The Amplitude of Circadian Body-Temperature Rhythms in 3 Rodents (Aethomys-Namaquensis, Thallomys-Paedulcus and Cryptomys-Damarensis) Along an Arboreal-Subterranean Gradient

Abstract

The maximum amplitudes of circadian rhythms of body temperature (R(t)) of three species of desert rodents inhabiting an arboreal-subterranean gradient were correlated with habitat-dependent thermoregulatory parameters such as minimal thermal conductance and the magnitude of ambient temperature tolerance by endotherms. It was shown that R(t) differed by 87-181% of expected values. The data for two rodents (Thallomys paedulcus and Aethomys namaquensis) that forage aboveground displayed higher-than-expected R(t) values, whereas the strictly subterranean species (Cryptomys damarensis) had lower-than-expected R(t) values. These data are interpreted in terms of the Endothermic Temperature Range Hypothesis, which argues that the large range of diel ambient temperature fluctuations found in desert habitats may account for the physiological parameters that generate the higher-than-expected body temperature rhythms, a low and fairly inflexible minimal thermal conductance and low resting metabolic rate. Further discussion centres on the possible functional significance of circadian energetic rhythms, particularly in terms of an endotherm's fitness. It is proposed that, at least, functions of the rhythms should be considered: diel thermoregulatory adjustments and energy conservation.