The attractiveness hypothesis predicts that females should bias the sex ratio of their offspring towards sons when mated to attractive males. Females of many socially monogamous bird species commonly engage in extra-pair fertilizations (EPFs). Assuming that extra-pair males are more attractive to females than their social partners, and that sons inherit superior traits from their fathers, extra-pair young should be more likely males. According to the maternal condition hypothesis male-biased sex ratio in offspring should be also associated with better female body condition. We evaluated these ideas in the scarlet rosefinch (Carpodacus erythrinus), socially monogamous songbird with moderate level of extra-pair fertilizations. Contrary to the attractiveness hypothesis we have found no significant effect of paternity (within-pair or extra-pair) on the sex of individual offspring. Furthermore, data did not suggest that females mated to males with more elaborated plumage colour were more likely to produce sons. However, consistently with the maternal condition hypothesis, high-quality females produced more sons than daughters. Our results indicate that scarlet rosefinch females may not be able to manipulate the primary sex ratio of their offspring in relation to the attractiveness of their mate, but they may adjust it according to their own condition.
While most mammal populations show equalized sex ratios in their offspring, some species reveal deviations as a reaction to environmental circumstances. The recent study focuses on mortality, dispersal and biased sex ratio in the offspring of the edible dormouse (Glis glis) and their influence on next year’s subadult population. Between 2002 and 2008 a male biased sex ratio in the offspring but an almost equal sex ratio of next year’s subadult population has been found. Our results reveal that this deviation from the 1:1 sex ratio compensates a higher mortality rate of young males. The data used for this study were obtained from a mark and recapture project of monitoring the population biology of the edible dormouse performed north-east of Frankfurt/Main, Germany.