A test of the thermal-stress and the cost-of-burrowing hypotheses among populations of the subterranean rodent Spalacopus cyanus

Subterranean mammals show lower than-allometrically expected-basal metabolic rates (BMR), and several competing hypotheses were suggested to explain how physical microenvironmental conditions and underground life affect subterranean mammalian energetics. Two of these are the thermal-stress and the cost-of-burrowing hypotheses. The thermal-stress hypothesis posits that a lower mass-independent BMR reduces overheating in burrows where convective and evaporative heat loss is low, whereas the cost-of-burrowing hypothesis states that a lower mass-independent BMR may compensate for the extremely high energy expenditure of digging during foraging activity. In this paper we tested both hypotheses at an intraspecific level. We compared seven populations of the subterranean rodent Spalacopus cyanus or cururo from different geographic localities with contrasting habitat conditions. We measured BMR and digging metabolic rate (DMR) through open flow respirometry. Our results support neither the thermal-stress nor the cost-of-burrowing hypothesis. Cururos from habitats with contrasting climatic and soil conditions exhibited similar BMR and DMR when measured under similar semi-natural conditions. It is possible that S. cyanus originated in Andean locations where it adapted to relatively hard soils. Later, when populations moved into coastal areas characterized by softer soils, they may have retained the original adaptation without further phenotypic changes.