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Conspecific presence and microhabitat features influence foraging decisions across ontogeny in a facultatively social mammal

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Abstract

Natural selection is expected to favor the evolution of threat-sensitive behaviors that permit individuals to adaptively detect and respond to danger. However, because time allocated to vigilance reduces the amount of time that is available for energy acquisition, foraging in the face of predation can impose an evolutionary trade-off. Optimal foraging theory therefore predicts that risk-taking decisions should vary in response to perceived levels of threat. Our goal here is to disentangle the relative contributions of conspecific presence, ecological factors, and individual traits on two measures of vigilance in free-living California ground squirrels (Otospermophilus beecheyi). This facultatively social and ecologically flexible rodent represents a major source of prey in California grasslands. Over a 5-year period, we collected 386 focal animal surveys on 156 free-living individuals residing at two colony sites. Individuals were most vigilant in conditions for which predation risk was highest, such as when foraging alone and on flat areas with low vegetative cover. In general, juvenile foragers were more gregarious but less vigilant than adult foragers. Although all animals—regardless of age or sex—generally decreased their intensity of vigilance as group size increased, only adults decreased their time allocated to vigilance in response to conspecific presence. Thus, grouping consistently buffered the costs of foraging for risk-averse adults, but the benefits of conspecific presence were less salient for juveniles. Taken together, our findings highlight the importance of context in shaping foraging decisions and offer insights into the suite of factors mediating decision-making in socially and ecologically variable environments.

Significance statement

Animals face trade-offs when foraging in the face of predation, but the capacity for individuals to respond appropriately to subtle changes in context-dependent threats remains poorly understood. We studied antipredator vigilance by the California ground squirrel, a flexible species that may forage alone or in small groups of various sizes and in habitats that vary in their protective value from predators and the ease at which prey may detect predators. Although adults are more sensitive to changes in group size, all animals detected context-dependent danger to some extent. Our findings offer insights into the multiple factors that prey respond to when making foraging decisions, suggesting that animals possess the ability to integrate disparate sources of information about danger to optimize energy gain.

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Acknowledgments

We thank Joseph Miller, Doug Bell, and the rest of the staff at the East Bay Regional Parks for allowing us to conduct research. We are also grateful to the numerous members of Team Squirrel for contributing to data collection for this study. Finally, we thank Ted Stankowich, Guy Beauchamp, and two anonymous reviewers for their helpful comments on earlier versions of this manuscript.

Funding

This work was supported by the Jill Barrett Research Program at Mills College (to CAO, ELP, KLN, JES). A Graduate Research Fellowship to CAO and a Research Opportunity Award (DEB grant 1456730) from the National Science Foundation (NSF) also supported this work. Finally, grants from the Animal Behavior Society (Diversity Fund Award), the Contra Costa Fish and Wildlife Committee (Propagation Fund), the W.M. Keck Foundation (Undergraduate Education Program), and Mills College (Sandra Greer Grant for Professional Development) supported this research.

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  1. Chelsea A. Ortiz

    Present address: Department of Environmental Science and Policy, University of California Davis, One Shields Avenue, Davis, CA, 95616, USA

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  1. Biology Department, Mills College, 5000 MacArthur Blvd, Oakland, CA, 94631, USA

    Chelsea A. Ortiz, Erika L. Pendleton, Kate Lee Newcomb & Jennifer E. Smith

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All methods were approved by the Animal Care and Use Committee at Mills College and consistent with guidelines of the American Society of Mammologists for the use of wild mammals in research (Sikes and Gannon 2016). We conducted this work with permission from the California Department of Fish and Wildlife and East Bay Parks.

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Ortiz, C.A., Pendleton, E.L., Newcomb, K.L. et al. Conspecific presence and microhabitat features influence foraging decisions across ontogeny in a facultatively social mammal. Behav Ecol Sociobiol 73, 42 (2019). https://doi.org/10.1007/s00265-019-2651-6

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