Daily Variation in the Body Temperatures of Free‐Ranging Garter Snakes

Abstract

This study describes how the body temperatures (Tb) of individual garter snakes varied on a daily basis under field conditions. Temperature—sensitive radio transmitters were surgically implanted into 10 Thamnophis elegans vagrans in eastern Washington. The snakes were released and monitored at 15—min intervals for periods of up to several days from June 1979 to May 1980. Operative environmental temperatures were estimated with painted metal models and were used to indicate possible Tb's. The combination of telemetered body temperatures with operative temperature data permitted detailed description of Tb variation and evaluation of the relative importance of the physical environment and behavior in determining that variation. The physical environment usually prevented the snakes from attaining preferred (selected) Tb's and thus limited when and where they could be active and potentially reduced their physiological performance. On cool days or at night, when operative temperatures (Te) were <15°C, the snakes remained underground and body temperatures changed gradually. On moderate days (15° < Te < 30°), the snakes usually emerged but were unable to maintain preferred body temperatures, and Tb variation was marked. On warm days (Te > 30°), the snakes selected Tb's within their preferred temperature range, and variation in Tb was small (e.g., range = 28.0°—32.5° X = 29.9°, SD = 1.32°). The snakes stayed within their preferred temperature range >90% of the time when Te > 30°. Their high thermoregulatory precision and thermal utilization presumably maintained high levels of physiological performance on warm days. Delays in emergence accounted for most instances in which the snakes did not utilize available thermal opportunities.