any of numerous species of small long-tailed South American monkeys (monkey). Similar in appearance to squirrels, marmosets are tree-dwelling primates (primate) that move in a quick, jerky manner. Claws on all the digits except the big toe aid them in scampering along branches, where they primarily eat insects in addition to fruit, tree sap, and other small animals. Marmosets are active during the day and live in small groups. The gestation period is four to six months, depending on species; twins are the norm, with single births being about as common as triplets. Marmosets have been kept as pets since the early 17th century, but they require knowledgeable care to remain healthy.

The “true” marmosets (genus Callithrix) have short lower canine teeth (short-tusked), whereas marmosets with relatively long lower canines (long-tusked) are known as tamarins (genera Saguinus and Leontopithecus). The pygmy marmoset (C. pygmaea) is the smallest “true” marmoset and lives in the rainforests of the Amazon River's upper tributaries. The length of the head and body of the pygmy marmoset is about 14 cm (6 inches), and the tail is somewhat longer. Adults weigh only about 90 grams (3 ounces), whereas other species of the family attain 600 grams (1.3 pounds) or more. The common marmoset (C. jacchus) lives in the scrub forest (caatinga) of northeastern Brazil. Weighing 400 grams, it is about 15–25 cm long, excluding the 25–40-cm tail. The marbled brown-and-white fur is dense and silky, and there are white tufts on the ears and black-and-white rings on the tail. Five Callithrix species live in different tropical forests (tropical rainforest) along Brazil's Atlantic coast. In the rainforests south of the Amazon River, there may be a dozen or more additional species—three were discovered in the 1990s, and several others were awaiting description; these vary widely in colour and in the amount of fur on the ears. The short canine teeth and long lower incisors of these marmosets are used to gnaw at tree bark and leave characteristic notches from which sap flows. “True” marmosets breed in monogamous pairs and live in a social organization in which the older young assist in feeding, carrying, and educating the infants. The presence of a breeding pair suppresses the sexual development of the young of both sexes until they leave the group.

Lion tamarins (genus Leontopithecus) are named for their thick manes, and all four species are endangered (endangered species), three of them critically; one (L. caissara) was first discovered in 1990. Lion tamarins are larger than “true” marmosets and have long, slender hands and fingers, which they use to hook insects from crevices. The golden lion tamarin (L. rosalia), found only in fragmented forest habitats in the Brazilian state of Rio de Janeiro, is particularly striking, with a thick mane, black face, and long silky, golden fur. The fur of the other three species is partly black. Lion tamarins appear to have a social organization similar to that of “true” marmosets, but reproductive suppression appears to be behavioral rather than physiological, and some tamarins appear to tolerate a polyandrous system in which two males share in the rearing of infants of a single female.

There are at least 12 species in the tamarin genus Saguinus. Although they lack the manes of lion tamarins, some have notable features. The emperor tamarin (S. imperator) of the southwestern Amazon basin, for example, has a long white mustache complementing its long grizzled fur and reddish tail, whereas the mustached tamarin (S. mystax) has a small white upswept mustache. The cotton-top tamarin (S. oedipus), found in Colombia and Panama, has a scruffy white crest of hair on the top of its head. The golden-handed tamarin, S. midas, is named for the mythological Greek king.