MULTIPLE BREEDING FEMALES IN FREE-RANGING GROUPS OF CALLITHRIX-JACCHUS

We present observations on the breeding patterns in a free-ranging pop ulation of common marmosets, Callithrix jacchus. We found evidence of the simultaneous presence of two reproductively active females in all three monitored groups. We recorded at least one full reproductive cyc le (conception-birth) for two females in each group, but other adult f emales did not breed. An overview of data from other callitrichid fiel d studies indicates that the observed pattern may be an additional cha racteristic of the flexibility of the social organization of common ma rmosets. Comparisons with data from other sites where callithrix has b een studied indicate that the presence of a second breeding female in a group may be related to high population density, though the underlyi ng mechanisms remain unclear.