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The Callitrichidae (previously called called Hapalidae/Hapalinae) is a family of New World monkeys, including marmosets and tamarins. At times this group of animals has been regarded as a subfamily, called Callitrichinae, of the family Cebidae.
This taxon was traditionally thought to be a primitive lineage, from which all the larger bodied platyrrhines evolved. However, some works argue that callitrichids are actually a dwarfed lineage. Ancestral stem-callitrichids would likely have been a "normal" sized ceboids that was dwarfed through evolutionary time. This may exemplify a rare example of insular dwarfing in a mainland context, with the "islands" being formed by biogeographic barriers during arid climatic periods when forest distribution became patchy, and/or by the extensive river networks in the Amazon Basin.
All callitrichids are arboreal. They are the smallest of the simian primates. They eat insects, fruit, and the sap or gum from trees; occasionally they will take small vertebrates. The marmosets rely quite heavily on exudates, with some species (e.g. Callithrix jacchus and Cebuella pygmaea) considered obligate exudativores.
Callitrichids typically live in small, territorial groups of about 5 or 6 unrelated animals, primarily adults. The groups are multimale-multifemale, and the number of adults of each sex is highly variable. Their social organization is unique among primates and is called "cooperative polyandry". In this communal breeding system, only one female is reproductively active in a group. Females may mate with more than one male. Care for the young of a group's breeding female is principally provided by adult males, an organization referred to as a "communal breeding system". There is a correlation between the number of males in a group and the number of surviving young.[7,8]
Callitrichids are the only primate group that regularly produce twins, which constitute over 80% of births in species that have been studied. Unlike other male primates, male callitrichids generally provide as much parental care as females. Parental duties may include carrying, protecting, feeding, comforting, and engaging in play behavior with offspring. Males generally provide care for the young. For example, in the cotton-top tamarin (Saguinus oedipus), males have been found to demonstrate a greater involvement in caregiving than females, particularly paternal males. Typical social groups seem to constitute breeding groups, with several previous offspring living in the group and providing significant help in rearing the young.