Saimiri boliviensis (I.Geoffroy Saint-Hilaire & de Blainville, 1834)

Common Name

This species, Saimiri boliviensis, is commonly known as the black-capped squirrel monkey.

Adult Weight

It shows clear sexual dimorphism: adult males typically weigh between 550 and 1,135 g (19.4 and 40.0 oz), while adult females weigh between 365 and 750 g (12.9 and 26.5 oz).

Newborn Weight

Newborn infants usually weigh between 80 and 140 g (2.8 and 4.9 oz).

Body Length

Head-to-tail-base length for adults ranges from 250 to 370 mm (9.8 to 14.6 in) for males and 225 to 295 mm (8.9 to 11.6 in) for females.

Tail Characteristics

The species' tail measures around 350 to 425 mm (13.8 to 16.7 in), matches the body's color, has a black tufted tip, and is not prehensile.

Coat Texture

The coat is short, soft, and dense.

Body Coloration

Most fur on the back ranges from grey to olive-brown, while the undersides are typically white, yellow, or ochre.

Head Coloration

The head is characteristically black, with white arches over the eyes.

Distinguishing Features Overview

Black-capped squirrel monkeys are physically very similar to many other squirrel monkey species, but can be distinguished by several unique features.

Eye Arch Pattern

The most noticeable are its dark black cap and the white 'Roman type' arch pattern over the eyes, which is narrower and more rounded than the 'Gothic type' eye arch pattern found on other squirrel monkey species.

Tail Width Comparison

The tails of 'Roman type' species, including this one, are also narrower than those of 'Gothic type' species.

Geographic Range

Black-capped squirrel monkeys with the characteristic 'Roman arch' facial pattern have been recorded across most of Bolivia, northern Peru, and the area between the Juruá and Purus Rivers in Brazil.

Habitat Type

This species lives in lowland tropical rainforests near water, in densely forested and swampy regions.

Arboreal Behavior

It is predominantly arboreal; while it uses all forest levels, it has been observed to stay mostly in lower canopies for traveling and foraging.

Matrilineal Troop Formation

It has been proposed that matrilineal troops of Saimiri boliviensis form because their habitat has an abundance of fruit and insects, a resource level not seen in the habitats of other Saimiri genus species.

Sexual Maturation Age

Females of this species typically reach sexual maturation around 3 years of age, while males reach it around 5 years of age.

Reproductive Cycle Drivers

Multiple researchers have observed that the yearly reproductive cycle of mature individuals is affected by several environmental factors, including rainfall cycles and illumination levels in their habitat.

Mating Season Timing

Mating season occurs during the dry season, and usually produces a single infant per mother.

Mating Season Cues

It has been suggested that adult females are more responsive to environmental cues that signal the start of mating season, and male response is partially attributed to behavioural and scent cues from females.

Male Mating Season Fat Gain

During mating season, males gain a large amount of subcutaneous upper body fat, resulting in a distinct 'fatted' appearance.

Testosterone Level Comparison

It has also been observed that testosterone levels in Bolivian and Peruvian populations of this species are noticeably higher during mating season than those of male Saimiri from Guayana.

Male Mating Season Behavior

Males become more irritable and aggressive, fight more frequently to establish dominance within the troop, and perform genital displays toward less dominant males.

Troop Hierarchy Fluctuation

Observations in both natural and laboratory settings show that troop hierarchy can change as often as three times per month.

Hierarchy Change Outcomes

These changes often lead to highly aggressive fights that may result in younger adult males being fully excluded from the troop.

Mating Approach Cues

A male uses female scent and behavioural cues to judge whether a female will be receptive to his mating approach.

Female Rejection Behavior

If the female is not receptive, she will usually chase the male away, sometimes with help from other nearby females.

Copulation Duration

Consorting and copulation can last between one minute and over an hour, depending on the presence of other monkeys and the surrounding environment.

Gestation Period

The gestation period is estimated to last between 160 and 170 days.

Newborn Care

During the first week after birth, an infant clings to its mother's back, rarely moves, and is only attended to by the mother when it needs assistance.

Diet Classification

Black-capped squirrel monkeys are omnivorous.

Diet Components

Their typical diet includes fruits, insects, eggs, small vertebrates, arachnids, leaves, flowers, nuts, and seeds; fungi are consumed rarely.

Diet Preference

They have been observed to prefer insects over fruit.

Foraging General Pattern

Mature individuals spend most of the day foraging.

Early Morning Foraging

They begin foraging 40 to 60 minutes before sunrise, spending the first part of the day actively feeding on fruits and any insects they catch while foraging for fruit.

Post-Morning Foraging

They then shift to slower feeding behaviors, spending the rest of the day resting and hunting for additional insects.

Midday Rest Behavior

When food is not scarce, they will stop to rest for an hour or two around midday when temperatures are too hot for continued activity.

Dry Season Fungal Consumption

When fruits are scarce during dry seasons, squirrel monkeys have been reported to consume Ascopolyporus, a fungus that parasitizes scale insects, which themselves parasitize local bamboo species.

Fungal Consumption Hypothesis

Researchers hypothesize this fungus acts as an alternative food source during dry seasons.