Adaptive physiological variation in nonshivering thermogenesis and its significance in speciation

Nonshivering thermogenesis (NST) magnitude was studied in the four chromosomal species of subterranean mole rats of the Spalax ehrenbergi superspecies in Israel. The four species show a distribution pattern which correlates with increasing aridity. The 2 n=52 species inhabits the cold and humid regions, 2 n=54 cold and dry, 2 n-58 warm and humid, and 2 n=60 the more arid regions in which temperatures fluctuate daily and annually. NST was measured as the ratio between maximal oxygen {Mathematical expression} due to noradrenaline injection and minimal {Mathematical expression} measured in anaesthetized animals. The chromosomal species 2 n=60 from semiarid and arid habitats has the highest NST value. This fact emerges from the low Basal Metabolic Rate (BMR) of this species relative to all the other three species. A linear correlation was found between NST magnitude and the average daily range of temperature during June and September. We conclude that speciation of the S. ehrenbergi complex in Israel has thermoregulatory correlates such as heat production by NST, among others. The level of NST appears to be an adaptive physiological characteristic in the ecological speciation of subterranean mole rats.