Evol Ecol Res 9: 705-716 (2007)     Full PDF if your library subscribes.

Foraging ecology of North American tree squirrels on cacheable and less cacheable foods: a comparison of two urban habitats

Marius van der Merwe,* Anna M. Burke and Joel S. Brown

Department of Biological Sciences, University of Illinois, 845 W. Taylor Street, Chicago, IL 60607, USA

Author to whom all correspondence should be addressed.
e-mail: duisendpoot@yahoo.com


Hypotheses: Animals often thrive in a variety of habitats where they face very different challenges and opportunities. We hypothesized: (1) As a consequence of higher squirrel density, the squirrels in an urban residential habitat (Oak Park, Illinois) experience greater food limitation than tree squirrels occupying a semi-natural habitat (Morton Arboretum in Lisle, Illinois). (2) Due to higher densities of natural predators, squirrels experience a higher foraging cost of predation at the Morton Arboretum than squirrels inhabiting Oak Park. Furthermore, (3) we expected the residential squirrels to place a higher premium on cacheable foods as a consequence of few oak and nut-producing trees in Oak Park compared with the much higher availability of acorns and nuts at the Arboretum.

Methods and predictions: By measuring giving-up densities in depletable food patches, we anticipated that at the Arboretum: (1) overall giving-up densities should be higher, (2) the difference in giving-up densities between risky and safe microhabitats should be higher, and (3) the difference in giving-up densities between cacheable food (hazelnuts in their shell) and less cacheable food (hazelnuts with shells removed) should be lower.

Results: Interestingly, the first prediction depended on the interaction between site and season. In Oak Park, giving-up densities were highest in summer and lowest in winter, the exact opposite of the Arboretum. Our second hypothesis was refuted, and the third supported.

Conclusions: A lack of cacheable food and an abundance of less cacheable human-derived summer food likely result in easy summers but hard winters for Oak Park squirrels. An abundance of cacheable foods and a summer lull in natural foods produces the opposite situation for the Arboretum squirrels. Less lethal, but more persistent, harassment by pets and people probably explain the higher cost of predation in Oak Park.

Keywords: food caching, foraging ecology, predation risk, Sciurus, urban ecology.

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