Bioenergetics and Distribution of Fossorial Spalacopus cyanus (Rodentia): Thermal Stress, or Cost of Burrowing

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Abstract
A study was made of the energetics of Spalacopus cyanus from high-altitude and cold burrows in the Andean Mountains and from low-altitude and warmer burrows near the Pacific Ocean in central Chile. Individuals from both populations are good thermoregulators at ambient temperatures between 2 and 30 C. They maintain a constant of 36.6 ± 0.2 (2 SE). The mass-specific BMR of S. cyanus from both populations is lower than expected from the Kleiber relationship, and it is significantly lower in individuals from high altitudes (the cooler sites) than in those from low altitudes (74% and 85%, respectively). Minimal thermal conductance (C) was 87% and 84% of that expected on the basis of for high-and lowland animals, respectively. Animals from high altitudes were larger than those from low altitudes; sexual size dimorphism occurred only at low altitudes. The combination of energetic parameters and determined a similar temperature differential between body and the lower limit of thermoneutrality (10.0 and 10.7 C for high and low altitudes, respectively). The energetic parameters, body mass, and distribution are discussed in relation to thermal stress by McNab, as is the cost-of-burrowing hypothesis by Vleck, for this and other fossorial species. It seems that the former hypothesis is more general, explaining a larger number of characteristics such as BMR, C, , and their interactions, as well as the distribution of these animals. However, the cost-of-burrowing hypothesis, as well as other factors, should be taken into account for a better understanding of patterns in fossorial mammals.