Daily rhythms of body temperature and activity in free-living Black-tailed Tree Rats ( Thallomys nigricauda) along an aridity gradient

Abstract

Since Black-tailed Tree Rats, Thallomys nigricauda, face variable thermal environments in their extensive range in southern Africa, variation in their physiological and behavioural responses to the wide range of ambient temperatures ( T as) was expected to play a role in the success and range of the species. Body temperature ( T b) and activity patterns of free-ranging Tree Rats were investigated during winter and summer at three study sites along an aridity gradient. We measured abdominal T b using implanted iButtons ®. All but three Tree Rats displayed significant 24 h T b rhythmicity. The T b range for Tree Rats was 32.33–40.63 °C ( n = 13) and 32.69–40.15 °C ( n = 17) in winter and summer respectively. Although there was variation in T b profiles, Tree Rats generally displayed a bimodal distribution of T b, with high and low T b values during scotophase and photophase respectively. Site had no effect on the range of T b in winter. Range in T b was significantly greater in winter, when Tree Rats reduced their minimum T b. Maximum amplitudes of daily rhythms of T b were 259.7% of expected values. Temperatures of cavities were more stable compared with operative temperature. In winter male Tree Rats spent a significantly greater proportion of the active phase away from their home cavity compared with females. It is suggested that the energy savings in Tree Rats are a result of a combination of physiological and behavioural mechanisms allowing them to maintain nocturnal activity in winter and overall energy balance.