Thermal ecology of Northern Water Snakes (Nerodia sipedon) in a northern wetland in central Michigan

Semi-aquatic north-temperate reptiles experience challenging environmental conditions for thermoregulation on both seasonal and daily bases. We studied the thermal biology of north-temperate Northern Water Snakes (Nerodia sipedon) in central Michigan by remote radiotelemetry monitoring of snake body temperature (Tb) using surgically implanted thermally-sensitive radio-transmitters and by measuring operative temperatures (Te) across a range of microhabitats using biophysical models. Our goals included evaluation of thermoregulatory capabilities at a locality that we viewed to be a fine-grained thermal environment, seasonal variations in patterns of thermoregulation, and the effects of sex on thermoregulation. During summer, snakes showed diel Tb cycling apparently using the open Sphagnum mat for late morning warming and shuttling among different microhabitats until early evening when a monotonic decline in Tb ensued and continued through the early morning hours. Snakes attained Tb within their laboratory-determined preferred body temperature range (Tset = 28–33 °C) mostly during late afternoon and with average percentages of Tb values for individual snakes within Tset range when permitted by operative temperatures (Ex) between 68 and 70% of the time depending on method of measurement. Relatively high investment in thermoregulation when thermal conditions were poor occurred only during September but declined thereafter as snakes prepared to overwinter. We did not detect differences in thermoregulation among reproductive females, non-reproductive females, and males. Relative to a population of N. sipedon at a higher latitude in Ontario our snakes showed a relatively high Tset range and thermoregulated more effectively, particularly during the daylight hours. It remains unclear how much inter-population variation in thermoregulation is due to potential adaptations to latitude, to habitat differences, or variations in methodologies. •The capacity for effective thermoregulation peaked between June-August and diminished during May, September, and October.•Snakes effectively thermoregulated during a 4–6 h timeframe during mid-afternoon.•Snakes used various microhabitats, especially the Sphagnum mat in direct sunlight, for thermoregulation.•No intersexual differences in thermoregulation were detected.•Snakes thermoregulated comparatively more effectively than did those previously studied in Ontario.