Behavioral responses of territorial red squirrels to natural and experimental variation in population density
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The relative scarcity of studies at the intersection of behavioral and population ecology is surprising given the presumed importance of behavior in density-dependent population regulation. Here we tested whether North American red squirrels (Tamiasciurus hudsonicus) adjust their behavior in response to local population density and whether they use rates of territorial vocalizations in their local neighborhood to assess density. We examined these relationships using 18 years of live trapping and 20 years of behavioral data that were collected across natural variation in local population density. To disentangle the effects of population density on behavior from those due to changes in per capita food abundance or changes in the frequency of antagonistic interactions with neighbors, we also experimentally manipulated population density with long-term food supplementation as well as perceived population density with long-term playbacks of territorial vocalizations. The frequency with which squirrels emitted territorial vocalizations was positively associated with local population density. In contrast, antagonistic physical interactions observed between squirrels and territorial intrusions were rare and the frequency of intrusions was weakly and negatively, not positively, associated with population density. Squirrels experiencing naturally and experimentally high density conditions spent less time in the nest and feeding but more time being vigilant. Similar density-dependent changes in behavior were observed in response to our manipulations of perceived population density, indicating that vocalization rates and not physical interactions or food abundance were the mechanism by which squirrels assessed and responded behaviorally to changes in local density.
KeywordsFood supplementation Playbacks Population density Population regulation Territoriality
We thank Adam Goble for supplying the squirrel vocalization recordings and Rudy Boonstra for providing comments on a previous version of this manuscript. This research was funded by student research grants to BD from the Animal Behavior Society, American Society of Mammalogists, and the Arctic Institute of North America, as well as funding from the Natural Sciences and Engineering Research Council of Canada (S. Boutin, M. M. Humphries, A. G. McAdam) and National Science Foundation (A. G. McAdam). This is publication number 60 of the Kluane Red Squirrel Project.
All research were approved by the University of Guelph (#09R006) and Michigan State University (# 04/08-046-00) Animal Care and Use Committees and complied with the ethical standards of the United States and Canada.
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