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Conception date affects litter type and foetal sex ratio in female moose in Estonia

Authors

  • Rauno Veeroja,

    Corresponding author
    1. Department of Zoology, Institute of Ecology and Earth Science, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia
    2. Centre of Forest Protection and Silviculture, Rõõmu tee 2, 51013 Tartu, Estonia
      E-mail: rauno.veeroja@mail.ee
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  • Anne Kirk,

    1. Department of Zoology, Institute of Ecology and Earth Science, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia
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  • Vallo Tilgar,

    1. Department of Zoology, Institute of Ecology and Earth Science, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia
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  • Sander Säde,

    1. Department of Zoology, Institute of Ecology and Earth Science, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia
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  • Merit Kreitsberg,

    1. Department of Zoology, Institute of Ecology and Earth Science, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia
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  • Jüri Tõnisson

    1. Centre of Forest Protection and Silviculture, Rõõmu tee 2, 51013 Tartu, Estonia
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E-mail: rauno.veeroja@mail.ee

Summary

1. The Trivers–Willard model of optimal sex ratios predicts that in polygynous species mothers in better condition should produce more male than female offspring. However, empirical support for this hypothesis in mammals and especially ungulates has been equivocal. This may be because the fitness of mothers has been defined in different ways, reflecting morphological, physiological or behavioural measures of condition. In addition, factors other than maternal condition can influence a mother’s fitness. Given that recent studies of wild ungulates have demonstrated the importance of the timing of conception and birth on offspring fitness, litters conceived at different stages of the rut might be expected to exhibit differences in types and embryonic sex ratio.

2. Based on a 6-year survey of the reproductive tracts of female moose harvested in Estonia, we investigated the effect of conception date on the types of litters produced and on the foetal sex ratio.

3. There was a clear relationship between conception date and litter characteristics. Overall, earlier conceived litters were more likely than those conceived late to contain multiple embryos and a high proportion of males. However, while foetal sex ratio varied nonlinearly with conception date in yearlings and subadults, no relationship was found in adults.

4. We conclude that female moose adjust foetal sex ratio and litter type/size depending on their age and the date of conception, and that these adjustments are in accordance with the Trivers–Willard hypothesis if females that conceive earlier are in better condition.

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