Validating infrared thermography for non-invasive estimation of internal body temperature in hatchling Mojave desert tortoises.

Abstract

Quantifying thermoregulatory parameters, such as preferred and realized body temperatures, is essential for predicting ectotherm responses to climate change, and accurate estimates of internal body temperature are central to these efforts. In reptiles, cloacal temperature serves as the primary field- and laboratory-standard proxy for internal body temperature. Infrared thermography (IRT) is increasingly used to estimate internal body temperatures of small ectotherms because it minimizes disturbance to animal behaviour, thereby improving data quality. Although IRT has advantages over traditional contact methods, it relies on correlations between external and internal body temperatures, which can vary across taxa. To evaluate IRT's efficacy for estimating internal temperature of small turtles, we compared surface and cloacal temperatures of hatchling Mojave desert tortoises (Gopherus agassizii) under warming, elevated temperature and cooling conditions on two laboratory thermal gradients spanning moderate to extreme temperature ranges. Among five measured shell and body regions, shell temperatures correlated most strongly with cloacal temperatures. IRT produced accurate estimates during elevated temperature and cooling phases but was less accurate during warming from low initial body temperatures, particularly under steeper thermal gradients. We also evaluated IRT in two additional contexts: hatchling rearing enclosures and a behavioural experiment. Shell and cloacal temperatures aligned closely in rearing enclosures but exhibited larger discrepancies in the behavioural experiment. Our findings suggest that IRT is a promising tool for non-invasive estimation of cloacal temperature in hatchling desert tortoises (25-101 g). This tool is most useful under moderate thermal conditions when body temperatures are near thermal equilibrium but should be used with caution during periods of rapid temperature change when surface and internal temperatures may diverge. Once validated within a given study system, IRT can provide a practical alternative to traditional methods for monitoring body temperature while reducing disturbance and handling stress that may alter natural thermal behaviour.