Thermal ecology and sexual size dimorphism in northern water snakes, Nerodia sipedon

We used more than 326 000 observations of temperature collected by radio telemetry from 38 individuals over three years to investigate thermoregulation and thermal relations of northern water snakes (Nerodia sipedon) near the northern limit of their distribution in Ontario, Canada. We tested hypotheses concerning the effects of feeding, season, sex, and reproductive condition on thermoregulation of individuals. The mean preferred body temperature (PBT) for captive snakes from the study population was 27.1°C, similar to that reported for other populations, and PBT range (defined as the 25th-75th percentiles of selected temperatures) was 25-30°C. When environmental conditions allowed, the mean and range of body temperature (T(b)) of free-living snakes were nearly identical to those observed in captivity. The typical daily pattern in snake T(b) was an increase in late morning to a plateau temperature in the preferred range, followed by a decrease in the evening with a nightime plateau at approximately the temperature of the water. We calculated indices of thermoregulation to relate T(b) data to available ambient temperatures. Despite the northern latitude of the study population, ambient conditions were favorable over much of the activity season and offered the potential for the snakes to maintain T(b) within the PBT range almost 24 h per day for much of the season. However, the thermoregulation indices indicated that N. sipedon are only moderate thermoregulators, and often do not exploit opportunities to achieve PBT, particularly during the day. Variation in annual ambient temperatures affected growth rates of the snakes, and was sufficient to affect the age of maturity of females. Feeding did not elicit a thermophilic response under laboratory conditions or when snakes were fed experimentally in the field, suggesting that the benefits of increasing T(b) after eating did not outweigh the costs. Reproductive females thermoregulated more carefully than nonreproductive females during the July-August gestation period. Pregnant females may increase thermoregulatory behavior to enhance the rate and efficiency of embryogenesis. Males thermoregulated less than both reproductive and nonreproductive females, particularly later in the activity season when thermoregulation by females increased. We hypothesized that males pursue a survival strategy following the spring mating season and thus remain cool and secretive during the later part of the activity season. This strategy by males clearly contributes to their slowed growth and smaller size relative to females.