Marmosets are New World monkeys of the genus Callithrix, which contains 18 species. The term marmoset is also used in reference to the Goeldi's Marmoset, Callimico goeldii, which is not part of the genus Callithrix and is not discussed in this article.
Most marmosets are about 20 cm long. Relative to other monkeys, they show some apparently primitive features: they have claws rather than nails, and tactile hairs on their wrists. They lack wisdom teeth, and their brain layout seems to be relatively primitive. Their body temperature is unusually variable, changing by up to 4 degrees Celsius (7 degrees Fahrenheit) in a day.
Marmosets are highly active, living in the upper canopy of forest trees, and feeding on insects, fruit and leaves. They have long lower incisors, which allow them to chew holes in tree trunks and branches to harvest the gum inside; some species are specialised feeders on gum.
Marmosets live in family groups of 3 to 15, consisting of one to two breeding females, an unrelated male, their offspring and occasionally extended family members and unrelated individuals. Their mating systems are highly variable and can include monogamy, polygyny and occasionally polyandry. In most species, fraternal twins are usually born, but triplets are not unknown. Like other callitrichines, marmosets are characterized by a high degree of cooperative care of the young and some food sharing and tolerated theft. Adult males, females other than the mother, and older offspring participate in carrying infants. Most groups scent mark and defend the edges of their ranges, but it is unclear if they are truly territorial, as group home ranges greatly overlap.