The hand and foot are elongated, and all digits bear pointed, sickle shaped nails, except the great toe, which has a flat nail. There are long circumauricular tufts, in front of, and often above and behind, the base of the ears. Black ear-tufted marmosets are diurnal, sleeping at night in tree holes or other shelters. They live in family groups of 4-15 individuals, having overlapping home ranges and do not defend territories. Dominance in the group is maintained through aggressive interaction.
Marmosets are usually docile and gentle in captivity.
Sudden noises or movements sometimes cause them to panic. Marmosets run quadrupedally along horizontal branches and leaping between thin terminal supports. They have acute sight, good hearing, and good smell. Facial expressions are indicated mainly by lip movements. Marmosets are fastidious in the care of their pelage and engage in both individual and mutual grooming, the former being more common.