Pre-ovulation control of hatchling sex ratio in the Seychelles warbler

Females of some bird species have a high degree of control over the sex ratio of their offspring at laying. Although several mechanisms have been put forward to explain how females might control the sex of their eggs, virtually nothing is known. As females are the heterogametic sex in birds, adjustm...

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Bibliographic Details
Published in:Proceedings of the Royal Society. B, Biological sciences Biological sciences, 2002-05, Vol.269 (1495), p.1067-1072
Main Authors: Komdeur, J., Magrath, M. J. L., Krackow, S.
Format: Article
Language:English
Subjects:
Animals
Birds
Breeding
Egg Sex Ratio
Eggs
Estrus - physiology
Female
Female animals
Gender bias
Hatching
Male
Male animals
Ova
Ovulation - physiology
Post-Ovulation Control
Pre-Ovulation Control
Sex Ratio
Seychelles Warbler
Songbirds - physiology
Warblers
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Summary:Females of some bird species have a high degree of control over the sex ratio of their offspring at laying. Although several mechanisms have been put forward to explain how females might control the sex of their eggs, virtually nothing is known. As females are the heterogametic sex in birds, adjustment of the clutch sex ratio could arise either by pre- or post-ovulation control mechanisms. The Seychelles warbler (Acrocephalus sechellensis) exhibits extreme adaptive egg sex ratio bias. Typically, warblers produce only single-egg clutches, but by translocating pairs to vacant habitat of very high quality, most females were induced to produce two-egg clutches. Overall, females skewed clutch sex ratios strongly towards daughters (86.6%). This bias was evident in the first egg, but critically, also in the second eggs laid a day apart, even when all absent, unhatched, or unsexed second eggs were assumed to be male. Although a bias in the first egg may arise through either pre- or post-ovulation mechanisms, the skew observed in second eggs could only arise through pre-ovulation control. Post-ovulation adjustment may also contribute to skewed hatchling sex ratios, but as sex-biased release of gametes is likely to be a more efficient process of control, pre-ovulation mechanisms may be the sole means of adjustment in this species. High fitness differentials between sons and daughters, as apparent in the Seychelles warblers, may be necessary for primary sex ratio adjustment to evolve.
ISSN:0962-8452
1471-2954
DOI:10.1098/rspb.2002.1965