Abstract
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.
Highlights
An individual’s experiences of the environment often lead to variation in its growth, condition and physiological state [1,2,3]
As a measure of effect sizes we used partial Eta-Square values for the factors and covariates tested with a General Linear Models (GLMs)
We tested whether yolk progesterone levels differed between eggs according to the future parental ability to feed the entire clutch in wild rockhopper penguins
Summary
An individual’s experiences of the environment often lead to variation in its growth, condition and physiological state [1,2,3]. In altricial and semi-altricial birds, asynchronous hatching generally leads to a size hierarchy among siblings, and to a competitive disadvantage for the youngest siblings compared to older ones [10,11,12] This size hierarchy is thought to optimize the fitness of the parents by facilitating brood reduction when resource availability does not enable parents to feed the entire clutch properly [13,14,15]. In this context, females should adjust prenatal maternal effects over the laying sequence, and increase or offset the chick’s disadvantage in hatching later, according to breeding conditions such as the parents’ future ability to feed the chicks. In precocial species where mothers would not benefit from a modulation of sibling quality, intra-clutch variation in yolk hormones may play little or no adaptive role (see for example [20,22])
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