HUMBOLDT'S SQUIRREL MONKEY

Saimiri cassiquiarensis

Geographic Distribution and Habitat
Humboldt’s squirrel monkeys are endemic to Brazil, Ecuador, Colombia, Venezuela, and northern Peru. Like most squirrel monkeys, Humboldt’s squirrel monkeys can adapt to many changes and disturbances in their environment and can thrive in a variety of rainforest types. However, they prefer riparian and secondary forests and sometimes venture into forests near human settlements. These New World monkeys tend to stay high in the trees, occasionally descending to the shrub layer and forest floor to forage for food and water. 

The Humboldt’s squirrel monkey (Saimiri cassiquiarensis) is the “parent species” of three subspecies that are differentiated primarily by region: the Colombian squirrel monkey (S. c. albigena) is endemic to the piedmont forests of the Colombian Llanos; S. c. cassiquiarensis is also referred to as “Humboldt’s squirrel monkey” and is endemic to the Amazon regions of Brazil and Venezuela; and the Ecuadorian squirrel monkey (S. c. macrodon) is distributed south of the Río Apaporis in the Colombian Amazon, throughout all of the Ecuadorian Amazon, and into the northern part of the Peruvian Amazon.

TAXONOMY IN TRANSITION

Until recently, the Humboldt’s squirrel monkey and its subspecies were either grouped among, or considered to subspecies of, the “common squirrel monkey” (Saimiri scuireus), within a very broad geographical range throughout the Amazon Basin. Genetic studies conducted throughout that range between 2009 and 2015 revealed that there are, in fact, several distinct squirrel monkey species and subspecies within that grouping. The distinctions were formally announced by the International Union for Conservation of Nature (IUCN) in April 2020 and work is still being done to describe the differences between and among the species. The commonly used moniker “common squirrel monkey” is obsolete. (Saimiri scuireus is now commonly called the Guinan squirrel monkey.) But old habits die hard and it will probably be some time before that term is no longer referenced. To confuse things further, there are few notable physiological differences between the species. As well as DNA, geographic location distinguishes the species.

Humboldt's Squirrel Monkey range, IUCN 2020

Size, Weight, and Lifespan
The head-to-body length of squirrel monkeys typically ranges between 9.8 and 13.8 inches (25–35 cm). Their tails add an extra 15 inches (38.1 cm) or so. They generally weigh between 1.6 and 2.4 pounds (750–1100 g). There is little sexual dimorphism between genders. The slight differences in Humboldt’s squirrel monkey genders are described below.

Squirrel monkeys can live between 15 and 20 years in the wild and up to 25 years in captivity. 

Appearance
Humboldt’s squirrel monkeys faces are pink with black muzzles. White hair encircles their dark eyes in a “gothic” arch. A gothic arch, in this context, is defined by the manner in which the high arch of white hair around the eyes and the darker hair on their heads forms a deep “V” shape between their eyes. Females have black temporal bands that run from the front of the ears to the nape of the neck and thick, dark sideburns. Males do not share these temporal bands; rather, their white face mask extends to the ears, and a pale agouti patch may be present in front of the ears. The pointed ears in both genders are wreathed with white fur. The crown of the head is olive or olive-gray with wisps of yellow mixed in, and the fur on the base of the crown is golden yellow.

The pelage on their back can vary in color from gray to olive to orange, their belly is white, and their arms are yellow-orange. Yellow fur also mixes with the gray on their torso.

Humboldt’s squirrel monkeys are small-bodied with very long black-tipped tails. Their tails are not prehensile but provide excellent balance while navigating through the trees. Their upper legs or thighs are shorter than their lower legs. This allows them to leap masterfully—as far as 4 feet from their starting point. Their slender and nimble fingers are equipped with nails that improve their ability to grasp and cling onto branches.

Males gain weight and take on a “fatted” upper body appearance just before breeding season. They can gain up to 20 percent of their body weight. The larger they are, the more attractive they are to the females.

What Does It Mean?

Allomothering:
Individuals other than the biological mother of an offspring performs the functions of a mother (as by caring for an infant temporarily).

Fission-fusion society:
A society in which the members of a social group break off into smaller groups and then rejoin as a larger group.  ​

Nonprehensile or Non-prehensile:
Incapable of grasping or gripping (opposite of prehensile: capable of grasping).

