Mechanisms of maternal investment by communal prairie voles, Microtus ochrogaster

Communal rearing enhances the quality of offspring weaned by some social vertebrates. Previously, we observed that prairie vole pups reared in groups containing two or more breeding females (plurally breeding groups) gained significantly more body mass than did pups reared by solitary mothers, but not more than pups reared in groups containing only one breeding female (singularly breeding groups). In this study, we tested three hypotheses that might explain these differences. We tested the enhanced thermoregulation hypothesis by determining the nest temperatures experienced by pups and the time that pups were left unattended during 6-h observations, and the core body temperature of pups. To test the enhanced tactile stimulation hypothesis, we determined the time that pups were licked or groomed by adults during 20-min observations. To test the milk quality hypothesis, we determined the total solids found in milk collected from mothers with 3- and 9-day-old pups. To test the increased milk volume hypothesis, we determined an index of milk intake for 4-, 8- and 13-day-old pups. Pups reared in plurally breeding groups were left alone less by caregivers and ingested more, but not higher-quality, milk than did pups reared by solitary mothers. Pups reared in singularly breeding groups experienced greater tactile stimulation and were left unattended less, but did not ingest more or higher-quality milk than did pups reared by solitary mothers. Pups reared in plurally breeding groups experienced slightly less tactile stimulation and similar thermal conditions, but ingested more milk than did pups reared in singularly breeding groups. Our results suggest that plural breeding enhances two forms of parental investment, thermoregulation and milk volume, that contribute to the enhanced weight gain of pups in plurally breeding groups.