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Maternal investment

Sex differences in avian yolk hormone levels


It has been suggested that female birds put more resources into eggs fathered by attractive males by laying larger eggs1 or by adding more testosterone2, but this inference could be undermined if eggs of different sex are provisioned differently, as these studies did not control for sex differences. Here we compare hormone concentrations in the yolks of male and female eggs and find that these are significantly different. Our results indicate that it is premature to conclude that female birds invest more in eggs sired by a preferred male, and raise the possibility that yolk sex steroids may be part of the sex-determining process in birds.

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Figure 1: Steroid concentrations (in pg per mg of yolk) in male (n = 43) and female (n = 45) eggs.


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Correspondence to Marion Petrie.

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Petrie, M., Schwabl, H., Brande-Lavridsen, N. et al. Sex differences in avian yolk hormone levels. Nature 412, 498 (2001).

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