BLACK SQUIRREL MONKEY
Geographic Distribution and Habitat
Black squirrel monkeys (Saimiri vanzolinii) are small primates endemic to the central Amazon in Brazil. They live in várzea forests, which are lowland floodplain areas along the Amazon River and its tributaries. Várzea forests also experience seasonal flooding that creates wet grasslands and floating meadows.
Also known as black-headed squirrel monkeys, they travel through all levels of the forest but primarily forage in the lower canopy. They don’t often venture to the higher levels of the forest canopy to avoid the risk of being seen by predators.
With an area occupancy of 336 square miles (870 km²), black squirrel monkeys have one of the smallest geographic distribution ranges of all South American primates. Their population is geographically split into three areas: the Mamirauá Sustainable Development Reserve, and the Capucho and Tarará islands. Even in their limited ranges, black squirrels monkey occur in high population densities ranging from 293 to 439 individuals per .4 square miles (1.0 km²).
Size, Weight, and Lifespan
Both male and female black squirrel monkeys are similar in appearance, but males are usually bigger. Their bodies grow to lengths between 11 and 13 inches (27–32 cm). Their tails are longer than their small bodies and grow to an average of 16 inches (41 cm).
Seasonally, the monkeys are sexually dimorphic. To appear more fattened and healthy, males’ weights increase two months before the breeding season begins, which makes them more attractive to females. Typically, males weigh between 1.4 and 2.7 pounds (649–1225 g) and females weigh between 1.4 and 1.9 pounds (649–898 g).
Monkeys in the genus Saimiri can live up to 15 years in the wild and up to 30 years in captivity.
Black squirrel monkeys are sexually dimorphic, with males being larger than females. At a quick glance, you’ll notice that black squirrel monkeys have small bodies and extremely long tails that are often curled over their shoulders. Their tails are used for balance while travelling throughout the forests, and are slimmer than those of other squirrel monkey species. The tip of their tail is bushy and looks like it was dipped in black paint.
Black squirrel monkeys got their name because of a strip of black that extends from their head to the end of their tail. Their back coats are blackish-gray and their forearms, limbs, and bellies have a vibrant yellow-brown hue. They have dark eyes that are outlined by a mask of white hair, and their ears and throats are white. The skin around their lips and nostrils is hairless and black, and their nails are very sharp.
Squirrel monkeys are differentiated by the shape of the arch of white fur over their eyes. Black squirrel monkeys have a low, rounded arch and the dark fur on their heads forms a shallow V shape between their eyes.
Black squirrel monkeys are omnivorous, which means they eat both plants and animals, but their food preferences depend on seasonal availability. Figs are available all year long and are a staple in their diet. When fruits are abundant, they enjoy small berries that are found in the forest’s lower and middle canopies. During fruit shortages, they eat insects like grasshoppers and caterpillars that they find resting on leaf surfaces.
If fruit and insects are scarce, black squirrel monkeys expand their diet to include flowers, seeds, nectar, leaves, and small vertebrates like bats or small birds. They’ll also eat eggs if available.
Behavior and Lifestyle
Black squirrel monkeys are diurnal (active during daylight hours) and arboreal (tree-dwelling) primates. They travel throughout the forest with exceptional speed by running on branches and leaping horizontal distances greater than 6.5 feet (2 m). Similar species, such as black-crowned Central American squirrel monkeys (Saimiri oerstedii) can travel as far as 2.6 miles (4.2 km) in one day, and it is believe that black squirrel monkeys may also travel similar distances.
More than half of their day is spent searching for insects and fruit. Black squirrel monkeys travel in groups and will sometimes form associations with other Amazon primates, like capuchin monkeys. Capuchins are known to cause disturbances in the forests, which brings out insects that black squirrel monkeys can then eat.
Since black squirrel monkeys spend their time primarily in their home ranges, disputes with other groups are infrequent. Multiple groups may forage for food together, but they will disperse back to their home ranges as soon as the search for sustenance is over.
Of all primates, black squirrel monkeys have the largest brain in proportion to their bodies. Their brain to mass ratio is 1:17.
Squirrel monkeys are sometimes called “death’s head monkeys” because of their black and white faces.
Female squirrel monkeys have a “pseudo-penis,” which is a significantly large clitoris that resembles a penis. Females will show this off to express dominance over other females in a troop.
