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.特里弗斯与威拉德提出，当以母体抚育能力定义的母体状态对雄性、雌性后代的进化适合度存在差异化影响时，子代性比应随母体状态发生变化。在一雄多雌制且存在性二态的物种中，状态优良的母体应优先产下雄性后代，而状态较差的母体则更倾向于产下雌性后代。尽管该假说逻辑上颇具说服力，但相关实证研究的支持结论始终缺乏一致性。子代性比变异可能受到环境条件、既往繁殖经历等额外因素的影响，而此类因素在过往实证研究中常被忽略。我们分析了时长39年的大角羊（Ovis canadensis）种群数据，该数据集完全符合特里弗斯-威拉德假说（Trivers-Willard hypothesis）的全部假设前提。仅当前一年成功将雄性幼崽抚养至断奶的母体，其产雄性后代的比例才会随母体状态提升而升高。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。母体状态与既往断奶抚育成功率对产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。母体与父本效应对后代性比方差的解释比例相近。在性分配相关研究中，除考量母体当前状态外，其繁殖历史也应被纳入分析范畴。