WHITE-CHEEKED SPIDER MONKEY

Ateles marginatus

Geographic Distribution and Habitat
The white-cheeked spider monkey (Ateles marginatus), also called the white-whiskered spider monkey, is endemic to the Amazon Basin in central Brazil. Their range is bound by the Rio Tapajós, Rio Teles Pires, Rio Xingu, and Rio Amazonas. They live in the lush, extremely biodiverse lowland rainforests for which the region is known.

White-cheeked spider monkeys share their genus with the six other species of true spider monkeys, so named because of their extremely long arms and legs that are adapted to life in the trees. White-cheeked spider monkeys are the least studied of the Brazilian spider monkeys.

White-cheeked spider monkey geographic range. Map: IUCN, 2020

Size, Weight, and Lifespan
​​On average, white-cheeked spider monkeys weigh about 13 lbs (5.8 kg) and are 13–20 inches (34–51 cm) in length, not including their tails. Their average life span is about 20 years.

Appearance
​White-cheeked spider monkeys are mostly dark brown or black, with white hair on their forehead, chin, and, as their name suggests, cheeks. Like all spider monkeys, they have extremely long arms and legs and a prehensile tail that helps them to smoothly swing from branches. Spider monkeys’ tails have more vertebrae than non-prehensile tails of other primates, and the vertebrae are smaller. This allows the tail to be more flexible and have more extension compared to the tails of other primates. To help them maintain their grip, the underside of their tail is hairless and textured like a fingerprint—in fact, their tails are so useful that they are often referred to as the monkeys’ “fifth limb” or “third hand!” Unlike most other primates, they lack external thumbs, instead using their hands like hooks. This is actually a specialized adaptation to their arboreal lifestyle—their ancestors had opposable thumbs, but in spider monkeys, the bones of their thumbs shrank as they evolved, such that they are not externally visible. Like other spider monkeys, white-cheeked spider monkeys do not exhibit much sexual dimorphism.

​Diet
​White-cheeked spider monkeys are highly frugivorous, with fruits comprising 83% of their diet. They supplement with young leaves and flowers, seeds, aerial roots, bark, decaying wood, honey, and occasionally small insects. They are more likely to consume these non-fruit items during periods of fruit shortages, such as at the beginning of the dry season.

Behavior and Lifestyle
White-cheeked spider monkeys are diurnal and exclusively arboreal. They spend most of their lives in the upper levels of the canopy. They sometimes descend to the middle and lower sections of the canopy but are almost never seen in the understory. Spider monkeys are well known for being suspensory—that is, they spend most of their time hanging from branches. This is particularly useful when foraging, as they can hang from their tail and use both hands to forage. They move mainly via brachiation, using their long, limber arms and legs to glide effortlessly through the trees.​

Daily Life and Group Dynamics 
White-cheeked spider monkeys live in large groups with an average size of 25 individuals. However, most of the time, white-cheeked spider monkeys break out into much smaller groups (usually two to four individuals) to travel, feed, and rest together, a social system called “fission-fusion.” Small groups can break out to forage together, but they still regularly associate with one another as part of the larger group. With the exception of a mother with her offspring, these small groups are fluid, with individuals moving about and forming new sub-groups frequently, even changing several times per day. The subgroups are led by a dominant female, and each female in the group has a core area within the group’s territory that she uses most.

This social system is directly related to the spider monkeys’ frugivorous diet, as the monkeys often have to cover a large area in order to find fruiting trees. During the months when food is most scarce, the subgroups tend to be smaller in size than when fruit is abundant. This suggests that the fission-fusion social structure may have evolved as a strategy to reduce competition for food within a group. Small groups that forage together compete less on a single tree than a large group that forages together, and they still have the benefit of increased mating opportunities and protection from predators that comes from associating with the large group.

Relations within a group are usually peaceful. When scraps do occur, they are typically a result of competition over food, and are brief and do not result in injury. When two larger social groups come in contact, the males in the groups display antagonistic behavior such as barking and calling. They rarely come in direct contact with each other, however, as territorial boundaries are usually respected.

What Does It Mean?

Arboreal:
Physically adapted to living primarily or exclusively in trees.

Biodiversity: 
​The level of variation of life in an ecosystem, a biome, or the entire planet. Biodiversity is a measure of the health and function of an ecosystem.

Brachiation:
Also called arm swinging, it’s a form of arboreal locomotion in which primates swing from tree limb to tree limb using only their arms.

Canopy:
Beneath the emergent layer, the canopy layer is the primary layer of the forest and forms a roof over the two remaining layers (the understory and forest floor). Many animals live in this maze of leaves and branches, where food is abundant.

Deforestation:
The permanent cutting, clearing, and removal of trees to convert forest land for other use, such as pasture, cropland, or plantations.

Diurnal:
Active during daylight hours.

​Endemic:
Native or restricted to a certain area or country.

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.  

Frugivorous:
Having a diet that consists of fruits.

Generation time:
The average time between successive generations in a population.

Gestation Period:
The time of pregnancy from conception until birth. 

Interbirth interval:
Refers to the period of time between successive births of an individual female.

