Comparative physiology of heat production in rodents under increasing salinity: The effects of habits and habitat

Small mammals inhabiting environments that are either seasonally or perpetually dry, such as Medi-terranean or desert ecosystems respectively, commonly have physiological capabilities that enable them to deal with water shortage. We compared results of thermoregulatory responses of rodent species inhabiting different habitats and having varying activity periods, when salinity increases in their water source, as often occurs in their natural habitats during the dry period. Experimental animals were maintained on a diet of dry soy-beans and an increased salinity of their water source (2% agar gel), from 0.9% to 3.5% NaCl in mesic species and up to 7% in xeric species. While desert species could cope with high salinities in their water source, mesic species could not. Desert-adapted species depending on their preferred micro-habitats differ in their thermoregulatory responses. Rock dwellers, such as the golden spiny mouse Acomys rus-satus and the bushy tailed gerbil Sekeetamys calurus, reduce their resting metabolic rates (RMR) and increase nonshiv-ering thermogenesis (NST) capacity in response to increasing salinity. In contrast, the deep burrowing fat jird Meri-ones crassus, increases RMR and only slightly increases NST-capacity. Our study suggests that species occupying different habitats vary their thermoregulatory capabilities, in relation to dehydration and increasing salinity in the water source. This may be a consequence of adaptation from the original ecosystem to the current environment in which a species inhabits.