Abstract
Within current theories on potential adaptive manipulation of offspring sex ratio, giving birth to a male or to a female is assumed to depend on the capacity of the mother to invest in offspring to maximize her fitness. The active role of the father in sex ratio bias at birth has been neglected until recently. The human sex ratio at birth is biased towards sons, although in occidental populations, the ratio has decreased regularly for 30 years and could be the consequence of the adverse effects of environmental chemicals on male hormones. In a Malagasy primate, the lesser mouse lemur, the potential effect of paternal testosterone levels on sex ratio bias at birth was tested on 130 litters (278 babies) produced in 52 mixed-sex groups. For each group, social dominance among males was characterized based on aggressive interactions and sexual behaviours. Using a multi correspondence analysis, high testosterone levels in grouped males, particularly those of the dominant male, were significantly correlated with more infants produced in male-biased litters, independent of the female condition. According to these results, predictions for sex ratio bias towards one sex or the other in mouse lemurs were discussed considering the influence of both parents.
Highlights
As a key for life history strategies, parental ability to control the sex of the offspring should provide an evolutionary advantage and has been studied in a variety of taxa
Besides the TW theory, an alternative hypothesis proposes that competition for local resource (LRC) which acts on individuals within groups may shape birth sex ratios of individuals based on their condition [2,3]
Experimental studies demonstrate that the social context in which females live before mating plays a major role in determining offspring sex ratio
Summary
As a key for life history strategies, parental ability to control the sex of the offspring should provide an evolutionary advantage and has been studied in a variety of taxa. The Trivers-Willard theory (TW) [1] predicts that the sex ratio of offspring (SR, male births/total births) should vary with the capacity of the mother to invest in her offspring to maximize her lifetime reproductive success. Besides the TW theory, an alternative hypothesis proposes that competition for local resource (LRC) which acts on individuals within groups may shape birth sex ratios of individuals based on their condition [2,3]. The theoretical scenario developed by Leinar [4] suggests that high quality females should produce the sex whose reproductive success is mostly influenced by maternal investment, when taking into account the lifetime reproductive values of sons or daughters.
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