Why does a grasshopper have fewer, larger offspring at its range limits?

Authors

  • M. HASSALL,

    1. Centre for Ecology, Evolution and Conservation, School of Environmental Sciences, University of East Anglia, Norwich, UK
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  • R. J. WALTERS,

    1. Centre for Ecology, Evolution and Conservation, School of Environmental Sciences, University of East Anglia, Norwich, UK
    2. Present address: Department of Zoology, Stockholm University, 106 91 Stockholm, Sweden
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  • M. TELFER,

    1. Biological Records Centre, CEH, Monks Wood, Abbots Ripton, Huntingdon, Cambridgeshire, UK
    2. Present address: RSPB, The Lodge, Sandy, Bedfordshire SG19 2DL, UK
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  • M. R. J. HASSALL

    1. Centre for Ecology, Evolution and Conservation, School of Environmental Sciences, University of East Anglia, Norwich, UK
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Mark Hassall, Centre for Ecology, Evolution and Conservation, School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK.
Tel.: +44-16-03592541; fax: +44-16-03591327;
e-mail: m.hassall@uea.ac.uk

Abstract

Analysis of size of offspring reared through three laboratory generations from populations of the field grasshopper Chorthippus brunneus from 27 sites around the British Isles showed that offspring were larger towards the cooler-wetter conditions in the western and northern limits of the range. This variation had a significant genetic component. There was a trade-off between clutch size and offspring size between and within populations. Under favourable thermal and feeding conditions maternal fitness was optimal when individuals produced the largest clutches of the smallest eggs, but under poor conditions maternal fitness was optimal when individuals produced small clutches of very large offspring. Calculation of geometric mean fitness over time indicated that having larger offspring near to the edge of the range could be advantageous as a conservative risk-spreading strategy. As well as geographic variation in egg size, significant environment–genotype interactions in egg size in relation to temperature were observed.

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