Relationship between urinary estrogen levels before conception and sex ratio at birth in a primate, the gray mouse lemur

BACKGROUND: In primates, including humans, bias of sex ratio at birth has been reported to depend on maternal condition at conception. In a Malagasy primate, the gray mouse lemur, male-biased sex ratio at birth occurred in captive parous females. The male bias was previously demonstrated to be pre-conceptual and independent of the female's nutritional state. It was hypothesized to be related to changes in hormonal state at ovulation. METHODS and RESULTS: The relationship between urinary estradiol (E-2) levels during the follicular phase until estrus and litter production (number and sex of newborns) was assessed in 91 females mated under controlled conditions. Changes in E-2 levels prior to ovulation followed the typical primate pattern characterized by a gradual rise during the 10 days preceding the sharp increase at estrus. A clear decline in E-2 levels occurred with ageing. Direction of the sex ratio bias was unrelated to E-2 levels at ovulation time but was significantly dependent on E-2 levels during the follicular phase. Reduced E-2 levels prior to estrus led to male-biased litters. CONCLUSION: This study suggests that hormonal stimulation during the follicular phase plays a role in shifting sex ratio at conception through changes in the local environment of the ova. This hypothesis deserves testing by assessing estrogen levels throughout the follicular phase in other primate species including humans.