Bolivian Squirrel Monkey, Saimiri boliviensis
BOLIVIAN SQUIRREL MONKEY
Saimiri boliviensis
Geographic Distribution and Habitat
The Bolivian squirrel monkey (Saimiri boliviensis), also known as the black-capped squirrel monkey, Peruvian squirrel monkey, or black-headed squirrel monkey, is a primate species that occurs in South America’s upper Amazon rainforest. Throughout Bolivia, Peru, and Brazil, Bolivian squirrel monkeys live in secondary forests, floodplain forests, or river edge forests. Additionally, they may inhabit inundated forests, or forests either seasonally or permanently flooded by freshwater.
Geographical borders for this species include the Rio Solimões-Amazonas, Rio Negro, and Río San Martín. The exact southern border of their established home range is unknown.
The average temperature for the upper Amazon ranges between 68 to 77 degrees Fahrenheit (20 to 25 degrees Celsius). Generally, the Amazon rainforest receives a significant amount of rainfall each year, particularly during the rainy season. For example, the average annual rainfall for this geographic region can reach 6 to 10 feet (1.8 to 3 meters)! When it rains, it really pours!
According to many researchers and primatologists, the squirrel monkey genus, Saimiri, is a “decades-long taxonomic and phylogenetic puzzle” especially when considering their DNA.
All squirrel monkey species have 44 chromosomes. Chromosomes are thread-like structures located in the nucleus of a cell that hold and carry the genetic information they’ve received from their parents. For squirrel monkeys, biological parents pass 22 chromosomes to each individual offspring in a unique genetic combination. These chromosomal markers are found on the symmetrical branches of a chromosome and communicate to the body specific instructions that help individuals grow and develop in unique ways.
Interestingly, unlike most animal species, squirrel monkey species have variations in the number of branches on their chromosomes. Additionally, in general, the length of their branches is uneven. Unfortunately, these genetic inconsistencies make it harder for scientists to determine genetic links between animals under the genus Saimiri. Although the number has historically ranged between one to 16 species, the true number of species under this genus remains uncertain.
Saimiri boliviensis is the scientific name for the Bolivian squirrel monkey. Additional Saimiri species include Humboldt’s squirrel monkey (S. cassiquiarensis), Collin’s squirrel monkey (S. collinsi), the black-crowned Central American squirrel monkey (S. oerstedii), the common squirrel monkey (S. sciureus), the bare-eared squirrel monkey (S. ustus), the black squirrel monkey (S. vanzolinii), etc.
Size, Weight, and Lifespan
On average, Bolivian squirrel monkeys weigh roughly 1.65 to 2.43 pounds (750 to 1100 grams) and grow to have a body length of 9.84 to 13.76 inches (25 to 35 centimeters), with an additional tail length of 13.78 to 16.54 inches (35 to 42 centimeters). From head to end of tail, their total average size is around 23.62 to 30.32 inches (60 to 77 centimeters).
In the wild, they may live up to 15 to 20 years.
Appearance
Bolivian squirrel monkeys are small, lithe primates with fur that varies in shades of olive, orange, brown, and gray. Typically Bolivian squirrel monkeys have olive coloring on their shoulders, while their back and four extremities are colored a combination of yellow and orange. Their throats and ears are white, while their muzzles and heads are black and to explain their unique appearance, some people refer to them as “skull monkeys”! Bolivian squirrel monkeys are distinguished from other squirrel monkey species by their “Roman”, or more modestly curved, eyebrow arch.
Like other squirrel monkey species, Bolivian squirrel monkeys have non-prehensile tails, which means that they are incapable of using their tails to grasp objects such as branches or food. Instead, they use their tails for balance.
Nonprehensile tails are especially important because they act as a counterweight. As they swing their tail from side to side, by applying an opposing force, the counterweight maintains their stability. As the forest is naturally uneven and unpredictable, although it cannot hold on to certain things, the nonprehensile tails of Bolivian squirrel monkeys play a significant role in upholding the animal’s safety.
