Saimiri boliviensis

Geographic Distribution and Habitat

Black-capped squirrel monkeys, also known as Bolivian squirrel monkeys, Peruvian squirrel monkeys, and black-headed squirrel monkeys, are endemic to Brazil, Bolivia, and Peru.

These New World monkeys can adapt to several habitat types including disturbed and edge forests. However, they are found primarily in lowland tropical rain forests and gallery forests throughout the Amazon basin. Within these habitats, black-capped squirrel monkeys are typically arboreal, staying in the canopy. Occasionally, they leave the canopy to visit the shrub layer below to scavenge for food.

Black-capped capuchin range, IUCN 2008
Size, Weight, and Lifespan

Females are smaller in size than males, measuring from slightly under 9 inches to almost 12 inches (22.3–29.5 cm) long. Females weigh between 0.88 and 1.6 pounds (0.4–0.72 kg) and have tails longer than their bodies at 14.5-17.5 in (36.8–44.5 cm). Males range from 9.8 to 14.6 inches (24.9–37.0 cm) long, weighing a minimum of over 1 to 2.5 pounds (0.54–1.13 cm), with tail lengths from 14.5 to 18.5 inches (36.8–47 cm).

Bolivian squirrel monkeys live between 20–30 years in captivity and typically live between 15–20 years in the wild.


Squirrel monkeys are all somewhat similar in appearance. Black-capped squirrel monkeys have dense, short, yellowish-tan hair, and a dark “crown” atop their heads. Sexual dimorphism is evident in the size and color of crown hair: it is gray in males and black in females. The hair on the underside of their limbs varies from yellow to orange to white.

White markings on the faces of squirrel monkeys create a mask-like design around the cheeks and eyes that are characterized as either “Roman” or “gothic” arches. A key defining feature of the black-capped squirrel monkey from other squirrel monkey species is their “Roman” arched brows, which are more rounded than the “gothic” arch and do not extend as high. Guianan squirrel monkeys (Saimiri sciureus), by comparison, have “gothic” arched brows, which are characterized by high white foreheads and a dark “v” between their eyes.

In general, squirrel monkeys have a short and blunt snout covered in dark hair. Ears are white and tufted and are relatively large compared to their head. Their tail has a black tip and is almost twice the length of the body. Black-capped squirrel monkeys and Guianan squirrel monkeys have noticeably thinner tails than the black squirrel monkey (Saimiri vanzolinii). Although the tail is non-prehensile, they have very dexterous fingers to help grasp onto branches, vines, and prey. They even possess a pseudo-opposable thumb, which assists them to grip (specifically between the thumb and index finger).

Males and females are seasonally sexually dimorphic, with apparent physical differences between the sexes only at certain times of year. Males fluctuate in weight throughout the year and gain weight two months prior to breeding season, taking on a “fatted” appearance from water and fat stored between the muscles and skin on their head, shoulders, humerus (upper arm), and ribs. During this period males can gain up to 20% of their body weight. The larger they appear, the more attractive they are to females.


Squirrel monkeys are mainly insectivores and frugivores, consuming fruits, nuts, flowers, seeds, leaves, insects, and arachnids. They occasionally consume eggs and small vertebrates, such as small birds and bats. Often, they forage on terminal branches in large groups, which increases their ability to disturb and capture insects. However, they rarely chase these insects and would rather capture stationary insects on plant surfaces.

Behavior and Lifestyle

Black-capped squirrel monkeys are mainly active during the day, making them diurnal. They are also arboreal and usually stay in the canopy moving quadrupedally through branches that are between 0.4 and 0.8 in (1–2 cm) in diameter. Occasionally they descend to the forest floor and forage bipedally. In addition, squirrel monkeys are avid leapers, but rarely leap horizontal distances greater than 6.5 ft (2 m).

Despite being considered “nervous primates,” they are very social and even establish dominance hierarchies. Males establish their dominance through fighting and urine-washing. Urine-washing is when an individual urinates on his or her hands, feet, and body, making sure wherever they travel it will leave a trace.

Black-capped squirrel monkeys participate in high levels of social play. In fact, they may participate in this behavior more so than other squirrel monkey species. Social play can vary by environment type. In some environments, young squirrel monkeys play up to three hours a day. In other cases, groups exhibit limited social play.

Play is common between immature individuals as well as between a mother and her offspring. Some adults play with other adults, which is rarely found in nature among other primates.

Fun Facts

Squirrel monkeys have the largest brains in proportion to their bodies of all primate species. 

Black-capped squirrel monkeys are one of the most highly prized species in the exotic pet trade.

Squirrel monkeys usually urinate on their hands to mark their territory; however, it also helps to control their body temperature.

Due to their small size, they are susceptible to many predators such as wild cats, hawks, and snakes.

