Invernizzi Lucas, Lemaître Jean-François, Douhard Mathieu
Laboratoire de Biométrie et Biologie Evolutive UMR 5558, Université Lyon 1, CNRS, Université de Lyon, Villeurbanne, France.
J Anim Ecol. 2025 Jan;94(1):20-44. doi: 10.1111/1365-2656.14207. Epub 2024 Oct 30.
In its initial form, the expensive son hypothesis postulates that sons from male-biased sexually dimorphic species require more food during growth than daughters, which ultimately incur fitness costs for mothers predominantly producing and rearing sons. We first dissect the evolutionary framework in which the expensive son hypothesis is rooted, and we provide a critical reappraisal of its differences from other evolutionary theories proposed in the field of sex allocation. Then, we synthesize the current (and absence of) support for the costs of producing and rearing sons on maternal fitness components (future reproduction and survival). Regarding the consequences in terms of future reproduction, we highlight that species with pronounced sexual size dimorphism display a higher cost of sons than of daughters on subsequent reproductive performance, at least in mammals. However, in most studies, the relative fitness costs of producing and rearing sons and daughters can be due to sex-biased maternal allocation strategies rather than differences in energetic demands of offspring, which constitutes an alternative mechanism to the expensive son hypothesis stricto sensu. We observe that empirical studies investigating the differential costs of sons and daughters on maternal survival in non-human animals remain rare, especially for long-term survival. Indeed, most studies have investigated the influence of offspring sex (or litter sex ratio) at year T on survival at year T + 1, and they rarely provide a support to the expensive son hypothesis. On the contrary, in humans, most studies have focused on the relationship between proportion of sons and maternal lifespan, but these results are inconsistent. Our study highlights new avenues for future research that should provide a comprehensive view of the expensive son hypothesis, by notably disentangling the effects of offspring behaviour from the effect of sex-specific maternal allocation. Moreover, we emphasize that future studies should also embrace the mechanistic side of the expensive son hypothesis, largely neglected so far, by deciphering the physiological pathways linking son's production to maternal health and fitness.
在其最初形式中，昂贵儿子假说假定，在雄性偏多的两性异形物种中，儿子在生长过程中比女儿需要更多食物，这最终会给主要生育和抚养儿子的母亲带来适应性成本。我们首先剖析昂贵儿子假说所基于的进化框架，并对其与性别分配领域提出的其他进化理论的差异进行批判性重新评估。然后，我们综合了目前对生育和抚养儿子对母体适应性组成部分（未来繁殖和生存）的成本（以及缺乏成本支持）的研究。关于未来繁殖方面的后果，我们强调，具有明显两性体型差异的物种在后续繁殖性能上，儿子的成本比女儿更高，至少在哺乳动物中是这样。然而，在大多数研究中，生育和抚养儿子与女儿的相对适应性成本可能是由于母体性别偏向的分配策略，而非后代能量需求的差异，这构成了严格意义上的昂贵儿子假说的一种替代机制。我们观察到，研究非人类动物中儿子和女儿对母体生存的差异成本的实证研究仍然很少，尤其是关于长期生存的研究。事实上，大多数研究调查了T年后代性别（或窝仔性别比例）对T + 1年生存的影响，它们很少为昂贵儿子假说提供支持。相反，在人类中，大多数研究关注儿子比例与母体寿命之间的关系，但这些结果并不一致。我们的研究突出了未来研究的新途径，这些途径应通过特别区分后代行为的影响与母体性别特异性分配的影响，来全面看待昂贵儿子假说。此外，我们强调，未来的研究还应关注昂贵儿子假说在很大程度上被忽视的机制方面，通过解读将儿子的生育与母体健康和适应性联系起来的生理途径。
