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Classical models of grooming predict that subordinate primates will direct grooming towards dominants to receive coalitionary support from them. In contrast, recent reviews suggest that grooming asymmetries can change with social system and ecological conditions and should reflect asymmetries in services provided by different members of the dyad. We studied grooming patterns between females in six wild groups of common marmosets, Callithrix jacchus, to investigate the relation between social structure and grooming between females in a cooperatively breeding species. We observed grooming frequently and consistently in all study groups. Breeding females groomed nonbreeding females more than vice versa, and grooming between breeding and nonbreeding females was not related to agonistic behaviour. Our results provide some support to the hypothesis that grooming asymmetries are related to differences in services provided by different group members. We suggest that, in cooperatively breeding systems, breeding females may use grooming as an incentive for helper females to stay in the group.

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The pygmy marmoset is a small South American primate with a complex social system based on cooperative breeding. Infant pygmy marmosets are extremely vocal; most of their calling is a repetitive pattern of mixed call types that is babbling-like. In a longitudinal study of vocal development in 8 infant pygmy marmosets, we recorded more than 750 calling bouts which occurred in a wide range of behavioural contexts. The infants used 16 different call types that we grouped into three categories: Adult-Like (acoustic structure consistent with that of adult calls), Adult-Variant (acoustic structure with some adult features and some variable features), and Infant (absent from the adult repertoire). The calling bouts were highly conspicuous in their duration (ranging up to more than 6.5 min/bout), complexity (up to 10 different call types/bout), and call rate with nearly 3 calls/s. When the infants were older, their call rate slowed and they shifted to using several of the Adult-Like calls with greater frequency, and used fewer Adult-Variant types. The infants did not use the Adult-Like call types appropriately when compared to the typical adult usage of those types. Caregivers were significantly more likely to respond to an infant when it was vocalizing than when it was not.