Body mass is a thermoregulatory adaptation of diurnal rodents to the desert environment

Most desert rodents are nocturnal and avoid the heat of their environment during the day hours. The diurnal fat sand rat Psammomys obesus is an exception. Sub-adult individuals less than 85 g body mass (W_b) have a body temperature (T_b) daily rhythm of a nocturnal rodent which changes to a diurnal pattern in adults with a W_b of above 120 g. We have assessed whether there are relationships between W_b, ambient temperature (T_a), and foraging duration. These were achieved by measuring the time six young (sub-adults) and six adult fat sand rats spent foraging, at two different T_as: 30°C - control and 44°C - experimental. A significant difference in the time spent close to the food was revealed between the two temperatures and the two age classes. The heavy adults spent more time close to the food than sub-adults at T_a of 30°C (3.9±0.9 min and 2.8±0.8 min, respectively) and at T_a of 44°C (10.3±0.9 vs. 7.0±1.0 s). Our results suggest that under extreme ambient temperature conditions, adults can spend more time foraging out of their burrow, and we suggest that this may emerge from their better ability to thermoregulate in a hot environment.