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Cooperatively breeding superb fairy-wrens show no facultative manipulation of offspring sex ratio despite plausible benefits

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Abstract

We report a long-term study of offspring sex ratios in the cooperatively breeding superb fairy-wren Malurus cyaneus. Detailed study of this species had revealed a suite of potentially strong selection pressures on the sex ratio. First, females gain substantial fitness benefits from the presence of helpers; so females without male helpers would benefit from any strategy that increased the probability of recruiting help, such as overproduction of sons (local resource enhancement hypothesis), but large numbers of helper males compete among themselves, favouring the production of daughters (local resource competition). Second, daughters fledged early in the season have far greater chances of recruitment to the breeding population than late-fledged daughters, so mothers would benefit from production of daughters early in the breeding season (early bird hypothesis). Third, extra-group mate choice imposes strong sexual selection on males, suggesting that females mating with attractive sires could benefit from investing in sons (sexual selection hypothesis). However, the predictions from these and other sex ratio hypotheses were rejected. The only convincing evidence for manipulation of the sex ratio was a slight bias towards sons (11 sons to 10 daughters) that occurred regardless of context. This result does not support current theory.

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Acknowledgments

Our research on fairy-wrens has been generously supported by the Australian Research Council. We are grateful for permits from the Australian National Botanic Gardens and the ANU Animal Experimentation Ethics Committee. Our ability to measure sex ratios was facilitated by the early efforts of Richard Griffiths, Angela Higgins and Raoul Mulder. Helen Osmond and numerous field assistants provided wonderful field support.

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Authors and Affiliations

  1. Evolutionary Ecology Group, School of Botany and Zoology, Australian National University, Canberra, ACT, 0200, Australia

    Andrew Cockburn & Michael C. Double

  2. Percy Fitzpatrick Institute of African Ornithology DST/NRF Centre of Excellence, University of Cape Town, Rondebosch, 7701, South Africa

    Andrew Cockburn

  3. Australian Antarctic Division, 203 Channel Highway, Kingston, Tasmania, 7050, Australia

    Michael C. Double

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  1. Andrew Cockburn
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  2. Michael C. Double
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Correspondence to Andrew Cockburn.

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Communicated by J. Lindström

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Cockburn, A., Double, M.C. Cooperatively breeding superb fairy-wrens show no facultative manipulation of offspring sex ratio despite plausible benefits. Behav Ecol Sociobiol 62, 681–688 (2008). https://doi.org/10.1007/s00265-007-0492-1

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  • DOI: https://doi.org/10.1007/s00265-007-0492-1

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