Unconfounded evidence of sex-biased, postnatal maternal effort by Mongolian gerbil dams.

In previous studies of effects of offspring sex on postnatal maternal effort there has often been unintentional confounding of the sex ratio of the pups that a dam reared with the sex ratio of the litter that she gestated. By controlling the sex ratio of litters that female subjects gestated, we showed that (1) the probability of dams retrieving pups was influenced by the sex ratio of the litters dams gestated, not by the sex of litters that dams reared and (2) both frequency of nursing and duration of vaginal closure were influenced by the sex of litters that dams reared, not by the sex ratios of litters that dams gestated.