Comparative metabolism, thermoregulation and morphology in two populationsof vlei rats (Otomys irroratus)

The Hogsback (32 degrees 33S 26 degrees 57E) and Alice (32 degrees 47S 26 d egrees 50E), Eastern Cape, South Africa, are separated by only 24 km but by 1000 m in altitude and fall into different climatic regions. Thermal respo nses (energy expenditure and body temperature) to ambient temperature were measured in a population of vlei rats (Otomys irroratus) from each of the t wo localities. We predicted that animals from the colder Hogsback would sho w differences in their thermal physiology and morphology consistent with be tter cold-resistance. Basal metabolic rates of the Hogsback population were slightly, but not significantly, higher than the Alice population (23.9 J g(-1) h(-1) vs 22.3 J g(-1) h(-1)), but the slope of the regression between energy expenditure and ambient temperature below the thermal neutral zone was significantly lower (-1.28 vs -1.60). Body temperature, although quite variable in both populations, was not significantly influenced by ambient t emperature in the Hogsback population, whereas that of Alice animals was. F ur length was longer and relative size of the ears and tail was smaller in the Hogsback population, which probably accounted for the slightly lower mi nimum thermal conductance (1.79 J g(-1) h(-1) degrees C-1 vs 1.91 J g(-1) h (-1) degrees C-1) in the Hogsback population. Vlei rats from the two sites also have different karyotypes that correlate with climate but there is ins ufficient evidence at present to suggest that the different karyotypes and the physiological parameters measured are adaptive.