Thermal variability in body temperature in an ectotherm: Are cloacal temperatures good indicators of tortoise body temperature?

Historically, studies of reptilian thermal biology have compared ambient temperatures (Ta) to body temperatures (Tb) from the animal under study, with Tb usually taken from the cloaca and various instruments being used to measure Tb. The advent of surgically implanted miniature temperature loggers has offered the opportunity to test the efficacy of cloacal Tb as a measurement in thermoregulatory studies. We expected that there was a difference between skin, cloacal, and core Tb's. Temperatures were measured from various positions on leopard tortoises (Stigmochelys pardalis) using thermocouples and miniature temperature loggers, including surgically implanted temperature loggers. Measurements of temperature from various positions on and in the tortoise were significantly different from Ta. Cloacal Tb's were significantly lower than all other body temperatures measured, and core Tb's were significantly different from cloacal Tb, skin and carapace temperatures. In addition, significant differences were found between measures of temperature from other parts of the body. The variations between core Tb, cloacal Tb and other measures of Tb indicated that there are large thermal gradients within the body of a relatively large tortoise at any given time with cloacal Tb not an accurate measure of core Tb.

► Reptilian thermal biology studies regularly investigate body temperatures (Tb).
► We tested the efficacy of cloacal Tb compared with core Tb in Stigmochelys pardalis.
► Cloacal Tb's were lower than all other body temperatures measured.
► Cloacal Tb may not be an accurate measure of core Tb.