A gerbil dam's fetal intrauterine position affects the sex ratios of litters she gestates.

The vaginally delivered litters of gerbil dams that, in utero, occupied intrauterine positions between two males fetuses (2M females) contain a greater proportion of sons than do the vaginally delivered litters of gerbil dams that were gestated between two female fetuses (2F females). This difference in the sex ratios of litters delivered vaginally by 2M and 2F gerbils might reflect sex-biased differences in perinatal mortality in the litters of 2M and 2F dams. However, the results of the present experiment, in which gerbil litters were delivered by caesarian section from females that developed in 2M and 2F intrauterine positions, demonstrated unequivocally that 2M female gerbils gestate male-biased litters, whereas 2F female gerbils gestate female-biased litters.