Department of Biology-Ethology, University of Antwerp, Wilrijk, Belgium.
PLoS One. 2011;6(11):e27765. doi: 10.1371/journal.pone.0027765. Epub 2011 Nov 16.
Hatching asynchrony in avian species generally leads to a size hierarchy among siblings, favouring the first-hatched chicks. Maternally deposited hormones affect the embryo and chick's physiology and behaviour. It has been observed that progesterone, a hormone present at higher levels than other steroid hormones in egg yolks, is negatively related to body mass in embryos, chicks and adults. A differential within-clutch progesterone deposition could therefore be linked to the size hierarchy between siblings and to the resulting brood reduction. We tested whether yolk progesterone levels differed between eggs according to future parental ability to feed the entire clutch in wild rockhopper penguins Eudyptes chrysocome. This species presents a unique reversed egg-size dimorphism and hatching asynchrony, with the larger second-laid egg (B-egg) hatching before the smaller first-laid egg (A-egg). Yolk progesterone levels increased only slightly with female body mass at laying. However, intra-clutch ratios were not related to female body mass. On the other hand, yolk progesterone levels increased significantly with the date of laying onset for A-eggs while they decreased for B-eggs. Early clutches therefore had proportionally more progesterone in the B-egg compared to the A-egg while late clutches had proportionally less progesterone in the B-egg. We propose that females could strategically regulate yolk progesterone deposition within clutches according to the expected food availability during chick growth, an adaptive strategy to adjust brood reduction to conditions. We also discuss these results, relating to yolk progesterone, in the broader context of other yolk steroids.
鸟类物种的孵化异步通常会导致兄弟姐妹之间出现大小等级，有利于先孵化的雏鸟。母体沉积的激素会影响胚胎和雏鸟的生理和行为。已经观察到，孕酮是蛋黄中存在的一种激素，其水平高于其他类固醇激素，与胚胎、雏鸟和成年个体的体重呈负相关。因此，在同一窝内沉积的孕酮差异可能与兄弟姐妹之间的大小等级以及由此导致的繁殖减少有关。我们测试了在野生跳岩企鹅 Eudyptes chrysocome 中，蛋黄中的孕酮水平是否因未来父母喂养整个窝卵的能力而在不同的卵之间存在差异。该物种表现出独特的反转蛋型二态性和孵化异步性，较大的第二枚产卵（B 卵）比较小的第一枚产卵（A 卵）先孵化。蛋黄中的孕酮水平在产卵时仅随雌体体重略有增加。然而，窝内的比例与雌体体重无关。另一方面，A 蛋的产卵日期与蛋黄中的孕酮水平显著相关，而 B 蛋的孕酮水平则下降。因此，早期的窝卵中 B 卵中的孕酮比例比 A 卵中的多，而晚期的窝卵中 B 卵中的孕酮比例比 A 卵中的少。我们提出，雌性可以根据雏鸟生长期间预期的食物供应情况，在窝卵内策略性地调节蛋黄中的孕酮沉积，这是一种适应策略，可以根据情况调整繁殖减少。我们还讨论了这些结果，将其与其他蛋黄类固醇中的蛋黄孕酮联系起来。
