Parental allocation of resources into male or female offspring and differences in the balance of offspring sexes in natural populations are central research topics in evolutionary ecology. Fisher (Fisher, R. A. 1930. The genetical theory of natural selection, Clarendon Press, Oxford, UK) identified frequency-dependent selection as the mechanism responsible for an equal investment in the sexes of offspring at the end of parental care. Three main theories have been proposed for explaining departures from Fisherian sex ratios in light of variation in environmental (social) and individual (maternal condition) characteristics. The Trivers–Willard model (Trivers, R., and D. Willard. 1973. Natural selection of parental ability to vary the sex ratio of offspring. Science 179:90–92) of male-biased sex allocation by mothers in the best body condition is based on the competitive ability of male offspring for future access to mates and thus superior reproduction. The local resource competition model is based on competitive interactions in matrilines, as occur in many mammal species, where producing sons reduces future intrasexual competition with daughters. A final model invokes advantages of maintaining matrilines for philopatric females, despite any increased competition among females. We used 29 yr of pedigree and demographic data to evaluate these hypotheses in the Colombian ground squirrel (Urocitellus columbianus), a semisocial species characterized by strong female philopatry. Overall, male offspring were heavier than female offspring at birth and at weaning, suggesting a higher production cost. With more local kin present, mothers in the best condition biased their offspring sex ratio in favor of males, and mothers in poor condition biased offspring sex ratio in favor of females. Without co-breeding close kin, the pattern was reversed, with mothers in the best condition producing more daughters, and mothers in poor condition producing more sons. Our results do not provide strong support for any of the single-factor models of allocation to the sexes of offspring, but rather suggest combined influences of relative maternal condition and matriline dominance on offspring sex ratio.

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哥伦比亚地松鼠的父母投资：性别分配模型的实证检验

父母将资源分配给雄性或雌性后代以及自然种群中后代性别平衡的差异是进化生态学的核心研究课题。Fisher（Fisher，RA 1930。自然选择的遗传理论，Clarendon Press，Oxford，UK）将频率依赖性选择确定为在父母照顾结束时对后代性别进行平等投资的机制。鉴于环境（社会）和个体（母体条件）特征的变化，已经提出了三种主要理论来解释与费舍尔性别比例的偏离。Trivers-Willard 模型（Trivers, R., and D. Willard. 1973. 父母能力的自然选择改变后代的性别比例。科学 179：90-92) 母亲在最佳身体条件下的男性偏向性别分配是基于男性后代未来获得配偶的竞争能力，从而获得优越的繁殖能力。本地资源竞争模型基于母系中的竞争性相互作用，正如许多哺乳动物物种中发生的那样，在这种情况下，生产儿子会减少未来与女儿的同性竞争。尽管雌性之间的竞争加剧，但最终的模型调用了维持亲缘雌性母系的优势。我们使用 29 年的谱系和人口统计数据来评估哥伦比亚地松鼠的这些假设（生儿子会减少未来与女儿的性内竞争。尽管雌性之间的竞争加剧，但最终的模型调用了维持亲缘雌性母系的优势。我们使用了 29 年的谱系和人口统计数据来评估哥伦比亚地松鼠的这些假设（生儿子会减少未来与女儿的性内竞争。尽管雌性之间的竞争加剧，但最终的模型调用了维持亲缘雌性母系的优势。我们使用了 29 年的谱系和人口统计数据来评估哥伦比亚地松鼠的这些假设（Urocitellus columbianus），一种半社会物种，其特征是强烈的雌性爱恋。总体而言，雄性后代在出生和断奶时比雌性后代重，表明生产成本更高。随着更多本地亲属在场，处于最佳状态的母亲将后代性别比例偏向男性，而状况较差的母亲则偏向于女性的后代性别比例。如果没有近亲共同繁育，情况就会逆转，条件最好的母亲生更多的女儿，而条件差的母亲生更多的儿子。我们的结果没有为任何分配给后代性别的单因素模型提供强有力的支持，而是表明相对母体状况和母系优势对后代性别比的综合影响。