Data from: Maternal condition and previous reproduction interact to affect offspring sex in a wild mammal

Trivers and Willard proposed that offspring sex ratio should vary with maternal condition when condition, meant as maternal capacity to care, has different fitness consequences for sons and daughters. In polygynous and dimorphic species, mothers in good condition should preferentially produce sons, whereas mothers in poor condition should produce more daughters. Despite its logical appeal, support for this hypothesis has been inconsistent. Sex-ratio variation may be influenced by additional factors, such as environmental conditions and previous reproduction, which are often ignored in empirical studies. We analysed 39 years of data on bighorn sheep (Ovis canadensis) that fit all the assumptions of the Trivers–Willard hypothesis. Production of sons increased with maternal condition only for mothers that weaned a son the previous year. This relationship likely reflects a mother's ability to bear the higher reproductive costs of sons. The interaction between maternal condition and previous weaning success on the probability of producing a son was independent of the positive effect of paternal reproductive success. Maternal and paternal effects accounted for similar proportions of the variance in offspring sex. Maternal reproductive history should be considered in addition to current condition in studies of sex allocation.

特里弗斯（Trivers）与威拉德（Willard）提出，当母体状况（此处定义为母体抚育后代的能力）对雄性和雌性后代的适合度造成不同影响时，子代的性别比例应随母体状况发生变化。在多配制且存在两性异形的物种中，状况良好的母体应优先产下雄性后代，而状况较差的母体则应产下更多雌性后代。尽管该假说具备逻辑合理性，但相关实证支持始终存在分歧。性别比例的变异可能受到环境条件、既往繁殖行为等额外因素的影响，而这些因素在实证研究中常被忽略。我们分析了符合特里弗斯-威拉德假说（Trivers–Willard hypothesis）全部假设条件的39年大角羊（Ovis canadensis）观测数据。仅当前一年成功抚育雄性幼崽至断乳的母体，其产下雄性后代的概率才会随自身状况提升而升高。这一关联大概率反映了母体承担雄性后代更高繁殖成本的能力。母体状况与既往断乳成功率对产下雄性后代概率的交互效应，不受父本繁殖成功所带来的正向影响的干扰。母体与父本效应对子代性别方差的解释占比相近。在性别分配相关研究中，除当前母体状况外，其既往繁殖历史也应被纳入考量范畴。