The black-capped squirrel monkey (Saimiri boliviensis) is a species of New-World monkey native to the upper Amazon basin in Bolivia, western Brazil and eastern Peru. They weigh between 365 and 1135 grams and measure, from the head to the base of the tail, between 225 and 370mm. Black-capped squirrel monkeys are primarily tree-dwelling and are found in both native and plantation forests as well as some farmed areas near running water. Their diet is omnivorous and mostly consists of flowers, fruit, leaves, nuts, seeds, insects, arachnids, eggs and small vertebrates. They mostly live in female-dominated troops of around 40 to 75 monkeys, with males having been observed to disperse to live in all-male troops after reaching sexual maturation. Their current conservation status according to the IUCN is 'Least Concern'. The species belongs to the genus Saimiri and has two subspecies, S. b. boliviensis (the Bolivian squirrel monkey) and S. b. peruviensis (the Peruvian squirrel monkey). Description The black-capped squirrel monkey displays sexual dimorphism, with males normally weighing between 550 and 1135 grams and females weighing between 365 and 750 grams. Infants typically weigh between 80 and 140 grams when they are born. Adults of the species measure in length (from the head to the base of the tail) between 250 and 370mm for males and 225 and 295mm for females. The coat of the monkey is short, soft and dense, and the majority of the fur covering the back of the monkey is a grey to olive-brown hue, while the undersides are typically white, yellow or ochre. The head is characteristically black with white arches over the eyes. The tail is the same colour as the body with a black tufted tip and is not prehensile; it usually measures around 350 to 425mm. Physically, the black-capped squirrel monkey is very similar to a number of other species of squirrel monkey, but is distinguishable from other species by a number of features. The most noticeable of these are the dark black cap and the white ‘Roman type’ arch pattern over the monkey's eyes, which is more narrow and rounded than the ‘Gothic type’ arch pattern over the eyes of the other species. The tail of the ‘Roman type’ species is also narrower than that of the ‘Gothic type’. Evolution Saimiri boliviensis is thought to have diverged from the Saimiri genus approximately 1.5 million years ago. It has been hypothesised that this