The thumb is not opposable. The hand and the foot are elongate, and all the digits bear pointed, sickle-shaped nails, except the great toe, which has a flat nail. A baculum is present in the males, and the females have two mammae in the chest region. Unlike squirrels, which range through the forest canopy mainly by ascending and descending large vertical trunks, callitrichids move by running quadrupedally along horizontal branches and leaping between thin terminal supports. Squirrels are adapted to feed on hard nuts, while callitrichids eat mainly insects, soft fruits, nectar, and plant exudates.
Marmosets have acute sight, good hearing, and apparently a good sense of smell.
Facial expression is indicated mainly by lip movements. Emotions are expressed by movements of the eyelids, ears, and the hairy adornments on the head if present. The voice has a variety of high-pitched trilling and staccato calls. Marmosets are diurnal, sheltering at night in tree holes and cavities. Most of their time is spent in trees or shrubs, though Callithrix argentata in the Matto Grosso sometimes goes into tall grass.