We tested externally applied, temperature-sensitive, radiotransmitters for determining the body temperature of big brown bats (Eptesicus fuscus) in various ambient temperatures (2- 260C). There was a slight, but significant, effect of ambient temperature on skin temperature (measured by the transmitters), but skin temperature accurately reflected rectal temperature in torpid and active bats, and it was never >3.30C below rectal temperature. External radiotransmitters are, thus, useful in studies of torpor in bats, even when only small decreases in body temperature occur.