Diet
Bolivian squirrel monkeys are insectivorous frugivores, meaning their diet primarily consists of fruit, seeds, or insects and their foraging occurs in the forest’s lower canopy, or the layer of vegetation beneath the main canopy of the forest. More often than not, Bolivian squirrel monkeys will spend nearly half of their day foraging for fruit and hunting for prey species.
Considering their geographical location, it is likely that they consume plant species such as achachairú fruit trees (Garcinia humilis), acai (Euterpe oleracea), camu-camu (Myrciaria dubia), and bacaba (Oenocarpus bacaba). Similarly, based on their home range, they probably consume insect and animal prey like the translucent jewel caterpillar (Acraga coa), the jewel caterpillar (Minacraga argentata), or the waxy monkey tree frog (Phyllomedusa sauvagii).
During the dry season, which occurs between the months of June and December, fruit is scarce because of the significant decreases in water availability. To make up for this and supplement their diet, compared to the rainy season, Bolivian squirrel monkeys eat an increased number of insects and animal prey.
Behavior and Lifestyle
The Bolivian squirrel monkey is a quadrupedal primate that walks and runs using all four limbs. Aided by their long legs, arms, and flexible spines, these animals are very speedy and can run up to 22 miles (35 kilometers) miles per hour!
As a diurnal species, Bolivian squirrel monkeys are most active during the day and remain asleep while the sun is down. Throughout the day and night, squirrel monkeys must stay on high alert in order to prevent predation. Because of their small size, many predators see them as easy prey, regardless of if they are hunted from above or below. The main predators of this species include: harpy eagles (Harpia harpyja), crested eagles (Morphnus guianensis), and black-hawk eagles (Spizaetus tyrannus). Snakes and felines also pose a serious threat to squirrel monkeys and examples include: boa constrictors (Boa constrictor), anacondas (Eunectes murinus), jaguars (Panthera onca), and ocelots (Leopardus pardalis).
While sleeping, squirrel monkeys sleep huddled together on branches with their tails wrapped around their bodies. By doing this, they are able to conserve their body heat to stay warm.
Did you know squirrel monkeys have the largest brain, proportionate to their bodies, of all primate species?
Brain-to-body ratio, also known as encephalization quotient (EQ), is a measure of how big an animal’s brain is compared to its body size. The encephalization quotient is used to compare the brain development and complexity of different species to one another.
The EQ in the Bolivian squirrel monkey (Saimiri boliviensis) is about 1:17. Comparing this ratio to other species, Sumatran orangutans (Pongo abelii) have an EQ of 1:150 and humans (Homo sapiens) have an EQ of 1:50.
Although the ratio is not the sole determinant of a species’s intelligence, it is an important indicator of a species’s cognitive abilities and evolutionary development. How cool is that!
Troops of Bolivian squirrel monkeys consist of between 45 to 75 individuals. On occasion, smaller groups combine to form large super-groups, sometimes reaching up to 100 individuals. Although the exact ratio of males to females is unclear, there is no debate that in all social troops, females are always more dominant than males.
While asserting their dominance, females keep males on the periphery of the group’s home range through bullying and harassment. Because males do not perform anti-predator vigilance, or protect their home range from predators, they spend most of their time here searching for food and looking for mates.
Female Bolivian squirrel monkeys are philopatric and follow a matrilineal social structure, meaning females form the strongest bonds with those they are closely related to and typically remain in the groups they were born into. Because of this, non-related females are likely to be met with aggression and confrontation from established troop members, especially if the opponents are competing for resources or mating opportunities.
Squirrel monkeys are very vocal animals and use over 20 different vocalizations to communicate. For example, Bolivian squirrel monkeys use twitters and clucks to lure prey out of their hiding spots, imitating their prey’s natural call. Recognizing these vocalizations as their own, insects and other small animals become susceptible to predation.
Other vocalizations examples include peeps, cackles, trills, etc. Some vocalizations act as warning sounds, while others convey feelings of excitement or happiness. Regardless of the intention, these calls are particularly useful when communicating to a large number of individuals, especially if in a super-group.
