Notes towards an explanation of the reported effect of mammalian breeding systems on offspring sex ratio

Chelini et al. ¹ reported that the breeding system affects offspring sex ratio in Syrian hamsters. These workers experimented with two such systems, viz (1) where two females were housed with one male before mating (harem system, HS) and (2) where females were housed individually before mating (monogamic temporary system [MTS]). Contrary to the prediction of these workers, the offspring sex ratio (proportion of male pups at birth) was higher in the HS system than in the MTS system. These workers found that litter size did not vary with breeding system, so they inferred that sex-selective fetal wastage was not responsible for their result, and they concluded that ‘sex ratio adjustment probably occurs around conception’. These workers cited my hypothesis that mammalian (including human) offspring sex ratios are partially controlled by parental hormone concentrations around the time of conception,^2,3 but were unclear about the explanation of such control. Here I briefly review findings that may point towards such an explanation.

Consistent with the finding of Chelini et al., ¹ it has been reported that offspring sex ratios vary inversely with the operational sex ratio (that of the adult breeding population) in a number of mammalian species, e.g. the black lemur, ⁴ the ring-tail lemur, ⁵ the human being,^6–8 the mouse lemur ⁹ and the wedge-capped capuchin monkey. ¹⁰ Though the data are not entirely consistent, one might expect such a phenomenon to be rather general because it would tend to correct operational sex ratios that, for some reason, had become biased away from the supposed optimum of one male to one female.

Females of some species react behaviourally and endocrinologically to the presence of other females. For instance, female house mice become aggressive and have high levels of testosterone when housed with other females. ¹¹ Isolated female mouse lemurs reportedly have shorter cycles than grouped females, and this difference was associated with progesterone levels. ¹² Moreover, it has been shown that exposing female mammals to the urinary chemosignals of other conspecifics affects offspring sex ratio: this has been reported in respect of the house mouse ¹³ and the mouse lemur. ¹⁴ In mouse lemurs, social communication primarily relies on olfactory cues. ¹⁵ Hence, there are strong grounds for supposing - as suggested by Chelini et al. ¹ – that sex ratio variation by mating system is mediated by endocrine means. Research attempts should continue to try identifying these endocrine details for each species. The point is important both for commercial breeding and for conservation of rare species.