KINGDOM = Animalia PHYLUM = Chordata SUBPHYLUM = Vertebrata CLASS = Mammalia ORDER = Primates FAMILY = Cebidae GENUS = Saimiri SPECIES = Saimiri sciureus Squirrel monkeys grow to 25 to 35 centimetres, plus a 35 to 42 centimetres tail. Squirrel monkeys weigh 750 to 1100 grams. Remarkably, the brain mass to body mass ratio for squirrel monkeys is 1:17 which gives them the largest brain, proportionately, of all the primates. Humans have a 1:35 ratio. Female squirrel monkeys have a pseudo-penis that they use to display dominance over smaller monkeys, much like the way the male squirrel monkeys display their dominance. Like most of their New World monkey relatives, squirrel monkeys are diurnal and arboreal. Unlike the other New World monkeys, their tail is not used for climbing, but as a kind of ‘balancing pole’ and also as a tool. Their movements in the branches are extremely fast. Squirrel monkeys live together in multi-male/multi-female groups with up to 500 individuals. These large groups can, however, occasionally break into smaller troops. Squirrel monkeys have a number of vocal calls, including warning sounds to protect themselves from large falcons, which are a natural threat to them. Squirrel monkeys small body size also makes them susceptible to predators such as snakes and big cats. For marking territory, squirrel monkeys rub their tail and their skin with their own urine. Squirrel monkeys are omnivores, eating primarily fruits and insects. Occasionally they also eat nuts, buds, eggs and small vertebrates. The mating of the squirrel monkeys is subject to seasonal influences. Females give birth to young during the rainy season, after a 150 – 170 day gestation period. The mothers exclusively care for the young. Squirrel monkeys are weaned by 4 months of age, while other species are not fully weaned until 18 months old. Female squirrel monkeys reach sexual maturity at age 3 years, while males reach sexual maturity around age 5 years. Squirrel monkeys live to about 15 years old in the wild, about 20 years in captivity.