Riparian: 
Of, relating to, or situated on the banks of a river.

​Sexual dimorphism:
Distinct differences in size or appearance between the sexes of an animal in addition to differences between the reproductive organs themselves.

Thermoregulation:
A process that allows your body to maintain its core internal temperature. 

Visit the Glossary for more definitions

Diet
Squirrel monkeys primarily eat fruits and insects, making them omnivores, with a considerable preference for fruit. When fruit is less plentiful, their diet includes more insects, seeds, young leaves, flowers, gum, shoots, nectar, spiders, lizards, and bird eggs.

Behavior and Lifestyle
Squirrel monkeys are arboreal (they are fully adapted for life in trees) and diurnal (active during daylight). Agile and fast-moving monkeys, they spend a good portion of their day running quadrupedally (on all four limbs) along branches in search of food. Advancing in groups, they travel single-file along frequently used treetop routes. At night, they sleep huddled on branches with their tails wrapped around their bodies.

Life in hot, humid tropical rainforests has its challenges. Squirrel monkeys can only sweat through the palms of their hands and the soles of their feet—not enough area to adequately cool down via thermoregulation. To cool down, they resort to clever and necessary behavioral tactics to get away from the heat of the day. These include seeking cooler shaded areas and modifying their postures to more effectively dissipate heat from their bodies. They also make use of a less seemly but convenient technique—urine washing. The monkeys urinate on their hands and rub the urine over the soles of their feet. The urine evaporates off their bodies in an efficient cooling process. Studies have shown that this behavior is mostly practiced during high temperatures, underscoring its significance as a thermoregulatory behavior.

Extreme high humidity brings more even challenges when it comes to maintaining healthful ion and hydration levels in their small bodies. When humidity reaches a threshold of roughly 95%, the monkeys intentionally ingest less water. This creates more concentrated urine, which balances the ion levels required for their good health.

Throughout the day, between feedings, groups rest and socialize. Young squirrel monkeys play with each other while adults groom one another.

Fun Facts

Squirrel monkeys are considered to be one of the cleverest monkeys due to having a large brain compared to the size of their body.

They are highly vocal and have at least 26 different types of calls.

Right before breeding season, males gain weight and take on a “fatted” appearance.

Daily Life and Group Dynamics
Some squirrel monkey species live in troops as large as 300–500 monkeys. Humboldt’s squirrel monkeys generally live in groups of 20–55 individuals. Large group sizes provide the small monkeys protection from predators and potential rivals. Larger groups may temporarily break into smaller subgroups from time to time—a form of fission/fusion social order—especially while foraging. Squirrel monkeys sometimes forage alongside capuchin monkeys troops for an added layer of protection.

Squirrel monkey groups consist of multiple males and females. Females form the central core of their troops and have their own dominance hierarchy. Males have a separate and looser hierarchy that, to some degree, may determine mating rights. 

Males emigrate from their natal groups at sexual maturity (at around 3–4 years of age), while females are philopatric—that is, they remain in their natal groups throughout their lives. When males disperse from their natal group, they temporarily form all-male bachelor groups. At around 5 years old (full maturity), males compete for dominance to join multi-male/multi-female groups. The bachelor males form alliances and work together to take over the highest positions in their new groups. The alliances remain strong if the males remain within the same group. 

Squirrel monkey species are very social. Juveniles begin engaging in playful activities at around two months of age. Social play helps the infants become independent from their mothers. Throughout the first year of life, young monkeys play with each other, often in the form of wrestling games.

Communication
Squirrel monkeys, in general, are among the most vocal of the New World monkeys. They use at least 26 varieties of calls to communicate with one another. When predators are nearby or if a group member feels threatened, an alarm call warns the rest of the group. Other vocalizations are used help to maintain the social organization of the troop and enhance group cohesion. 

Their calls have been 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.

Squirrel monkeys have high olfactory sensitivity. For example, males use scent to identify receptive mates and it is posited that females use scent to identify individual infants. Both males and females use scent-marking to identify themselves and their territory. The urine-washing, described earlier, serves more than thermoregulation. When the monkeys urinate on their hands and feet, they leave scent-trails on branches, leaves, and other substrates. This identifies individuals and their locations, and warns other squirrel monkey troops that they may be venturing into an occupied territory. There are typically no territorial disputes among squirrel monkey troops as a result.

