marmoset, name for many of the small, squirrellike New World monkeys of the family Callithricidae. Members of this family are all found in tropical South America, with one species found also in Central America. They range in size from the pygmy marmoset, which is 8 in. (20 cm) long including the tail and weighs 3 oz (85 g), to species about the size of house rats. Many of the larger species are called tamarins. Most marmosets and tamarins are brightly colored, and many are ornamented with manes, ear tufts, or mustaches. Their tails are long and furry. Day active, gregarious animals, they scurry through trees and chatter in shrill voices. They feed on plant matter as well as on insects and other small animals. Females usually bear twins, and it is claimed that in some species the male takes a large part in the care of the young. Most spectacular is the golden lion marmoset, with flaming, golden fur and a luxuriant mane. Marmosets have long been valued as pets. They are classified in the phylum Chordata, subphylum Vertebrata, class Mammalia, order Primates, family Callithricidae.

Any species of arboreal, diurnal, long-tailed South American monkey (family Callitrichidae) classified in two groups: eight species with short tusks (lower canine teeth), called marmosets, and 25 with long tusks, called tamarins. Marmosets move in a quick, jerky manner and eat insects and sometimes fruit and small animals. Members of the common marmoset genus Callithrix are 6–10 in. (15–25 cm) long, excluding the 10–16-in. (25–40-cm) tail. The dense, silky fur is white, reddish, or blackish; the ears are generally tufted. Marmosets have been kept as pets since the early 17th century.

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