Sex-specific fitness returns are too weak to select for non-random patterns of sex allocation in a viviparous snake |
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Authors: | Jean-Pierre Baron Thomas Tully Jean-François Le Galliard |
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Institution: | 1. CNRS/UPMC/ENS UMR 7625, écologie et évolution, école Normale Supérieure, 46 rue d’Ulm, 75005, Paris, France 2. IUFM de Paris, Université Paris 4-Sorbonne, 10 rue Molitor, 75016, Paris, France 3. CNRS/ENS UMS 3194, CEREEP, écotron IleDeFrance, école Normale Supérieure, 78 rue du Chateau, 77140, St-Pierre-lès-Nemours, France 4. CNRS/UPMC/ENS UMR 7625, Université Pierre et Marie Curie, 7 Quai St Bernard, 75005, Paris, France
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Abstract: | When environmental conditions exert sex-specific selection on offspring, mothers should benefit from biasing their sex allocation
towards the sex with the highest fitness in a given environment. Yet, studies show mixed support for such adaptive strategies
in vertebrates, which may be due to mechanistic constraints and/or weak selection on facultative sex allocation. In an attempt
to disentangle these alternatives, we quantified sex-specific fitness returns and sex allocation (sex ratio and sex-specific
mass at birth) according to maternal factors (body size, age, birth date, and litter size), habitat, and year in a viviparous
snake with genotypic sex determination. We used data on 106 litters from 19 years of field survey in two nearby habitats occupied
by the meadow viper Vipera ursinii ursinii in south-eastern France. Maternal reproductive investment and habitat quality had no differential effects on the growth and
survival of sons and daughters. Sex ratio at birth was balanced despite a slight female-biased mortality before birth. No
sexual mass dimorphism between offspring was evident. Sex allocation was almost random apart for a trend towards more male-biased
litters as females grew older, which could be explained by an inbreeding avoidance strategy. Thus, a weak selection for facultative
sex allocation seems sufficient to explain the almost equal sex allocation in the meadow viper. |
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