperatures (Te) and T set. Because ectotherms regulate body temperatures through choice of habitat and behavioural adjustments, we also examined the link between thermoregulation, habitat use and behaviour. During 2003–2004, we located 25 individuals 890 times, and recorded their T b. Thermal quality was lower in the spring and fall than in the summer, and was lower in forests than in open habitats. Milk snakes thermoregulated more effectively in the spring than in the summer and fall, and more effectively in the forest than in open habitats. Milk snakes had a strong preference for open habitats in all seasons, which was likely to facilitate behavioural thermoregulation. The preference for open habitats was equally strong in all seasons and, therefore, the higher effectiveness of thermoregulation was not a result of altered habitat use. Instead, milk snakes modified their behaviour and were seen basking more and moved less in the spring than in the summer.
Snakes caught opportunistically from hibernacula, had radio-transmitters implanted which tracked location and temperature