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Density-dependent investment in costly anti-predator defences: an explanation for the weak survival benefit of group living

Authors

  • Derek Daly,

    1. Department of Evolution, Faculty of Health and Life Sciences, Institute of Integrative Biology, Ecology and Behaviour, Biosciences Building, University of Liverpool, Crown Street, Liverpool L69 7ZB, UK
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    • These authors contributed equally to this work.

  • A. D. Higginson,

    Corresponding author
    1. School of Biological Sciences, University of Bristol, Bristol BS8 1UG, UK
      E-mail: a.d.higginson@bris.ac.uk
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    • These authors contributed equally to this work.

  • Dong Chen,

    1. Department of Evolution, Faculty of Health and Life Sciences, Institute of Integrative Biology, Ecology and Behaviour, Biosciences Building, University of Liverpool, Crown Street, Liverpool L69 7ZB, UK
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  • G. D. Ruxton,

    1. Institute of Biodiversity, Animal Health & Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow G12 8QQ, UK
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  • M. P. Speed

    1. Department of Evolution, Faculty of Health and Life Sciences, Institute of Integrative Biology, Ecology and Behaviour, Biosciences Building, University of Liverpool, Crown Street, Liverpool L69 7ZB, UK
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E-mail: a.d.higginson@bris.ac.uk

Abstract

Ecology Letters (2012)

Abstract

A central explanation for group living across animal taxa is the reduced rate of attack by predators. However, many field observations show a weak or non-existent effect of group size on per capita mortality rates. Herein we resolve this apparent paradox. We found that Pieris brassicae larvae defended themselves less readily when in groups than when alone, in that they were more reluctant to regurgitate in response to simulated attacks and produced less regurgitant. Furthermore, a simple model demonstrates that this reluctance was sufficient to cancel out the benefit from being in a group. This conditional strategy can be understood in terms of the costs and benefits of defences. For grouped individuals, defence is less often required because attack rates are lower and the costs of defence may be higher due to competition for resources. These phenomena are likely to be widespread in facultatively gregarious species that utilise anti-predator defences.

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