Sexual dimorphism of egg size in the European Blackbird Turdus merula |
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Authors: | Rafa? Martyka Joanna Rutkowska Anna Dybek-Karpiuk Mariusz Cichoń and Kazimierz Walasz |
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Institution: | (1) Institute of Environmental Sciences, Jagiellonian University, Gronostajowa 7, 30-387 Krak?w, Poland |
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Abstract: | Recently, a number of studies have shown that female birds are able to control the sex of their progeny at the stage of the
gamete. There is also some evidence that females adjust their investment in offspring depending on the sex of the embryo during
egg formation. Differential maternal investment to the eggs depending on their sex is usually interpreted as an adaptive strategy,
by which females can increase competitive abilities of the smaller sex, or preferentially invest towards the sex with the
potentially higher fitness returns. Here, we studied variation in egg size in relation to embryo sex and laying order in the
European Blackbird Turdus merula. We found male and female eggs to differ in size, with larger eggs containing male embryos, as well as a significant interaction
between embryo sex and laying order. This interaction resulted from the fact that egg size increased with the laying sequence
among eggs bearing females but did not change with laying order among eggs bearing males. There was no relationship between
offspring sex and the laying sequence within a clutch. We suggest that sexual dimorphism in egg size recorded in the European
Blackbird may reflect favouritism of the sex which may give higher fitness returns. |
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