The squirrel monkey eyes are close together, the big ears are clearly visible, the long tail is unable to grasp the branches of the trees. The top of the head is oblong, dark (black in ordinary squirrel monkey, olive-gray in red-headed squirrel monkey), back greenish-gray or red (in red-headed squirrel monkey), limbs reddish-yellow, belly light, tip of tail black. The brain is small (weight 24 g), smooth, without convolutions, but, nevertheless, its relative weight is considerable. This is the brain of the present primate, suitable for electro-physiological studies.
Squirrel monkey is found in the forests of South America (from Costa Rica to Paraguay and from the Atlantic coast to the Cordillera). They live in large groups of up to 100 or more individuals along the banks of rivers. Food for them are fruits, flowers, nuts, plant buds, berries, but the main food is arthropods, chicks and small animals. Groups of squirrel monkeys often break up and form again, sometimes same-sex communities of these monkeys arise.
And the behavior of squirrel monkey varies depending on the size of this group. In one of the groups there were 23 red-headed Squirrel monkeys; it consisted of two adult males (one of them in the last stage of puberty), six adult females, five toddlers, seven young adolescents, two immature males. Every day this group moved along trees at an altitude of 3-12 m at a distance of about 2.5-4.2 km, sometimes they all descended to the ground and walked on two legs.
In the behavior of squirrel monkey some features are noted. So, when they meet, they touch their mouths, as if kissing, and when they establish domination, they show their sexual organs. A male of a higher rank rolls on the back on the floor than, probably, confirms his higher position and in such an unusual way gathers around himself monkeys of lower ranks. The herd hierarchy is dominated by adult males, but there are no individual leaders in squirrel monkeys. Adult monkeys dominate the rest, but between different sexes and young growth the relationship is quite stable.
Squirrel monkey is widely used in communicating a rich set of sound signals. Depending on the situation, these monkeys can scream, squeak, trash and even cackle.