Rainfall is the most important factor limiting primary production, and hence food availability, in ecosystems. This study tests the thesis that, under conditions where rainfall is unpredictable and food supply varies spatially and temporally from year-to-year, physiological energy-saving mechanisms should evolve in animals for which migration is not an option to avoid periods of low habitat productivity. Apart from the normally erratic rainfall patterns characteristic of arid regions, unpredictable seasonal rainfall may also occur in mesic zones as a consequence of climatic phenomena such as the El Niño Southern Oscillations (ENSO). Using two hypothetical models, this study attempts to determine whether the respective evolutionary history of both aridity and ENSO can be detected in the various phenotypic physiological traits of Rhabdomys pumilio from across southern Africa. Model 1 predicts that a gradient of conservatism of physiological traits related to metabolism, will be detectable along an aridity gradient. Model 2 predicts an increase in the conservatism of similar traits correlated with both the aridity gradient and the gradient of negative rainfall anomalies that are associated with the ENSO. Measurements of circadian rhythms of body temperature and oxygen consumption of individuals of R. pumilio collected from six sites across southern Africa showed that the geographic variation in the physiological traits and proximate responses of this species was explained better by Model 2 than by Model 1. Furthermore, for some traits and proximate responses of this species it was shown that, although the stresses associated with unpredictable rainfall occur less frequently in non-desert ENSO-zones, they are nevertheless sufficient to mimic the selection pressures of rainfall unpredictability in arid zones. This study also highlighted the low basal rates of metabolism and the other conservative physiological traits of this species as a whole. The basal metabolic rate of 0.819 mlO₂g/h and the minimum wet thermal conductance of 0.110 mlO₂/g/h/ºC are lower than that predicted by allometry. The circadian amplitude of body temperature of 2.52°C and the circadian amplitude of oxygen consumption of 1.39 mlO₂/g/h were higher than those predicted by allometric equations. These deviations from predicted values were attributed to the adaptedness of this species to the physiological energy stresses associated with unpredictable resource availability in southern Africa. / Thesis (M.Sc.)-University of Natal, Pietermaritzburg, 1998.