Tactile communication, such as grooming one another and huddling together, is important for enhancing social bonds in squirrel monkey groups.

Reproduction and Family
Squirrel monkeys are polygynandrous, meaning females and males mate with multiple partners. Though there is a loose hierarchy among males that determines who has first mating rights, the considerable size of their groups makes regulating and enforcing these conventions difficult.

Mating usually occurs between September and November, with one baby produced after a 5- to 7-month gestation period. Birthing season coincides with the rainy season and the abundance of resources that it brings.  

Mothers are almost exclusively the primary caretaker of their infants. A mother nurses her infant and carries him or her on her belly for the first few weeks and then on her back until the child is more independent. Other females in the troop assist in raising young. This is known as “aunting,” or more formally as allomothering. Aunting behavior consists of carrying an infant on their back, retrieval of a wayward toddler, and cleaning the youngsters. 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. It helps the mother in many ways. Not only does she have more “downtime,” but it helps with a quicker weaning process, which allows her to invest time in her next offspring. Fathers are not involved in childrearing. 

Females reach maturity before males, at around 2.5 to 3 years of age. Males fully mature at five years of age. 

Photo credit: Edmundo Mascarenhas de Andrade Filho/Creative Commons

Ecological Role
Due to their preferred frugivorous diet, squirrel monkeys play an important role in the ecosystem. By consuming fruit, they act as agents for seed dispersal throughout the forest. When they consume insects, they help to keep the insect population in check.

Conservation Status and Threats
The International Union for Conservation of Nature lists the Humboldt’s squirrel monkey as Least Concern (IUCN, 2015), appearing on the IUCN’s Red List of Threatened Species. Though they are relatively widespread in their region, their populations are declining due to habitat loss and habitat fragmentation. The principal pressures driving population loss are agricultural activities and the rapid conversion of forest land to palm oil plantations, as well as new areas of petroleum exploration in the monkeys’ distribution area. Other threats within and near their range are hunting and trapping for illegal pet trade.

The Colombian squirrel monkey subspecies (Saimiri cassiquiarensis ssp. albigena) is Vulnerable (IUCN, 2015) due to habitat loss and fragmentation.

Conservation Efforts
The Humboldt’s squirrel monkey is listed on the Convention on International Trade in Endangered Species (CITES) Appendix II. There are no documented conservation efforts on their behalf, if there are any at all. The good news is that the monkeys have been found in some protected areas in Brazil: Pico da Neblina National Park, Jaú National Park, Juamí-Japurá Ecological Station, Maracá Ecological Station, Niquiá Ecological Station, and Caracaraí Ecological Station. In Colombia, they are found in Nukak Natural National Reserve and Puinawai Natural National Reserve, also protected areas.

References:

  • https://www.iucnredlist.org/species/160940148/192585552
  • https://olaw.nih.gov/sites/default/files/Squirrel_Monkeys.pdf
  • https://www.sciencedirect.com/science/article/pii/S1055790314003169
  • Carretero-Pinzon, X., Ruiz- Garcia, M., Delfer, TR. 2013. Conservation of Saimiri sciureus albigena, a Colombian endemic subspecies of squirrel monkey. Primates Colombianos en Peligro de Extinción. Asociación Primatológica Colombiana, Bogotá D.C. 249-258.
  • Godoy, I. and Perry, SE. 2019. Mating Systems of New World Monkeys. Encyclopedia of Animal Behavior. 563-567. 
  • Stone, AI., 2014. Is Fatter Sexier? Reproductive Strategies of Male Squirrel Monkeys (Saimiri sciureus). International Journal of Primatology. (35):628-642. 
  • https://animaldiversity.org/accounts/Saimiri_sciureus/
  • https://www.newworldencyclopedia.org/entry/Squirrel_monkey
  • https://a-z-animals.com/animals/squirrel-monkey/
  • https://primate.wisc.edu/primate-info-net/pin-factsheets/pin-factsheet-squirrel-monkey/

Written by Tara Covert, July 2021