Sex allocation theory predicts that the proportion of daughters to sons will evolve in response to ecological conditions that determine the costs and benefits of producing each sex. All else being equal, the adult sex ratio (ASR) should also vary with ecological conditions. Many studies of subterranean species reported female-biased ASR, but no systematic study has yet been conducted. We test the hypothesis that the ASR becomes more female-biased with increased isolation from the surface. We compiled a data set of ASRs of 35 species in the subterranean amphipod Niphargus, each living in one of three distinct habitats (surface-subterranean boundary, cave streams, phreatic lakes) representing an environmental gradient of increased isolation underground. The ASR was female-biased in 27 of 35 species; the bias was statistically significant in 12 species. We found a significant difference in the ASR among habitats after correction for phylogeny. It is most weakly female-biased at the surface-subterranean boundary and most strongly female-biased in phreatic lakes. Additional modelling suggests that the ASR has evolved towards a single value for both surface-subterranean boundary and cave stream-dwelling species, and another value for 9 of 11 phreatic lake dwellers. We suggest that a history of inbreeding in subterranean populations might lower inbreeding depression such that kin selection favours mating with siblings. This could select for a female-biased offspring sex ratio due to local mate competition among brothers. The observed patterns in sex ratios in subterranean species make them a group worthy of more attention from those interested in sex allocation theory.

中文翻译：

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洞穴中没有雄性的空间：Niphargus 属地下片脚类动物的雌性性别比例。

性别分配理论预测，女儿与儿子的比例将随着生态条件的变化而变化，生态条件决定了生产每种性别的成本和收益。在其他条件相同的情况下，成人性别比 (ASR) 也应随生态条件而变化。许多地下物种的研究报告了女性偏见的 ASR，但尚未进行系统研究。我们测试了这样一个假设，即随着与表面的隔离增加，ASR 变得更加偏向女性。我们汇编了地下片脚类动物 Niphargus 中 35 种 ASR 的数据集，每一种都生活在三个不同栖息地（地表-地下边界、洞穴溪流、潜水湖）中的一个，代表了地下隔离度增加的环境梯度。ASR 在 35 个物种中的 27 个中偏向雌性；偏差在 12 个物种中具有统计学意义。我们发现在对系统发育进行校正后，栖息地之间的 ASR 存在显着差异。它在地表-地下边界最弱地偏向女性，在潜水湖中最强烈地偏向女性。额外的模型表明，ASR 已经演变为地表-地下边界和洞穴溪流栖息物种的单一值，以及 11 个潜水湖居民中的 9 个的另一个值。我们认为，地下种群的近亲繁殖历史可能会降低近亲繁殖的衰退，从而使亲属选择有利于与兄弟姐妹交配。由于兄弟之间的本地配偶竞争，这可能会选择偏向女性的后代性别比。