Exogenous passive heating during torpor arousal in free-ranging rock elephant shrews, Elephantulus myurus

In the laboratory rock elephant shrews (Elephantulus myurus; mean body mass 56.6 g) displayed the lowest torpor T _b min yet recorded (ca. 5°C) in a placental daily heterotherm. It was unknown whether these low T _bs were characteristic of daily heterothermy in free-ranging animals. It was also unclear how cost effective these low T _bs were since considerable energy is required to arouse from low T _bs on a daily basis. We continuously measured body temperature once every hour for 85 days in 13 free-ranging E. myurus from May to August 2001 (winter) in Weenen Game Reserve, KwaZulu-Natal, South Africa. We recorded a total of 412 torpor bouts. Free-ranging E. myurus had a high propensity for torpor with females displaying higher torpor frequency than males. The lowest T _b recorded was 7.5°C at T _a=2.7°C and the minimum torpor T _b was strongly correlated with ambient temperature. Torpor arousal was tightly coupled with ambient temperature cycles. Low torpor T _b min at low T _as was therefore cost-effective because the animals offset the high cost of arousal through exogenous passive heating. Laboratory studies under constant ambient temperatures may therefore underestimate the energetic benefits of torpor in free-ranging small mammals that inhabit regions where seasonality is moderate.