Helping by juvenile pine voles (Microtus pinetorum), growth and survival of younger siblings, and the evolution of pine vole sociality

We quantified the behavior of juvenile pine voles (Microtuspinetorum) in families maintained in outdoor enclosures protectedfrom rain and predators and determined the effects of numberof juveniles present (0–3) on their younger siblings andtheir parents. Juveniles exhibited behaviors characteristicof helpers in cooperative breeding species: brooding, grooming,and retrieving younger siblings, and maintaining nests, runways,and food caches. The number of juveniles had no effect on weights,growth rates, or survival of younger siblings or on survivalof parents, but families with three juveniles had shorter interlitterintervals than other families. There was a trend toward lowerweights for younger siblings when juveniles continued to nursefrom their mothers. Juveniles brooded and groomed younger siblingsmore when their mothers were absent than when mothers were present.In families with juveniles, younger siblings were alone significandyless often than in families without juveniles, but number ofjuveniles in the family had little effect on duration or frequencywith which juveniles brooded or groomed younger siblings. Thesedata are consistent with cooperative breeding having evolvedvia individual selection for delayed dispersal. We argue thatthe limited availability of vacant tunnel systems and the highcost of digging new tunnel systems constrains dispersal in pinevoles and has led to the evolution of cooperative breeding.