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. They are active and agile, running, jumping, and occasionally leaping among trees and shrubs. Leontopithecus is said to bound from tree to tree with remarkable rapidity. Movements are usually quick and jerky. When at rest, marmosets sometimes draw the fingers inward, so that the clawlike nails pierce the bark of the support, and rest with the belly in contact with the branch, all four limbs hanging on either side.