The mating season for Bolivian squirrel monkeys is typically dependent on seasonal influences, but more often than not, females give birth to their young during the rainy season, between the months of December and May. During the mating season, between September to November, males gain weight in their shoulders, arms, and torso in a process known as “fattening”, or “bulking”. To determine who they believe the best reproductive match will be, females pay close attention to this process in males. For female squirrel monkeys, the more weight a male builds, the more attractive he becomes!
The gestation length, or time interval between conception and birth, for Bolivian squirrel monkeys is between 150 and 170 days. After this period, females give birth to one infant that weighs only around 0.44 pounds (200 grams). Unlike other primate species, females are responsible for nearly all infant care and because males do not actively protect their children, they remain on their home’s perimeter.
As soon as they are born, infants cling to their mother’s backs, essentially becoming backpacks! In close resemblance to their parents, Bolivian squirrel monkey infants are born with short, brown, and fuzzy fur that darkens with age.
During the first few weeks of an infant’s life, adult females practice allomaternal behavior. Allomothering, or allomaternal infant care and handling, is performed by any group member other than an infant’s natural mother. Allomothering can have a strong influence on the life of an infant, in addition to the individual with which the infant interacts. Sampling research indicates that during the first 6 months of an infant’s life, 30% of their time is spent holding on to their allomothers.
Sexual maturity is different between females and males. While females reach sexual maturity between 2 to 3 years old, males reach maturity between the ages of 4 and 5 years. Unlike females, when males reach sexual maturity they leave their natural-born troops to form “bachelor” troops. Although the troop membership may change every couple of years, these groups allow non-breeding males to live healthy social lives without the fear of competition or conflict.
Bolivian squirrel monkeys play an integral role in maintaining environmental health and diversity. Not only are they seed dispersers, but squirrel monkeys maintain insect populations.
As mentioned previously, Bolivian squirrel monkeys are frugivorous primates, so more than 50% of their diet is made of fruit and seeds. Whether the seeds are spit in one location, or defecated in another, squirrel monkeys are capable of moving seeds across large distances. Once a seed travels away from its original location, the natural plant may establish new populations. Overall, this process increases a plant species’ survival rate and increases their ability to successfully reproduce.
Insect predation is an important behavior that impacts plant health and habitat biodiversity. Once insect populations become overpopulated, they have the potential to damage crops, deplete food sources for other animals, spread disease, and damage soil health. By maintaining a balanced insect population, primates such as the Bolivian squirrel monkey enhance the vitality of the plants around them. Their impact is truly unbelievable!
The International Union for Conservation of Nature lists the Bolivian squirrel monkey as Least Concern (IUCN, January 26, 2015), appearing on the IUCN Red List of Threatened Species.
According to the IUCN Red List of Threatened Species, this species is mostly hunted for food in areas that have suffered the loss of larger primate species. Habitat loss is also a significant threat, especially since deforestation in the Amazon rainforest has increased over time. Not only does deforestation create rainforest fragmentation, but cattle ranching, logging, and infrastructure development often have disastrous effects on an environment’s overall health.
The Bolivian squirrel monkey is listed in Appendix II of the Convention on International Trade in Endangered Species (CITES), an international agreement between governments whose goal is to ensure that international trade in specimens of wild animals and plants does not threaten their survival.
They are found in many protected areas across the upper Amazon, particularly in the countries of Bolivia, Peru, and Brazil. In Bolivia, they occur in Madidi National Park, Manuripi National Reserve, Beni Biosphere Reserve, Amboro National Park, etc. In Peru, they occur in Tambopata National Reserve, Bahuaja-Sonene Community Reserve, Manu National Park, and Yanachaga-Chemillen National Park. Lastly, in Brazil, the Bolivian squirrel monkey occurs in Serra do Divisor National Park and Rio Acre Ecological Station.
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Written by Brooklynne Mitchell-Arno, February 2025