Thermal ecology of massasauga rattlesnakes (sistrurus catenatus) near their northern range limit

Using populations at their northern range limits, we compared the thermoregulatory behaviour of Massasauga Rattlesnakes (Sistrurus catenatus (Rafinesque, 1818)) to published results for Ratsnakes (Elaphe obsoleta (Say in James, 1823)) and Northern Watersnakes (Nerodia sipedon (L., 1758)) to test the hypothesis that given similar benefits of thermoregulation, costs associated with foraging ecology should shape thermoregulatory behaviour. More than 32000 body temperature measurements from 34 individual snakes over 4 years were used to quantify thermoregulation by Massasauga Rattlesnakes using standard indices and a new index (%MaxE) that describes how much of the thermoregulatory potential available to a snake is realized. On average, Massasaugas were much cooler (d_b = 6.9 °C) than their preferred body temperature range (30-33.6 °C) but warmer (d_e d_b = 3.3 °C) than were they not thermoregulating. Massasaugas realized more than half of their environmental potential for effective thermoregulation (%MaxE = 64%). Consistent with there being less conflict between foraging and thermoregulation for ambush predators than active foragers, Massasaugas were more effective thermoregulators than Ratsnakes or Watersnakes during the day. All three species were effective thermoregulators at night, supporting the assumption that species in a cool environment will thermoregulate when there are few competing interests.