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Abstract：
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.
摘要：
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中心研究课题。费希尔(费希尔，R.A.1930年。自然选择的遗传理论，克拉伦登出版社，牛津,英国）将频率依赖性选择确定为在父母照料结束时对后代性别进行平等投资的机制。根据环境（社会）和个人（母亲状况）特征的变化，提出了三种主要理论来解释与渔民性别比例的偏离。特里弗斯-威拉德模型(特里弗斯，R、还有D.Willard.1973.父母的自然选择能力改变后代的性别比例。科学179：90-92）母亲在最佳身体状况下的男性偏见性别分配是基于男性后代的竞争能力，以便将来获得配偶并因此获得更好的繁殖。本地资源竞争模型基于母系的竞争互动，就像许多哺乳动物一样，生儿子减少了未来与女儿的性交竞争。最终模型调用了为哲学女性维持母系的优势，尽管女性之间的竞争有所增加。我们使用29年的血统和人口统计数据来评估哥伦比亚地松鼠（Urocitelluscolumbianus）的这些假设，一种半社会物种，以强烈的女性哲学为特征。总的来说,雄性后代在出生和断奶时比雌性后代重，这意味着更高的生产成本。有更多的当地亲属在场，处于最佳状态的母亲使后代的性别比例偏向男性，和母亲在恶劣的条件下偏向女性的后代性别比例。没有近亲的同养,模式颠倒了，母亲在最好的条件下生育更多的女儿，和母亲在恶劣的条件下生产更多的儿子。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，而是建议相对母体状况和母系优势对后代性别比的综合影响。
