Altitudinal Zonation of Chipmunks (Eutamias): Energy Budgets

Eutamias alpinus, E. speciosus, E. amoenus, and E. minimus are contiguously allopatric and altitudinally zoned into habitats with different energy relationships. The total energy budgets of the species were constructed and compared to see if physiological adaptations could partially explain the observed habitat segregation. Radiating surface area and coefficients of convection were measured through the use of internally heated metal casts of the animals which were covered with pelts. Metabolic rates were measured as O_2 consumption. The climate space of an animal is described by all combinations of absorbed radiation, ambient temperature, and wind velocity which a species can tolerate and remain in neutral energy balance. The climate space for each species is constructed by putting realistic mutually compatible limiting values for body temperature, metabolic rate, evaporative water loss, and coefficient of convection into the energy budget equation. The equation is solved for the combinations of radiation absorbed, ambient temperature, and wind velocity which balance the equation. The upper limits of the climate space of E. speciosus are lower than the upper limits of the other three species' climate spaces. The sagebrush habitat is frequently outside of the climate spaces of all four species. E. minimus makes use of hyperthermia, retreats to its burrow, and probably uses transient excursions beyond is average steady—state limits so that its fundamental niche includes the sagebrush desert habitat whereas the fundamental niches of the other three species do not. The accuracy and predictive potential of the climate spaces are revealed by comparisons with field observations.