Parental behaviour of prairie voles (Microtus ochrogaster) and meadow voles (M. pennsylvanicus) in relation to sex of offspring

Monogamous parents are predicted to invest equally in male and female offspring whereas polygynous parents in good condition are predicted to invest more in male than female offspring. Sex-biased parental investment can occur in three ways: (1) mothers and fathers invest different amounts of care in their offspring (effect of parent sex); (2) parents invest different amounts of care in male and female offspring (effect of offspring sex); and (3) one parent, but not the other, invests different amounts of care in male and female offspring (interaction between parent sex and offspring sex). Studies of parent–offspring interactions in rodents have focused on either effect of parent sex or effect of offspring sex, but not the interaction between parent sex and offspring sex, and most studies have examined only one species. We studied prairie voles (Microtus ochrogaster), a monogamous species, and meadow voles (M. pennsylvanicus), a polygynous (or promiscuous) species, under laboratory conditions designed to simulate field conditions. For each species, we recorded the frequency and duration with which mothers and fathers licked their male and female offspring. We found that meadow vole fathers licked male offspring for longer durations than female offspring. However, prairie vole fathers, prairie vole mothers, and meadow vole mothers did not lick male and female pups for different durations. From the standpoint of the pups, male prairie vole pups, female prairie vole pups, and female meadow vole pups were licked for longer durations by their mothers than by their fathers. In contrast, for male meadow vole pups there was no difference in the duration with which they were licked by mothers and fathers. We also detected effects of litter size: as litter size increased, the frequency and duration of pup licking decreased for mothers and increased for fathers. For duration (but not frequency) of pup licking, these changes were more dramatic in meadow voles than in prairie voles. Our data are generally consistent with predictions that monogamous parents, such as prairie voles, should invest equally in male and female offspring whereas polygynous (or promiscuous) parents, such as meadow voles, should invest more in male offspring when conditions are favourable. Our data also highlight the complexity of parent–offspring interactions in rodents and emphasize the need to examine whether male and female offspring within a species differ in their behaviour or ability to obtain parental care.