Behavior and dispersal of yearling Columbian ground squirrels

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Abstract
Behavior of yearling Columbian ground squirrels (Spermophilus columbianus) was studied through intensive observations of an isolated, high-density population in southwestern Alberta from 1979 to 1981. All members of this population were individually marked. A greater proportion of yearlings disappeared during the active season than that of any other age-class. This loss appeared to be due to emigration, and was greater for males than for females. Some 2-year-old males may also have dispersed. Male yearlings played more than females, but both sexes displayed a similar frequency of aggressive interactions with other yearlings. As the season progressed, the frequency of play behaviors between yearlings decreased, and by the time of presumed emigration almost no play was seen. There was no clear relationship between a yearling's dominance status and whether it emigrated. Adults of both sexes behaved aggressively towards yearlings, particularly yearling males. Adult females were most aggressive just preceding the time of presumed emigration, and at this time yearlings changed their dispersion pattern within the meadow, apparently in an attempt to avoid female aggression. Postponement of dispersal to the yearling age appears to be an adaptation to a short active season. Adult squirrels may benefit from expelling yearlings by diminishing future competition, but the adaptive advantage of emigration for the yearlings themselves remains unclear.