Sex-specific strategies in a sex-biased world

PhD Ceremony: dhr. S.P.M. Michler, 14.45 uur, Academiegebouw, Broerstraat 5, Groningen

Dissertation: Sex-specific strategies in a sex-biased world

Promotor(s): prof.dr. J. Komdeur, prof.dr.ir. J. Tinbergen

Faculty: Mathematics and Natural Sciences

Contact: Stephanie Michler, tel. 00 41 (0)62 535 3798, e-mail: s.p.m.michler@rug.nl

Stephanie Michler investigated experimentally how changes in the social environment affect competition between and within the sexes and if parents alter their investment in the sexes consequently. She manipulated simultaneously density and sex ratio of juvenile great tits (Parus major) in forest plots and investigated the effects on survival, dispersal and the sex allocation patterns produced the following year.

Generally we observe equal numbers of males and females in a population, however, some families produce mainly daughters and others mainly sons. One explanation for this phenomenon is that under certain conditions, which vary between families, one offspring sex has a higher reproductive success than the other. Sex-specific competition pressure is such a condition and it takes place when mainly one sex competes for locally restricted resources or when one sex dominates the other during competition.

The results of Michler showed that male and female young reacted differently to local density and sex ratio and that sex allocation the following year was altered. High plot density seemed to indicate high competition (for territories) for young males as they moved away further from such plots than young females. Latter moved away from plots with many young males in autumn probably to avoid competition with them. Sex allocation the following year was adjusted to the previously experienced social environment indicating that more of the dispersing sex (females) was produced when competition (density, sex ratio) was high. These results provide valuable insights in how selection acts on sex allocation by highlighting the importance of social traits.

Considering competition between the sexes might help to understand why certain species show declining population sizes and might also be relevant for the success of reintroduction programs.