In Callitrichidae, reproduction in subordinate females is generally inhibited but occurs in rare cases, possibly in association with the presence of an unrelated male, important food resources or low dispersal opportunities. This study investigates the occurrence of groups with multiple breeding females in captive golden-headed lion tamarins (Leontopithecus chrysomelas), the factors leading to their formation and the consequences for the group. Information obtained from studbook data on the world captive population during 1984-1998 revealed that polygynous groups in captivity are very rare: only 7 cases were discovered. Family groups in which daughters started breeding with a related male were larger than average, had a high number of sexually mature sons and eldest offspring that were well past the age of sexual maturity. Following a breeding attempt, severe aggression frequently occurred, especially if the infants were liveborn. Polygynous groups composed of two related females and an unrelated male tended to remain stable for a longer period than families with breeding daughters. Competition for infant care is probably an important factor determining whether the polygynous situation can persist and for how long. Copyright (C) 2001 S. Karger AG, Basel.