Behavioral Factors in the Altitudinal Zonation of Chipmunks (Eutamias)

Behavioral factors important in the determination of the lines of contact between 3 altitudinally zone chipmunks, Eutamias minimus, Eutamias amoenus, and Eutamias speciosus, were investigated at a field site in the Sierra Nevada, California. These species are contiguously allopatric and restricted to particular vegetational communities. Extensive mark—release—recapture studies and a removal experiment suggested that the E. minimus—E. amoenus border results from a physiological limitation of E. amoenus and competitive exclusion of E. minimus from E. amoenus habitat. In areas where E. amoenus were removed, E. minimus captures increased; the converse did not occur. Behavioral observations coupled to vegetation surveys showed that E. amoenus had a much stronger preference for the vicinity of trees than the other species. Eutamias minimus, because it is physiologically more capable of handling dry heat loads than E. amoenus, showed a weaker orientation to trees. Eutamias speciosus shows even less orientation to trees, even though it is the most vulnerable of the 3 to heat stress. Field observations of interactions revealed that all 3 species are highly aggressive intraspecifically, and that a clear—cut interspecific dominance hierarchy exists: E. speciosus dominates both E. amoenus and E. minimus, and E. amoenus in turn dominates E. minimus. The evolution of aggressiveness is explained in terms of the hibernation strategy of the animals and the concomitant need to storage large quantities of food for the winter. Interspecific aggression is not selected against because of large overlaps in food utilization among the species. It is concluded that physiological and behavioral factors interact to produce the contiguously allopatric distribution of these chipmunks: E. speciosus aggressively excludes other species from its forest habitat, to which it is restricted by physiological considerations and habitat selection; E. amoenus tolerates the more severe conditions in arid woodlands, from which it excludes E. minimus; E. minimus has extensive thermoregulatory adaptations enabling it to inhabit the extremely hot, dry sagebrush, which is intolerable for the more aggressively dominant species.