Daily Life and Group Dynamics 

Squirrel monkeys, especially the black-capped species, live in multi-male groups; however, larger groups may be multi-male/multi-female. Groups can range from 10 to 75 individuals, with an average size of 30–40 individuals. Black-capped squirrel monkeys travel in the largest groups of any squirrel monkey species with as many as 300 individuals in a group.

Males emigrate from their natal groups at sexual maturity (at around 3–4 years), while females are philopatric—they remain in their natal groups throughout their lives. When males disperse from their natal group, they form all-male bachelor groups. These males form alliances and work together to take over the highest positions in their new groups. The alliances remain strong if the males stay within the same group. At around 5 years old, males compete for dominance and join multi-male/multi-female groups. Aggression often occurs towards immigrating males. Males become increasingly aggressive with one another during mating season.

Females form stable, kin-based groups. They are dominant over all males within a group. Female black-capped squirrel monkeys spatially segregate males to the perimeter of the group through agonistic encounters and display aggressive behaviors to other females, except those who are their relatives.

The distribution of available resources affects the levels of intragroup (within group) feeding competition. Black-capped squirrel monkeys forage in the largest fruit patches with the highest densities of fruit. Where there are higher densities of fruit, within-group competition increases, since group members are motivated to protect the patch of food they are searching. Between-group aggression is not common in black-capped squirrel monkeys, but is more so found in the common squirrel monkey.


One of the most vocal squirrel monkey species is the black-capped squirrel monkey. Vocalizations help to maintain the social organization of troops. Scientists and researchers have found 26 identifiable calls including chirps, peeps, squawks, purrs, barks, and screams. Chirps and peeps are used as alarm calls; squawks and purrs are used during mating and birthing season; barks are aggression calls; and screams are used when an individual is in pain.

Olfactory signaling is another form of communication shared between squirrel monkeys in a social group. This includes urine washing, which releases pheromones by females and males during mating and birthing seasons.

Tactile communication consists of grooming as well as play and huddling.

Visual cues are made to assert dominance or reduce tension between individuals. These include facial expressions (associated with fear or aggression) and penile display (considered a dominance gesture).

Reproduction and Family

Black-capped squirrel monkeys are polygynandrous, meaning females and males have multiple partners. Specific to this species, one or two males in the group typically have more frequent copulation opportunities than the other males. During mating season, males with the highest rank (most dominant) get to interact and mate with the females. Once females reach sexual maturity, around 2.5–3 years old, they begin to mate. They may mate with several males during the three-month mating season. When the mating season ends, females once again segregate the males to the periphery of the group.

Within a group, females other than the mother will assist in raising young. This is known as “aunting,” where mothers cooperate with other females to look after their young. Aunting behavior consists of carrying an infant on their back, retrieval, and cleaning. Aunts can be any female in the group; however, they are usually females who the mother spent a lot of time with prior to birthing or a female the mother previously acted as an aunt for. This aunting behavior is thought to maintain group relationships over long periods of time.

In each breeding season, a female has one offspring. The gestation period (average length of pregnancy) lasts between 152 and 172 days. Weaning—or when the mother stops nursing her young—occurs between four to six months after birth.

Female black-capped squirrel monkeys invest most of their time taking care of their young. After birth, the mother protects her offspring and provides entirely for him or her. Some responsibilities include: carrying, cleaning, retrieving, and nursing their young.

​Ecological Role

Due to their diet, black-capped squirrel monkeys play various important roles in the ecosystem. By consuming fruit, they act as agents for seed dispersal throughout the forest. They also consume insects, which keeps the insect population under control.

Conservation Status and Threats

Black-capped squirrel monkeys are listed as Least Concern by the International Union for Conservation of Nature, Red List of Threatened Species (IUCN, 2015), however, their populations are decreasing. Threats such as hunting for subsistence and capturing them for the pet trade or medical research occur frequently in Peru. Recently, there was a large trade in squirrel monkeys in the United States—for both biomedical research and entertainment, including zoos and pet markets. Agricultural threats (farming, livestock farming, ranching, logging) and transportation threats (building roads and railroads) severely affect the ecosystems in which they live due to habitat destruction by mass clearing forests.

Conservation Efforts

This species is listed on Appendix II of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES). They are found in many protected areas throughout Bolivia, Brazil, and Peru. 

In order to help protect black-capped squirrel monkeys and their habitats, land management, water management, and site management are all needed. Additional research and monitoring this species’ population size, distribution, and trends will help determine the best conservation techniques to be used.

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  • Napier, J., P. Napier. 1967. A handbook of living primates. London: Academic Press.
  • Nowak, R. 2000. Walker’s Primates of the World. Baltimore: John Hopkins.
  • Baldwin, J.D., 1985. The Behavior of Squirrel Monkeys (Saimiri) in Natural Environments. Handbook of Squirrel Monkey Research. 35-53.

Written by Tara Covert, July 2019