The effects of season and dietary salt content on body temperature daily rhythms of common spiny mice from different micro-habitats

We compared body temperature (T_b) daily rhythms in two populations of common spiny mice, Acomys cahirinus, during summer and winter months in relation to increasing dietary salt content. Mice were collected from the North and South facing slopes (NFS and SFS) of the same valley, that are exhibiting mesic and xeric habitats, respectively. During the summer, whilst mice were offered a water source containing 0.9% NaCl, SFS individuals had T_b peak values at 24:00, whereas NFS individuals had peak values at 18:00. When the salinity of the water source was increased, from 0.9 to 2.5% and then 3.5%, the difference between maximal and minimal T_b of both populations increased. In addition, with increased salinity, the T_b daily peak of SFS mice shifted to 18:00. During the winter, the mean daily T_b values of both populations of mice were lower than during the summer. At 0.9% salinity, the NFS mice exhibited a daily T_b variation with a peak at the beginning of the night. However, we did not detect any significant variation in daily T_b in the SFS mice. At 2.5% salinity, the difference between the mean daily T_b of mice from the two slopes increased. In winter we were unable to increase the salinity to 3.5% as the animals began to lose weight rapidly. We suggest that common spiny mice that inhabit these two micro-habitats are forming two discrete populations that respond differently to the environmental pressures prevailing in each habitat, by evolving different physiological capacities.