Sex Ratio Manipulation in Birds (Summary)
B.Sc. Dissertation
Eszter Szász
Supervisor: Dr. Balázs Rosivall
Budapest 2010
The aim of my dissertation is to review the ultimate and proximate causes of sex ratio manipulation in birds,
as well as to draw a short conclusion regarding its practical relevance to conservation biology and its human
relations. Sex ratio manipulation is the phenomenon when the sex ratio of the offspring is not random, but
depends on many parental and environmental factors. Sex ratio manipulation is an adaptive ability of the
individuals, since under certain conditions fitness return from parental investment in sons and daughters
differs, because of sex differences in survival and/or reproductive prospects. Some of the explanatory
hypotheses focus on the sex-dependent costs, while others on the sex-dependent benefits of producing male
and female offspring. I discuss both groups of the hypotheses in details. Evidently, costs and benefits can
only be interpreted together, since parents optimize the sex ratio of their offspring to maximize their net
return. To test the hypotheses successfully, long-term field studies and well designed experimental studies
are needed. The precise mechanism of sex ratio manipulation is unknown, and we cannot presume that a
single mechanism has spread throughout in all avian species. However, only mechanisms for which the
benefits overcome the costs, are expected to evolve. The most cost-efficient mechanism would be the
control of sex-determination, namely segregation distortion. As in birds, female is the heterogametic sex,
and fertilization and egg formation occurs in the body of the female, the sex ratio of the progeny is probably
under maternal control. In practice, we distinguish primary sex ratio manipulation, which occurs before egg
laying, and secondary sex ratio manipulation, which occurs after egg laying. I discuss the potential
mechanisms of both in details. I also mention that steroid hormones may act as mediators between external
conditions and the direct mechanisms, since their level and rate changes rapidly in response to changes in
environmental conditions and egg yolk contains a great amount of them. They may play a role also in
segregation distortion, because yolk is sythesised before the sex-determination. To successfully investigate
the mechanisms, first we need to get a more precise picture of oocyta maturing, ovulation, sperm storing and
steroid hormone metabolism.