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Previous studies suggest three hypotheses to explain the importance of dispersal in small mammal populations: (1) regulation of density, (2) promotion of outcrossing, and (3) avoidance of competition for mates. These hypotheses were examined in an experimental study of the California ground squirrel (Spermophilus beecheyi). One ground squirrel colony was supplied with supplemental food for 3 mo, while two other colonies were monitored for reference. More adult ground squirrels moved to the supplemented colony than to the reference colonies. All young males disappeared from their natal colonies, even on two areas with different adult sex ratios (.09 and .56 male per female). Most young females remained in their natal colonies through the last month of the study. More young male immigrants were trapped at a reference colony than at the supplemented colony. More young females moved to the supplented colony than to the reference colonies; both via exploratory visits from nearby colonies and by immigration. All young resident males of the colony nearest to the supplented colony disappeared from both colony areas. The supplemented colony received visits from several young females who resided at the nearest adjacent colony. Eleven of these females took up a regular "commute" between their natal colony and the supplemental food. These results support the importance of the regulation of density relative to critical resources via young female dispersal, and the promotion of outcrossing via young male dispersal.
F. Stephen Dobson (Sat,) studied this question.