and the Osa Peninsula area of Costa Rica (including Corcovado National Park), while S. o. citrinellus lives in the Central Pacific portion of Costa Rica. The largest estimate (most recently in 2003) is that the remaining wild population of S. o. citrinellus is only 1,300 to 1,800 individuals. Physical description The Central American squirrel monkey differs in coloration from South American squirrel monkeys. While South American squirrel monkeys tend to be primarily greenish in color, the Central American species has an orange back with olive shoulders, hips and tail, and white undersides. The hands and feet are also orange. There is a black cap at the top of the head, and a black tip at the end of the tail. Males generally have lighter caps than females. The face is white with black rims around the eyes and black around the nose and mouth. The two subspecies are similar in coloration, but differ in the shade of the cap. The northern subspecies, living in Central Pacific Costa Rica, has a lighter cap than the southern subspecies, which lives in Panama and in parts of Costa Rica near Panama. The southern subspecies also has more yellowish limbs and underparts. Adults reach a length of between , excluding tail, and a weight between . The tail is longer than the body, and between in length. As with other squirrel monkeys, there is considerable sexual dimorphism. On average, males weigh 16% more than females. Males have an average body weight of and females average . Squirrel monkeys have the largest brains of all primates relative to their body size; the Central American squirrel monkey's brain weighs about , or about 4% of its body weight. Unlike larger relatives, such as the capuchin, spider and howler monkeys, Central American squirrel monkeys do not have a fully prehensile tail, except as newborn infants, and the tail is primarily used to help with balance. Behavior Social structure The Central American squirrel monkey is arboreal and diurnal, and most often moves through the trees on four legs (quadrupedal locomotion). It lives in groups containing several adult males, adult females, and juveniles. The group size tends to be smaller than that of South American squirrel monkeys, but is still larger than for many other New World monkey species. The group generally numbers between 20 and 75 monkeys, with a mean of 41 monkeys. Groups in excess of 100