When the fitness costs and benefits of sons and daughters differ, offspring sex ratio manipulation could be an important reproductive tactic. We explored the effects of environment and maternal caring ability on offspring sex to test four adaptive sex ratio modification hypotheses: the extrinsic modification hypothesis (EMH), carrying capacity hypothesis (CCH), Trivers-Willard hypothesis (TWH), and cost-of-reproduction hypothesis (CRH). The EMH and CCH propose that environmental conditions shape offspring sex ratios, directly or in interaction with maternal condition. The TWH and CRH predict a positive relationship between maternal condition and production of the costlier sex. The TWH predicts that mothers with superior caring ability should produce more of the sex that can provide the greatest fitness returns from additional maternal allocation, and the CRH proposes that females with limited caring ability should reduce fitness costs by producing the cheaper sex. Repeated measures on 83 known-age eastern gray kangaroos, polygynous marsupials with strong sexual dimorphism, revealed that offspring sex ratio was independent of per capita forage, supporting neither the EMH nor CCH, but was dependent on maternal mass, consistent with the TWH and CCH. Our results, however, cannot clearly identify the ultimate cause of the relationship between maternal mass and greater production of sons. One of the three assumptions of the TWH could not be verified, and mothers of sons suffered only marginal additional fitness costs. Sex ratios in higher vertebrates are likely not solely explained by factors dependent on maternal control.

中文翻译：

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袋鼠后代性别的决定因素：多重假设的检验

当儿子和女儿的健身成本和收益不同时，对子代性别比例的操纵可能是一项重要的生殖策略。我们探索了环境和母体关怀能力对后代性别的影响，以测试四个适应性性别比例修正假设：外在修饰假设（EMH），承载力假设（CCH），Trivers-Willard假设（TWH）和成本繁殖假说（CRH）。EMH和CCH提出，环境条件直接或与孕产妇状况相互作用会影响后代性别比例。TWH和CRH预测孕产妇状况与较昂贵的性行为之间呈正相关。TWH预测，具有较高照护能力的母亲应该产生更多的性别，这些性别可以通过增加产妇分配来提供最大的健康回报，CRH提出，照顾能力有限的女性应该通过产生更便宜的性别来降低健身成本。对83个已知年龄的东部灰袋鼠（具有强性二态性的多齿有袋动物）进行的重复测量显示，后代性别比与人均饲草量无关，既不支持EMH也不支持CCH，但取决于母体质量，与TWH和CCH一致。然而，我们的结果不能清楚地确定孕产妇数量与儿子高产之间关系的最终原因。TWH的三个假设之一无法得到证实，儿子的母亲仅承受了少量的额外健身费用。高等脊椎动物中的性别比可能不仅仅由依赖母体控制的因素来解释。