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Squirrel monkeys
Common squirrel monkey
Saimiri sciureus
Scientific classification
Kingdom: Animalia
Class: Mammalia
Order: Primates
Family: Cebidae
Genus: Saimiri
Linnaeus, 1758

Squirrel monkeys live in Central and South America. They grow to 25 to 35 cm, and their long tail is 35 to 42 cm. long. Squirrel monkeys are small enough to go on the smallest branches to get food. They eat fruits and insects. When fruit is scarce, they will drink nectar. Large falcons and snakes eat them so they have lots of different warning calls.

Description

A squirrel monkey's fur is short and close, coloured black at the shoulders and yellowish orange on its back and extremities. The upper parts of their heads are hairy. This black-and-white face gives them the name "death's head monkey" in several Germanic languages (e.g., German Totenkopfaffen, Swedish dödskalleapor, Dutch doodshoofdaapjes) and Slovenian (smrtoglavka).

Squirrel monkeys grow from 25 to 35 centimetres (9.8 to 13.8 in) long, plus a 35 to 42 centimetres (14 to 17 in) tail. Male squirrel monkeys weigh 750 to 1,100 grams (26 to 39 oz). Females weigh 500 to 750 grams (18 to 26 oz). Both males and females are equipped with long and hairy tails, flat nails, and pointed claws.

Female squirrel monkeys have pseudo-penises, which they use to display dominance over smaller monkeys, in much the same way that the male squirrel monkeys display their dominance.

Behaviour and ecology

Like most of their New World monkey relatives, squirrel monkeys are diurnal and arboreal. Unlike 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 can be very rapid.

Squirrel monkeys live together in multi-male/multi-female groups with up to 500 members. These large groups, however, can occasionally break into smaller troupes. The groups have a number of vocal calls, including warning sounds to protect the group from large falcons, which are a natural threat. Their small body size also makes them susceptible to predators such as snakes and felids. 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 seeds, leaves, flowers, buds, nuts, and eggs.

Squirrel monkey mating is subject to seasonal influences. Females give birth to young during the rainy season, after a 150- to 170-day gestation. Only the mothers care for the young. Saimiri oerstedti are weaned by 4 months of age, while S. boliviensis are not fully weaned until 18 months old. Female squirrel monkeys reach sexual maturity at age 2–2.5 years, while males take until age 3.5–4 years. They live to about fifteen years old in the wild, and over twenty years in captivity. Menopause in females probably occurs in the mid-teens.

Colour vision

Colour vision in squirrel monkeys has been extensively studied as a stand-in for human ailments. In humans, two genes for colour vision are found on the X chromosome. Typically, one gene (OPN1LW) produces a pigment that is most sensitive to the 564 nm wavelength, while the other gene (OPN1MW) produces a pigment most sensitive to 534 nm. In squirrel monkeys, there is only one gene on the X chromosome but it exists in three varieties: one is most sensitive to 538 nm, one to 551 nm, and one to 561 nm. Since males have only one X chromosome, they are dichromatic, although with different sensitivities. Females have two X chromosomes, so some of them can have copies of two different alleles. The three alleles seem to be equally common, leading to one-third of females being dichromatic, while two-thirds are trichromatic. Recently, gene therapy has given the human OPN1LW gene to adult male squirrel monkeys, producing behaviour consistent with trichromatic colour vision.

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