Male-biased sex ratios in broods of the cooperatively breeding bell miner Manorina melanophrys |
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| Authors: | Michael F. Clarke David A. Jones John G. Ewen Raleigh J. Robertson Richard Griffiths Jodie Painter Peter T. Boag Ross Crozier |
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| Affiliation: | 1 Department of Zoology, La Trobe University, Bundoora, 3086 Victoria, Australia,;
2 Biology Department, Queen's University, Kingston, Ontario, Canada K7L 3N6;
3 Molecular Laboratory, DEEB, Graham Kerr Building, Glasgow University, Glasgow G12 8QQ, UK;
4 Department of Genetics and Human Variation, La Trobe University, Bundoora, 3086 Victoria, Australia. |
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| Abstract: | We examined the sex ratios of adults and nestlings in the cooperatively breeding bell miner Manorina melanophrys . Males were over-represented among helpers (mean of 6.8 male helpers per nest compared to 0.3 female helpers). 58% of nestlings sampled were identified as male using a molecular genetic marker. This was a significant departure from parity, yet the magnitude of the bias varied between years. The beneficial and male-biased nature of helping behaviour in this species and the similar size of male and female nestlings suggest the net cost of raising males is lower than the cost of raising females. Consequently, the male-biased sex ratio of nestlings we observed is consistent with the predictions of the repayment hypothesis that females may bias the production of their young towards the more helpful sex. Difficulties of generating quantitative predictions from repayment models that can be tested in the field are discussed. |
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