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Timing of foetal growth spurts can explain sex ratio variation in polygynous mammals


  • M.C. Forchhammer

    1. Population Ecology Group, Department of Landscape Ecology, National Environmental Research Institute, Grenåvej 14, Kalø, DK-8410 Rønde, Denmark, and, Large Animal Research Group, Department of Zoology, University of Cambridge, Downing Street, Cambridge CB2 3EJ, UK.
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Mads C. Forchhammer E-mail: mcf@dmu.dkE-mail:


The prediction from sex ratio theory that natural selection on sexually dimorphic mammals should favour an excess of male offspring only when mothers are in good condition, has been tested extensively but with little consistency in results. Although recent studies have shown that environmental variations may cause some of the discrepancy, there have also been reports of contrasting sex ratios under similar environmental settings. Here it is suggested that variation in timing of environmental stress and sex-specific differences in foetal growth pattern in relation to maternal condition, may explain such seeming contradictions in sex ratio variation of polygynous mammals.