Offspring sex ratio in hamsters is correlated with vaginal pH at certain times of mating

Golden hamsters (Mesocricetus auratus) are sexually receptive for 19-21 h around the time of ovulation. Vaginal pH was measured immediately before mating in four groups of animals each mated at a different time during the female's receptive period. Litter size and sex ratio (percentage male offspring) were recorded at parturition. Mean litter size decreased while offspring sex ratio increased in matings occurring later in estrus. Significant negative correlations were found between vaginal pH and sex ratio of subsequent litters when matings occurred during midestrus. Fluctuations in vaginal pH may differentially affect longevity, motility, or fertilizing capacity of X and Y sperm and/or may reflect other physiological changes in the female which occur over the receptive period. Such a prefertilization mechanism for sex ratio regulation could be highly adaptive since little or no parental reproductive potential is lost.