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Population regulation and annual cycles of activity and dispersal in Arctic ground squirrels Green, Jeffrey Emil


Dispersal has been implicated as one of the potentially important factors of population regulation. Interactive behaviours, particularly aggression, have been suggested as the ultimate cause of dispersal. To determine if social behaviour is related to dispersal and to population regulation, I conducted a study of the population changes and behaviour of two populations of Arctic ground squirrels, Spermophilus undulatus, in Kluane National Park, Yukon Territory, Canada. Within each study site, I established a control area and one experimental area in which resident animals were removed at regular intervals. On one site, I conducted two additional experiments. One involved the provision of an additional food supply and the other, a continuous removal of all adult males. The annual activity pattern of Arctic ground squirrels in the Kluane region was characterized by a 7-8 month period of hibernation, emergence, establishment of territories, a short breeding period, development and emergence of young, restoration of fat deposits, establishment of fall territories and entry into hibernation. Two peaks of aggressive behaviour, as evidenced by interaction rates and wounding occurred, both coincident with the establishment of territories. The number of resident breeding adults changed little during the two and a half years of this study. Some fluctuations in the total population occurred as a result of mature animal and juvenile dispersal and recruitment of young. Four possible sources of in situ loss were investigated; predation, disease. starvation and dispersal. Loss due to disease or starvation was negligible. Interspecific predation accounted for an estimated 10-15% of the total annual loss. The three measures of dispersal; immigration to removal and control areas and emigration to control areas showed similar seasonal peaks. The correlation between the three measures indicates that dispersal is the major cause of in situ loss. Mature females dispersed roost in flay and June. Dispersal of mature females was related to reproductive success. Juvenile male and female dispersal was highest in August. Juvenile male dispersal was related to body size. Larger juvenile males tended to disperse first. A hypothesis is proposed suggesting that body size is related to the initiation of adult : juvenile aggression and the subsequent dispersal of juveniles. Behavioural comparisons of dispersing and resident animals indicated few differences. Overall, dispersing animals tended to initiate fewer and receive more agonistic behaviours than resident animals. Animals of one class received and initiated most agonistic behaviours with animals of the same age and/or sex. This suggests that aggression among animals of the same class may be an important cause of dispersal, particularly in juveniles. Further, related animals tended to show higher rates of amicable behaviour and lower levels of physical aggression than unrelated animals. A graphical model of population regulation in Arctic ground squirrels based on behavioural and demographic relationships observed in this study, is proposed.

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