Long-term study of infant-carrying behavior in captive common marmosets (Callithrix jacchus): Effect of nonreproductive helpers on the parents' carrying performance

Infant-carrying behavior in four families of Callithrix jacchus was investigated over a period covering six to eight litters. We evaluated the effect of the total number of helpers and the number of adult helpers on the parents' carrying performance for the total carrying period and for three age stages of the infants. The carrying performance of the parents reached an asymptote beginning with litters 3 to 5. In two groups, the carrying performance of the parents was significantly negatively correlated with group size. However, considerable differences existed within each group and between the groups. The mother was the primary caregiver in the early weeks of the infants' lives. In this phase the helpers' contribution to carrying was less than might be expected. The relief of the parents from infant-carrying was greatest when the infants were 4 to 7 weeks old. Breeding males benefited most and from every helper, while breeding females benefited most from adult helpers, i.e., the carrying performance of the breeding female was considerably reduced until the respective group had grown to 8 to 10 members (infants not included). This group size is considered to be ideal for maximal relief of the parents from infant carrying. An increasing number of adult nonreproductive helpers (>4 or 5) does not induce a further reduction of the parents' carrying performance. Infant-rearing experience of nonreproductive helpers seems to be more important for the parents' and, especially, the breeding female's relief from infant-carrying than the overall number of helpers is. The data also strongly evidence that a group member's participation in infant-carrying is influenced by housing conditions and the demographic history of the group.