Sex-specific fitness returns are too weak to select for non-random patterns of sex allocation in a viviparous snake.

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.