]ournalofAridEnvironments (1995) 31:431-440 Comparative physiology of thermoregulation in rodents: adaptations to arid and mesic environments A. Haim & I. Izhaki University of Haifa at Oranim, P. 0. Kiryat Tivon, 36006, Israel (Received 28 March 1994, accepted 12 May 1994) Metabolic and thermoregulatory variables of six rodent species inhabiting a desert environment were compared with those of six species inhabiting mesic environments. The results of such a comparison show that species originating in deserts are more efficient in conserving energy and presumably water when compared with those species originating in mesic habitats. Reduced resting metabolic rates (RMR) which characterize desert species are an important adaptation. However, the ability to inhabit mesic or very cold environments by such species is achieved through different morphological, physiological or behavioural adaptations, and not by an increase in RMR values. Non- shivering thermogenesis (NST) seems to be an important physiological adaptation in such desert rodents. The relatively high values of NST capacity found in desert species seem to compensate for their low RMR values. ©1995 Academic Press Limited Keywords: ecophysiology; resting metabolic rate; non-shivering thermogenesis; high altitudes Introduction The desert ecosystem is characterized by low rainfall which is highly variable and unpred!ctable. Water is the most important controlling factor for many biological processes occurring in this ecosystem (Noy-Meir, 1973). Deserts like the Negev in Israel or those of Southern Africa are also characterized by high ambient temperatures with significant daily and seasonal fluctuations. In order to cope with such a hostile environment small mammals like rodents with a high surface to body volume ratio are adapted to nocturnal activity and select suitable shelters to avoid the hot hours of the day. Such shelters can be under rocks or slits in cliffs or burrows. Such a behavioural pattern is an important adaptation for conserving water in small mammals (Schmidt- Nielsen, 1964). However, a few species such as the Golden spiny mouse Acomys russatus (Shkolnik, 1971), the striped mouse Rhabdomyspumilio, the ground squirrels Xerus inauris and X. princeps (Skinner & Smithers, 1990) and the fat sand rat Psammomys obesus (Harrison & Bates, 1991) are diurnal. Desert rodents, as do other desert mammalian species, have resting metabolic rates (RMR) at their thermoneutral zone, which are lower than those predicted from their body mass according to allometric equations such as Kleiber's equation (Kleiber, 1961). In some species overall thermal conductance (C) is higher than that expected 0140-1963/95/040431 + 10 $12.00/0 © 1995 Academic Press Limited