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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