Daily activities for the black squirrel monkey always take place around a source of water. About 60% of their day is spent foraging for insects and fruit, and the remainder of their day is spent socializing and self-grooming. Black squirrel monkeys prefer to groom themselves over allogrooming, a bonding social activity among most primate species in which one is groomed by and grooms companions.
Troops are comprised of multiple males and females, with group sizes averaging 20 to 75 individuals. Groups of up to 300 individuals have been reported. Females with no infants often form aunt-like relationships with the young of other mothers. Among males, there is a dominance hierarchy, but further details are undocumented.
Black squirrel monkeys use scent-marking and between 25 to 30 different vocalizations to communicate with group members. They also communicate non-verbally with facial expressions and body postures.
Their vocalizations are divided into six different groups, including “peeps,” “twitters,” “chuck-chucks,” “cackles,” “pulsed calls,” and “noisy calls.” “Chuck-chucks” are the most commonly used vocalizations and are used when descending rapidly from trees, during mother-child interactions, and during sexual activities. As small, easily preyed upon primates, they also use specific warning calls to alert group members of nearby predators or dangers.
To scent-mark, squirrel monkeys urinate on their hands or feet and rub the strong scent into their bodies as a way to leave a scent trail for wandering monkeys to find their way back to their group. Their urine can also let others know of their reproductive status.
Squirrel monkeys are polygamous and form large groups that include multiple males and females. During their mating season (September to November), males gain weight to make themselves appear robust and healthy to attract the attention of females. Males compete for female attention, which can lead to physical fights and injuries, but subordinate males are typically ostracized from the competition.
Females become sexually mature by the age of 2.5 and males are sexually mature between 2.5 and 4 years of age. Estrus (the period of female fertility) only lasts for 12 to 36 hours, but if a pregnancy doesn’t occur the cycle will increase to 6 to 25 days. Pregnant females have a 140- to 170-day gestation period and will birth a single offspring. The birthing season—between February and April—occurs during the Amazon’s greatest period of rainfall and food availability. A year after the birth, females may produce another offspring. They remain fertile until they reach menopause in their mid-teens.
Newborn black squirrel monkeys are vulnerable tiny creatures that only weigh 3.5 ounces (100 g). The first 5 to 10 weeks of an infant’s life is spent under close protection by the mother, who is responsible for all care. Infants are often seen clinging to their mother’s back as she travels throughout the canopy. By the time a black squirrel monkey is one year old, they are independent enough to take care of themselves.
The impact of black squirrel monkeys on their environment has not been fully studied or documented. Since they eat seeds and fruits, they are likely valuable seed dispersers throughout their forests. Their appetite for insects and small vertebrates is also thought to help with population control. Black squirrel monkeys also serve as food for predators, like birds of prey and snakes.
The International Union for Conservation of Nature lists black squirrel monkeys (Saimiri vanzolinii) as Endangered (IUCN, 2020), appearing on the IUCN Red List of Threatened Species.
There are several primary threats to the black squirrel monkey population. Their small size makes them easy targets for predators, but they have adapted to resist becoming prey by traveling in large troops.
Climate change is their most imminent threat, especially since black squirrel monkeys live in a very small range with a high human population density. Annual precipitation cycles in the Amazon have been disrupted due to changing temperature patterns, causing extreme flooding and severe habitat degradation. Their habitats are also vulnerable to selective logging.
Although black squirrel monkeys live in protected areas, they are often hunted and captured for use in biomedical research and to be sold locally as pets. With these threats, it is projected that the monkeys will experience a population decrease of at least 50% over the next 30 years.
The black squirrel monkey is listed on Convention on International Trade in Endangered Species (CITES) Appendix II.
The monkeys live in the Mamirauá State Sustainable Development Reserve, which was the first conservation in Brazil to involve traditional communities by teaching them how to hunt and fish sustainably without causing harm to the vulnerable creatures within the habitat. Despite living in a reserve, they are still hunted and used as pets or as test subjects for research projects. More oversight is needed to prevent them from being captured.
Climate change is the biggest threat to black squirrel monkeys and many other animals in the Amazon. As temperatures rise and ecosystems change, preserving migration routes to new habitats for Amazonian animals like the black squirrel monkey is crucial for their survival.
Written by Maria DiCesare, December 2022