Primate Behavioral Ecology Lab, Instituto de Neuroetología, Universidad Veracruzana, Xalapa, Veracruz, Mexico.
Am J Phys Anthropol. 2020 May;172(1):48-57. doi: 10.1002/ajpa.24035. Epub 2020 Mar 6.
Frequency-dependent selection is expected to maintain infant sex ratios around parity over evolutionary time. However, over ecological time periods, infant sex ratios vary, and it has been proposed that this variation may reflect adaptive processes. In primates, there are consistent patterns of variation in infant sex ratios, although their adaptive significance remains contentious. In addition to design issues, contrasting results could have derived across primates from variation in the fitness benefits accrued through sons or daughters associated with the specific social, ecological, and demographic context of populations. Thus, different sex allocation tactics could occur within species over time and space.
We reviewed the literature to describe variation in infant sex ratio in howler monkeys (genus Alouatta) and to examine whether such a variation could be associated with adaptive sex allocation. We found 26 studies that provided data for this review. These studies yielded 96 infant sex reports, corresponding to 1,477 sexed infants.
Infant sex ratio across howler monkey species tends to parity, but females produce more sons under high group densities and more daughters when rainfall increases.
Based on these results, as well as on information on howler monkey dispersal patterns, demography, and within-group genetic relatedness, we speculate that, depending on population growth stage, sex allocation is explained by (a) local resource enhancement, that is, more cooperative philopatric daughters are produced when populations are growing; and (b) local resource competition, that is, more dispersing sons are produced when populations are saturated. Thus, there is evidence suggestive of adaptive variation in infant sex ratios in howler monkeys.
在进化过程中，频率依赖选择预计会使婴儿性别比例接近生育次数。然而，在生态时间范围内，婴儿性别比例会发生变化，有人提出这种变化可能反映了适应过程。在灵长类动物中，婴儿性别比例存在一致的变化模式，尽管其适应意义仍存在争议。除了设计问题外，由于与特定社会、生态和人口背景相关的儿子或女儿所带来的适应益处的差异，跨灵长类动物的对比结果也可能有所不同。因此，不同的性别分配策略可能会在不同时间和空间内在同一物种中发生。
我们回顾了文献，描述了吼猴（Alouatta 属）的婴儿性别比例的变化，并探讨了这种变化是否与适应性的性别分配有关。我们发现了 26 项研究，为这项综述提供了数据。这些研究共提供了 96 份婴儿性别报告，对应于 1477 名已确定性别的婴儿。
吼猴物种的婴儿性别比例倾向于接近生育次数，但在高群体密度下，雌性产生更多的雄性后代，而在降雨量增加时，雌性产生更多的雌性后代。
基于这些结果，以及关于吼猴扩散模式、种群动态和群体内遗传相关性的信息，我们推测，根据种群增长阶段，性别分配可以通过以下两种机制来解释：（a）局部资源增强，即当种群增长时，会产生更多的合作性的留居雌性后代；（b）局部资源竞争，即当种群饱和时，会产生更多的扩散性的雄性后代。因此，有证据表明吼猴的婴儿性别比例存在适应性变化。
