Within-clutch egg size asymmetry covaries with embryo sex in the yellow-legged gull Larus michahellis

Behavioral Ecology and Sociobiology (Impact Factor: 2.75). 63(12):1809-1819. DOI: 10.1007/s00265-009-0808-4

ABSTRACT Non-random sex allocation may occur whenever the expected reproductive value of sons and daughters differs, as is the case
when the sexes differ in susceptibility to environmental conditions or maternal effects (e.g. egg size and hatch order). Under
such circumstances, covariation between egg and clutch characteristics and egg sex may be expected, and this covariation should
vary with maternal state or ecological conditions. In this 2-year study (2007–2008), we examined sex allocation in relation
to egg and clutch traits in the yellow-legged gull Larus michahellis, a species where male chicks are larger and more susceptible to harsh rearing conditions than female ones. In 2008, eggs
were more likely male early in the season in two- but not three-egg clutches, and large eggs were more likely males late in
the season. No egg/clutch traits predicted egg sex in 2007. Within-clutch egg mass asymmetry (the difference in egg mass between
the first- and last-laid eggs) predicted sex in both years. In 2007, clutches with smaller egg mass variation were more likely
to contain males, while in 2008 this relationship held for the last-laid egg and was reversed for the preceding egg(s). Laying
order and sex of the previous egg did not predict egg sex, providing no evidence of sex-specific oocyte clustering. Thus,
the relationships between egg sex and egg/clutch traits differed among years, suggesting a phenotypically plastic response
of females to extrinsic conditions, and involved within-clutch egg mass asymmetry, a trait likely reflecting variation in
maternal quality and/or reproductive tactics, which has been largely neglected in previous studies of sex allocation.

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    ABSTRACT: Parental decisions can determine offspring experience of environmental conditions. Such ‘maternal’ effects act both before and after hatching via, e.g., egg quality or the social milieu predisposed by parents. Resource availability may constrain the expression of adaptive maternal effects, and the specific pattern of allocation of these effects among offspring depending on their sex or birth order can result in different fitness payoffs to parents. Declining egg mass with laying order observed in several bird species may constitute an adaptive strategy of parental favouritism towards early hatching offspring with larger reproductive value but may also result from nutritional constraints on laying effort. A previous study has shown that the small size of the third, last laid (c-)egg in yellow-legged gull (Larus michahellis) clutches depends on food availability and that food-supplemented mothers increase the size of their female but not male c-eggs. Here, we show that increased mass of c-eggs laid by females supplemented with food after clutch initiation depends on increased albumen mass, which, in turn, enhances the size of daughters at hatching. Because asynchronous hatching results in a competitive disadvantage of c-chicks, present results suggest that mothers relieved from nutritional constraints enhance the size of daughters to compensate for their larger susceptibility to hatching last. The study also confirms the role of egg albumen content in determining hatchling size, previously experimentally detected only in one species in the wild. The effect of increased egg mass on offspring size persisted at least until day 8 after hatching, when, however, it did not vary with sex, suggesting intense negative selection on small female c-chicks in control broods. Hence, maternal effects mediated by egg albumen content had persistent effects on offspring size. KeywordsAlbumen mass-Brood reduction-Egg mass-Food provisioning-Hatching order-Laying order-Maternal effects-Offspring size-Sex allocation
    Behavioral Ecology and Sociobiology 01/2010; 64(11):1813-1821. · 2.75 Impact Factor
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    ABSTRACT: Maternal effects through albumen quality are largely unexplored, despite the fundamental role that albumen exerts as source of proteins and water, as well as for antimicrobial defence of the embryo. We analysed the variation of two major albumen antimicrobials, avidin and lysozyme, by extracting samples from freshly laid eggs of the yellow-legged gull (Larus michahellis) and by correlating their levels to egg features. Lysozyme concentration increased along the laying sequence, while avidin concentration decreased. Both antimicrobials declined during the season. In addition, avidin concentration declined from first- to last-laid male eggs, whereas the opposite was true among the female eggs. We also analysed chick body mass and size and immune response, in relation to albumen antimicrobial levels in their original egg while controlling for potential covariation between egg quality and rearing conditions by cross-fostering eggs between nests. Tarsus length decreased with avidin concentration, particularly early in the season. Avidin concentration negatively predicted tarsus length of chicks and the phytohaemagglutinin response of females, but not males. However, chick phenotype did not covary with lysozyme albumen concentration. This is the first study where maternal effects mediated by albumen antimicrobials are investigated in relation to both sex and egg features in any wild bird species. Whether the observed patterns of variation in antimicrobial concentration are the by-product of maternal physiological constraints, or reflect adaptive allocation strategies, cannot be ascertained. The covariation between chick cell-mediated immunity and albumen avidin concentration might be causal, according to the documented effects of albumen proteins on immunity in other species. KeywordsAlbumen-Antimicrobial proteins-Avidin-Lysozyme-Maternal effects-Yellow-legged gull-Cross-fostering
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    ABSTRACT: Maternal effects may allow parents to increase their own fitness by adjusting progeny phenotype to pre- and post-natal conditions depending on the sex of individual offspring. In birds, sex-specific maternal effects can be mediated by predictable gradients of rearing conditions due to hatching asynchrony, egg laying date, order and mass, and brood sex ratio. In this study, we analysed the growth and survival consequences of experimentally reduced hatching asynchrony and order per se and in conjuction with other maternal effects (laying date, egg mass and sibling sex ratio) in yellow-legged gull (Larus michahellis) chicks of either sex. Survival of male chicks in synchronised broods was reduced compared to that of their sisters and that of male chicks in control broods, whereas survival of females but not that of males declined with hatching order independently of hatching synchronisation treatment. Survival of female but not male chicks declined with egg mass. In addition, survival of individual chicks, independently of their sex, was positively predicted by the proportion of females among their siblings. Hatching synchronisation resulted in larger average body size and in a weaker dependence of body mass on hatching order, suggesting that by modulating hatching asynchrony parents can effectively tune the size hierarchy among the offspring. Thus, survival of yellow-legged gull chicks of either sex was differentially affected by maternal effects as mediated by hatching asynchrony, hatching order as well as egg mass, which constitute predictable gradients of pre- or post-natal environmental conditions potentially under parental control.
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