Natal dispersal in Belding's ground squirrels (Spermophilus beldingi)

Abstract

Dispersal from the natal site was documented in two populations of Belding's ground squirrels (Spermophilus beldingi) living at different altitudes in the Sierra Nevada of North America. Distance dispersed and age at dispersal were monitored by a combination of observation, trapping, radio telemetry, and examination of road kills. Dispersal was sexually dimorphic in both populations (Tables 1 and 2). All surviving males emigrated before they were 55 weeks of age, with most dispersing midway through the juvenile summer (Fig. 1). By contrast, most females remained within the boundaries of their mothers' home range (Fig. 2 and 3). Those very few females that did emigrate moved distances from their natal burrows similar to those travelled by dispersing males (200–450 m; Table 2), but females tended to disperse at a slightly older age. Significant differences between the study populations were found in distances moved by juveniles of both sexes (Fig. 2). Body weights of juvenile male dispersers were significantly greater than were those of juvenile males of equivilent ages that had not yet dispersed (Fig. 4). The results were considered in light of Shields' (1982) discussion of dispersal and inbreeding. I concluded that if dispersal is related to reproductive success, then dispersal distances selected for use in tests of evolutionary hypotheses should be measured just prior to the subject animals' first reproduction. Finally, although not tested in the present study, the evolutionary hypothesis most consistent with my data suggests that dispersal in S. beldingi may function to minimize nuclear family incest.