Effect of Pregnancy on Body Temperature Regulation and Habitat Use of Northern Watersnakes (Nerodia sipedon) in Middle Tennessee

Abstract

Snakes of several species select and maintain higher and less variable body temperatures (Tbs) during pregnancy by means of behavioral thermoregulation. The leading hypothesis for this behavior is that the rate of embryonic development is facilitated by warmer temperatures, shortening gestation time, and there are optimal temperature ranges for successful embryonic development which are typically warmer than baseline body temperatures. Therefore, maintaining a warm and relatively constant body temperature could be more beneficial than costly to a reproductive snake. Body temperature selection can be accomplished through behavioral actions such as modifying body postures, choosing different habitats and moving throughout the environment. To test this hypothesis, in 2021 and 2022 I implanted reproductive and non-reproductive female Nerodia sipedon from middle Tennessee with radio transmitters and temperature loggers to continuously record their body temperatures throughout the gestation season. With a sample size of 12 female snakes (7 reproductive, 5 non-reproductive) and a total of 7440 Tbs (from 7 July – 14 August for 2021 and 2022), daily (24-hour) mean Tb was 27.1 ± 0.03 °C. Non-reproductive snakes overall 24-hour mean Tb (27.0 ± 0.05°C) was the same as that of reproductive snakes (27.2 ± 0.04°C). After running a generalized additive mixed model with body temperature as the response variable and pregnancy status as the main predictor variable and accounting for time of day (in hours), air temperature, water temperature, month, snake ID (individual snakes) and site as random variables, pregnancy appeared to have no effect on body temperature selection. Because habitat use and snake movement can influence body temperature selection, I also collected data on movements and habitat use. Reproductive and non-reproductive females had similar daily movements, generally moving less than 50 m between each telemetric location. However, snakes did show differences in habitat use with reproductive snakes predominately choosing rocks out of water and forbs while non-reproductive females chose rocks within the river or close to the water. Comparison of this study to others suggests the possibility that N. sipedon may exhibit thermal plasticity in body temperature selection based on geographic variation and that pregnant N. sipedon in warmer climates may not need to thermoregulate as strongly to maintain body temperatures at appropriate levels for embryonic development.