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Behavioral responses of territorial red squirrels to natural and experimental variation in population density

Abstract

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.

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Acknowledgements

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.

Ethical Standards

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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Correspondence to Ben Dantzer.

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Communicated by A. Schulte-Hostedde

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Fig. S1

Pearson’s correlations between local squirrel density (squirrels/hectare) measured from 25 to 300 m away from the midden of interest and the frequency of territorial intruders, territorial vocalizations, and behavioral response variables (PC1 and PC2). We calculated local density by considering a circle with a radius ranging from 25 to 300 m around the midden of interest and counting the number of squirrels defending a territory within these areas. Values shown on the y-axis are absolute values of Pearson’s correlations. We used these data to determine that 150 m is an appropriate scale at which to measure local population density. At this scale, the Pearson correlation between local population density and three of the response variables (intruders, rattles, and PC1) is near the highest relative to the other scales. While the Pearson correlation between local population density measured at this scale and PC2 is not near the highest relative to the other scales, we are still likely to gain similar inferences measuring density at this scale compared to others because the Pearson correlation remains either weakly or strongly positive at all scales (PDF 187 kb)

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Dantzer, B., Boutin, S., Humphries, M.M. et al. Behavioral responses of territorial red squirrels to natural and experimental variation in population density. Behav Ecol Sociobiol 66, 865–878 (2012). https://doi.org/10.1007/s00265-012-1335-2

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  • DOI: https://doi.org/10.1007/s00265-012-1335-2

Keywords

  • Food supplementation
  • Playbacks
  • Population density
  • Population regulation
  • Territoriality