Determinants of female fecundity in a simultaneous hermaphrodite: the role of polyandry and food availability

Classical sexual selection theory assumes that the reproductive success of females is primarily limited by the resources available for egg production rather than by the number of mating partners. However, there is now accumulating evidence that multiple mating can entail fitness costs or benefits for females. In this study we investigated the effect of polyandry (i.e., the mating with different mating partners) and food availability on the reproductive output of the female sex function in an outcrossing simultaneous hermaphrodite, the free-living flatworm Macrostomum lignano. We exposed virgin worms to different group sizes, a treatment that has previously been shown to affect the level of polyandry in this species. Moreover, we manipulated the food availability throughout the subsequent egg laying period, during which the worms were kept in isolation. The number of offspring produced was used as an estimate of female fecundity. We found that food availability, but not group size, had a significant effect on female fecundity. Additionally, female fecundity was positively correlated with the number of stored sperm in the female sperm-storage organ at the time of isolation, but it was not correlated with body or ovary size of the worms. Our results suggest that female fecundity in M. lignano is primarily determined by the resources available for egg production, and not by the level of polyandry, confirming classic sexual selection theory for simultaneous hermaphrodites

Extra-pair paternity, offspring mortality and offspring sex ratio in the socially monogamous coal tit (Parus ater)

Dietrich-Bischoff V, Schmoll T, Winkel W, Krackow S, Lubjuhn T. Extra-pair paternity, offspring mortality and offspring sex ratio in the socially monogamous coal tit (Parus ater). BEHAVIORAL ECOLOGY AND SOCIOBIOLOGY. 2006;60(4):563-571.Females of many socially monogamous bird species commonly engage in extra-pair copulations. Assuming that extra-pair males are more attractive than the females' social partners and that attractiveness has a heritable component, sex allocation theory predicts facultative overproduction of sons among extra-pair offspring (EPO) as sons benefit more than daughters from inheriting their father's attractiveness traits. Here, we present a large-scale, three-year study on sex ratio variation in a passerine bird, the coal tit (Parus ater). Molecular sexing in combination with paternity analysis revealed no evidence for a male-bias in EPO sex ratios compared to their within-pair maternal half-siblings. Our main conclusion, therefore, is that facultative sex allocation to EPO is absent in the coal tit, in accordance with findings in several other species. Either there is no net selection for a deviation from random sex ratio variation (e.g. because extra-pair mating may serve goals different from striving for 'attractiveness genes') or evolutionary constraints preclude the evolution of precise maternal sex ratio adjustment. It is interesting to note that, however, we found broods without EPO as well as broods without mortality to be relatively female-biased compared to broods with EPO and mortality, respectively. We were unable to identify any environmental or parental variable to co-vary with brood sex ratios. There was no significant repeatability of sex ratios in consecutive broods of individual females that would hint at some idiosyncratic maternal sex ratio adjustment. Further research is needed to resolve the biological significance of the correlation between brood sex ratios and extra-pair paternity and mortality incidence, respectively

The evolution of eusociality

International audienc

Inclusive fitness theory and eusociality

Arising from M. A. Nowak, C. E. Tarnita & E. O. Wilson 466, 1057-1062 (2010); Nowak et al. reply. Nowak et al. argue that inclusive fitness theory has been of little value in explaining the natural world, and that it has led to negligible progress in explaining the evolution of eusociality. However, we believe that their arguments are based upon a misunderstanding of evolutionary theory and a misrepresentation of the empirical literature. We will focus our comments on three general issues