Pronghorn male spatial organization: population differences in degree of nonterritoriality

Abstract

Pronghorn (Antilocapra americana) vary in spatial organization. A literature review revealed that males occupied undefended home ranges in 12 populations and maintained territories in 11 populations. Low-productivity habitats and high or low population density could preclude territoriality. Using activity budgets, interaction rates, and home-range overlap, male social organization was described for a translocated pronghorn population in central California and compared with that of another population studied at Sheldon National Wildlife Refuge, Nevada, to determine if males were organized differently. Interaction rates were highest during spring, decreased in summer, then increased in late summer coincident with the rut. Home-range overlap ranged from 0 to 85% and averaged 30.6%; small amounts of overlap resulted from geographical features rather than from behavioral interactions. Males joined groups of females and fawns during summer; few behavior patterns associated with territoriality were observed. Males occupied undefended home ranges and this spacing system may have been influenced by food abundance and distribution, population density, or a combination of these factors. Comparisons between Sheldon and Carrizo males indicated that, while neither population was territorial, the behavior of Sheldon males was closer to the territoriality end of a continuum between territoriality and undefended home range than was that of Carrizo males. Behavior patterns were very similar between the populations but some occurred more frequently among Sheldon males. Both populations lived in semi-arid habitats, where low primary productivity would make the costs of maintaining territories greater than the benefits. Differences between the populations could be based on differences in population density. The Carrizo population was small and widely dispersed and male–male competition appeared low; therefore, the opportunity to interact with other males was lower than in areas of higher population density. Comparisons with other populations were difficult to make because of a lack of quantified data on behavioral and ecological variables. Such data are required if more is to be learned about the effects of ecology and demography on social organization.