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The effects of castration, sex ratio and population density on social segregation and habitat use in Soay sheep

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Abstract

We analysed 16 years of census data gathered on the island of Hirta (archipelago of St. Kilda) to investigate the effects of castration, population density, sex ratio, season and group type on habitat use and social segregation of Soay sheep. From 1978 to 1980, 72 male lambs were castrated. We used this experiment to study how a change in reproductive status could affect sociality and habitat choice of these males. Males, females and castrates were all segregated outside the rutting season in autumn. Castrates were the least segregated from females in spring and summer but were most segregated from them during the pre-rut. The more equal the sex ratios, the higher was the degree of social segregation. The three sex classes used similar habitat types, namely, Holcus agrostis, Agrostis festuca and Calluna habitats. Holcus agrostis and Agrostis festuca were top- and second-ranked in female and castrate habitat use, while Holcus agrostis and Calluna were the two top habitat types used by rams. It is unclear why males included Calluna heath habitats, but it cannot be excluded that they might have shifted their use depending on forage availability. A lack in differences in habitat use between castrates and females suggests that body size differences alone cannot be the driving factor for habitat segregation in male and female Soay sheep and that there are reasons other than body size that could motivate reproductive males to use additional habitat types, such as Calluna heath. Although habitat use shifted from one habitat type to the next between low- and high-population-density years and between seasons, there was no clear link between population density and how different groups (male, female or castrate) used these areas. We discuss effects of reproductive status, population density and sex ratio on social segregation and habitat use and suggest that these factors need to be taken into account when investigating causes of sexual segregation in ungulates.

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Acknowledgements

We thank all the people, including volunteers, who helped tag sheep and collect census data on Hirta, especially Tony Robertson and Ian Stevenson. We thank the National Trust for Scotland and Scottish Natural Heritage for permission to work on St. Kilda, and the Royal Artillery Range (Hebrides), the Royal Corps of Transport, DERA and SERCo for logistical assistance. Luca Borger kindly digitalised the map of St. Kilda and calculated habitat availability. We thank Tim Coulson for help with the compilation of Soay sheep data, Tim Coulson, Giacomo Tavecchia, Ian Stevenson, Kavita Isvaran and Andy Russell for statistical advice and Josephine Pemberton, Peter Neuhaus, Johan duToit, Anne Carlson, Ian Stevenson and anonymous referees for comments on an earlier draft of this manuscript. This paper was written while K.E. Ruckstuhl was a post-doctoral fellow at Cambridge University, paid through a PDF of Natural Sciences and Engineering Research Council of Canada and a 2000 Canada–UK Millennium Research Award. An NSERC discovery grant to K.E.R. currently held at the University of Calgary further supported the writing of this manuscript. The UK Natural Environment Research Council mainly funded collection of the Soay sheep data reported on here. The castration experiments had been approved and complied with the laws of Scotland, where they were performed.

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Authors and Affiliations

  1. Department of Zoology, University of Cambridge, Cambridge, UK

    K. E. Ruckstuhl, A. Manica & T. H. Clutton-Brock

  2. Department of Zoology, University of British Columbia, Vancouver, Canada

    A. D. C. MacColl

  3. Institute of Evolutionary Biology, University of Edinburgh, Edinburgh, UK

    J. G. Pilkington

  4. Department of Biological Sciences, University of Calgary, 2500 University Drive, T2N 1N4, N. W., Alberta, Canada

    K. E. Ruckstuhl

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  1. K. E. Ruckstuhl
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  2. A. Manica
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  3. A. D. C. MacColl
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  4. J. G. Pilkington
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Correspondence to K. E. Ruckstuhl.

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Communicated by C. Nunn

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Ruckstuhl, K.E., Manica, A., MacColl, A.D.C. et al. The effects of castration, sex ratio and population density on social segregation and habitat use in Soay sheep. Behav Ecol Sociobiol 59, 694–703 (2006). https://doi.org/10.1007/s00265-005-0099-3

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