Comparative thermoregulatory adaptations of field mice of the genus Apodemus to habitat challenges.

Thermoregulatory abilities, which may play a role in physiological adaptations, were compared between two field mouse species (Apodemus mystacinus and A. hermonensis) from Mount Hermon. While A. hermonensis is common at altitudes above 2100 m, A. mystacinus is common at 1650 m. The following variables were compared in mice acclimated to an ambient temperature of 24 degrees C with a photoperiod of 12L:12D, body temperature during exposure to 4 degrees C for 6 h, O2 consumption and body temperature at various ambient temperature, non-shivering thermogenesis measured as a response to a noradrenaline injection, and the daily rhythm of body temperature. Both species could regulate their body temperature at ambient temperatures between 6 and 34 degrees C. The thermoneutral zone for A. mystacinus lies between 28 and 32 degrees C, while for A. hermonensis a thermoneutral point is noted at 28 degrees C. Both species increased O2 consumption and body temperature as a response to noradrenalin. However, maximal VO2 consumption as an response to noradrenaline and non-shivering thermogenesis capacity were higher in A. mystacinus, even though A. hermonensis is half the size of A. mystacinus. The body temperature rhythm in A. hermonensis has a clear daily pattern, while A. mystacinus can be considered arhythmic. The results suggest that A. hermonensis is adapted to its environment by an increase in resting metabolic rate but also depends on behavioural thermoregulation. A. mystacinus depends more on an increased non-shivering thermogenesis capacity.