Evidence of sex-biased postnatal maternal investment by Mongolian gerbils

Three experiments were undertaken examining the relative costs to female Mongolian gerbils, Meriones unguiculatus, of rearing four male or four female pups from birth to weaning. In experiment 1, it was observed that gerbil dams rearing all-male litters were almost twice as likely as gerbil dams rearing allfemale litters to be found: (1) with all their young in the nest throughout a 20-min observation period; and (2) with one or more pups attached to their nipples. In experiment 2, (1) dams rearing all-male litters delivered significantly fewer young as a consequence of impregnation during postpartum oestrus than did dams rearing all-female litters and (2) dams rearing all-male litters exhibited both significantly greater periods of vaginal closure (preventing copulation) and significantly greater inter-litter intervals than did dams rearing all-female litters. In experiment 3, it was found that vaginal closure also provides an index of whether a gerbil dam will transfer milk to her pups. Consequently, the greater durations of vaginal closure observed in dams rearing all-male litters than in dams rearing all-female litters (experiment 2) suggest a longer period of milk transfer from dams to their male than to their female young before weaning. The data were interpreted as consistent with the hypothesis that rearing male pups was more costly to gerbil dams than was rearing female pups. Difficulties in determining the relative cost to dams of rearing male and female young were discussed.