Maternal age and offspring sex ratio variation in reindeer (Rangifer tarandus)

Different evolutionary hypotheses have been proposed to explain variation in offspring sex ratios among mammalian populations. The Trivers and Willard and local resource competition hypotheses, which are based on adaptive modification by maternal investment, are two opposing hypotheses commonly used for ungulates, but empirical patterns often do not fit either hypotheses' expectations. We investigated sex ratio variation in 1658 reindeer (Rangifer tarandus) calves in relation to their mothers' mass and age, while accounting for potential density-independent factors such as climate. The most parsimonious model included only the effect of maternal age, the proportion of males increasing with increasing maternal age. Similar models in terms of parsimony included (in addition to maternal age) the winter temperature or the North Atlantic Oscillation (NAO) index when the mothers were in utero, indicating that climatic conditions experienced by mothers in their year of birth, have the potential to influence variation in a cohort's offspring sex ratio. Thus, a combination of both density-dependent and density-independent factors may influence sex ratio variation in reindeer and perhaps in other ungulates. We suggest that, if an adaptive maternal strategy is present, it may be induced and/or inhibited by, or interact with other factors such as climate.