[4][5] Squirrel monkeys live in the tropical forests of Central and South America in the canopy layer.

Most species have parapatric or allopatric ranges in the Amazon, while S. oerstedii is found disjunctly in Costa Rica and Panama.

[8] Squirrel monkeys have determined breeding seasons which involve large fluctuations in hormones and there is evidence of sexual dimorphism between males and females.

[6][14] Until 1984, all South American squirrel monkeys were considered part of a single widespread species, and many zoologists considered the Central American squirrel monkey to be a member of that single species as well.

[17][18][19][20] The crown group of the extant squirrel monkeys appears to have diverged around 1.5 million years ago.

[21] Diversification of squirrel monkey species appears to have occurred during the Pleistocene Epoch, likely due to climatic changes associated with interglacial periods in South America at the time.

The origin of squirrel monkeys in Central America is unclear, but a possible hypothesis is human transport.

[clarification needed] A squirrel monkey's fur is short and close, coloured black at the shoulders and yellowish orange on its back and extremities.

Both males and females are equipped with long and hairy tails, flat nails, and pointed claws.

Unlike other New World monkeys, their tail is not used for climbing but as a kind of "balancing pole" and also as a tool.

These size changes are caused by seasonal fluctuations in androgen hormones synthesized in the hypothalamus, pituitary, adrenal and gonadal axes.

The fluctuations include increases in the concentrations of testosterone, androstenedione, and dehydroepiandrosterone levels in sexually mature males during the breeding season, peaking in January.

Sweating in these areas alone does not provide enough cooling for the monkeys to survive in the high temperature environments of South and Central America, requiring them to use other methods to thermoregulate.

They will use behavioral tactics such as seeking out shaded areas sheltered from the sun and also make use of postural changes to better dissipate heat from their body.

When reaching approximately 95% humidity, the monkeys have more drastic changes in osmoregulation in order to maintain homeostasis.

Studies have been done to suggest that female squirrel monkeys show disadvantageous inequity aversion as it pertains to food rewards.

[25] Colour vision in squirrel monkeys has been extensively studied as a stand-in for human ailments.

The three alleles seem to be equally common, leading to one-third of females being dichromatic, while two-thirds are trichromatic.