Onset of plural cooperative breeding in common marmoset families following replacement of the breeding male

Common marmosets, Callithrix jacchus, are usually characterized as singular cooperative breeders, with only a single, dominant female reproducing in each group. Anecdotal reports, however, have described two females breeding concurrently when an unrelated male joins their group. We tested the hypothesis that incorporation of an unrelated adult male into a family systematically leads to the onset of reproductive activity in a daughter and investigated the underlying mechanisms. We collected hormonal and behavioural data from mothers and their eldest daughters before and after the father was removed from the family and either replaced by an unrelated male (N=11) or immediately returned to the family (N=7). Variation between daughters in the occurrence of ovulatory cyclicity was not associated with the presence of an unrelated male but was closely linked to daughtersʹ relationships with their mothers: only anovulatory daughters behaved submissively towards their mothers. Daughters never engaged in sexual behaviour with their fathers, but most did so with unrelated males. Similarly, daughters never conceived in intact natal families but did so in eight of 11 families containing an unrelated male. In six of these families, the mother and daughter bred concurrently. Thus, incorporation of an unrelated adult male into a marmoset family may frequently lead to the onset of plural breeding by activating sexual behaviour in a daughter; however, daughters ovulate only if they are not behaviourally subordinate to their mothers. Therefore, both inbreeding avoidance and rank-related reproductive suppression may constrain reproduction in marmoset daughters and contribute to maintenance of singular breeding.