Data from: Cortisol advantage of neighbouring the opposite sex in utero

Population sex ratios naturally fluctuate around equality. It is argued that the production of an equal number of male and female offspring by individual parents should be favoured by selection, if all costs and benefits are equal. Theoretically, an even-sex ratio should yield the highest probability for a fetus to be adjacent to a fetus of the opposite sex in utero. This may cause developmental costs or benefits that have been overlooked. We examined the physiological and developmental parameters associated with in utero sex ratios in the nutria (Myocastor coypus), an invasive wildlife species with a strong reproductive output. Using hair-testing, we found that litters with even-sex ratios had the highest average cortisol levels. Fetuses neighbouring the opposite sex exhibited longer trunks than those neighbouring the same sex, which might imply better lung development. Our results are the first to link intra-utero sex ratios and fetal cortisol and suggest that fetal cortisol might be a mechanism by which even-sex ratios are maintained via developmental advantages.

种群性比天然地围绕均等水平波动。有研究指出，若所有成本与收益均保持一致，自然选择应会青睐那些能产生等量雄性与雌性后代的个体亲本。理论上，均等的性比可使子宫内胎儿与异性胎儿相邻的概率达到最高。这可能会引发此前被忽视的发育成本或收益。我们以繁殖能力极强的入侵野生动物物种海狸鼠（Myocastor coypus）为研究对象，探究了其子宫内（in utero）性比相关的生理与发育参数。通过毛发检测，我们发现性比均等的同窝胎仔平均皮质醇水平最高。与同性胎儿相邻的胎儿相比，与异性胎儿相邻的胎儿躯干更长，这或暗示其肺部发育更为良好。本研究首次将子宫内性比与胎儿皮质醇水平建立关联，并提示胎儿皮质醇可能是通过发育优势维持均等性比的潜在机制。