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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(a)s) 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 (R). All but three Tree Rats displayed significant 24 h T-b rhythmicity. The T-b range for Tree Rats was 32.33-40.63 degrees C (n = 13) and 32.69-40.15 degrees 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. (C) 2009 Elsevier Inc. All rights reserved.