Lowland forest:
An equatorial evergreen rainforest, commonly known as a tropical rainforest, which receives high rainfall (80 in, or 2m) throughout the year.

​​Natal group:
The group into which an animal is born.

​Opposable thumb:
A thumb that can be placed opposite the fingers of the same hand. Opposable thumbs allow the digits to grasp and handle objects and are a characteristic of primates.

Polygynous:
A mating system in which one male mates and lives with multiple females.

Prehensile tail: 
The tail of an animal that has adapted to be able to grasp or hold objects.

Visit the Glossary for more definitions

Communication
​White-cheeked spider monkeys ward off predators by barking at them and shaking nearby tree branches. If the predator isn’t scared off by that, they break off and throw small branches at them. Other forms of communication are not well documented in white-cheeked spider monkeys, but the closely related black spider monkey is known to make sounds such as whoops, whinnies, sobs, trills, twitters, and squeaks to communicate with groupmates.

Reproduction and Family
​Unfortunately, the reproductive system of white-cheeked spider monkeys is not well studied. They are believed to be polygynous, with males mating with multiple females, but females mating with only one male. After a gestation period of almost eight months, they give birth to a single offspring. In the wild, the interbirth interval is likely about 2–4 years. Offspring reach sexual maturity at 4–5 years of age. This late maturation, long gestation time, and long interbirth period results in a relatively long generation time of 15 years. This makes it difficult for a population to recover from hunting and other threats. Females are the primary caregivers of young, carrying infants on their stomachs for the first six months of life, then on their backs for at least another six months. Offspring then become increasingly independent from their mothers, and by 15 months of age they rely on her for transportation only when they’re fatigued or to cross a particularly long gap between trees. By three years of age, spider monkey young are fully weaned, yet they remain with their mother for another year or two. Upon reaching independence, females disperse to new groups to find mating opportunities and males typically remain with their natal group.

Fun Facts

Spider monkeys exhibit a behavior called “bridge-gapping,” in which an adult monkey helps a juvenile cross a large gap between trees by forming a bridge between branches with their body and allowing the juvenile to climb across them.​

Ecological Role
​As frugivores, white-cheeked spider monkeys play an important role as seed dispersers. One study found that these monkeys deposit the seeds of 138 plant species, a full 94% of the species they consume, making them highly effective disperers and extremely important to the life cycle of these plant species. Because of their relatively large bodies, spider monkeys are typically not subject to the same levels of predation as smaller primates. However, white-cheeked spider monkeys are vulnerable to predation by raptors, jaguars, and large snakes.

Conservation Status and Threats
White-cheeked spider monkeys are considered Endangered by the International Union for Conservation of Nature, appearing on the IUCN’s Red List of Threatened Species (IUCN, 2019). This categorization is based on a predicted population loss of about 50% between 2006 and 2050. This prediction is based on the most recent generation (from 2006–2020) of the species, which was impacted by habitat declines and high levels of hunting. Because hunting pressure remains high and habitat reduction is expected to continue, the next two generations (covering the time span of 2021–2050) are expected to be similarly impacted, negatively impacting the species as a whole.

The most urgent threat against white-cheeked spider monkeys is habitat loss. This habitat loss is mostly due to deforestation to clear space for soybean farming, cattle ranching, and urbanization. Most of this deforestation occurs in what’s known as the “arc of deforestation”: a region in the southeastern portion of the Amazon rainforest that is subject to approximately half of Brazil’s annual forest loss. A 2006 study found that nearly 100 mammal species in the Brazilian Amazon will lose more than 40% of their protected habitat and up to 80% of their unprotected habitat by 2050. This predicted loss of habitat is based on the expansion of the cattle and soy industries, and the expansion of highway networks throughout the region.

Another significant habitat threat against white-cheeked spider monkeys is hunting pressure, as some indigienous groups in Brazil consider spider monkey meat a delicacy. In some areas, spider monkeys are hunted heavily, and their slow rate of reproduction makes recovering from hunting pressure very difficult. Population surveys from 1987–2005 in areas where spider monkeys are hunted heavily show a population density drop of up to 50%. The expansion of highway networks through the white-cheeked spider monkey range means that hunting pressure will likely only increase as the monkeys become more accessible.

Conservation Efforts
​White-cheeked spider monkeys occur in a number of protected areas throughout their range, including Altamira National Forest, Terra do Meio Ecological Station, and Jamanxim National Forest. They are listed on Appendix II of the Convention on International Trade in Endangered Species (CITES).

References:

  • ​Ravetta, A.L. and S.F. Ferrari. 2009. Geographic distribution and population characteristics of the endangered white-fronted spider monkey (Ateles marginatus) on the lower Tapajós River in central Brazilian Amazonia. Primates 50:261.
  • https://www.iucnredlist.org/species/2282/17929907
  • https://earthobservatory.nasa.gov/images/4385/deforestation-patterns-in-the-amazon
  • http://animalia.bio/white-cheeked-spider-monkey
  • http://www.earthsendangered.com/profile.asp?gr=&view=c&ID=11&sp=1895
  • http://pin.primate.wisc.edu/factsheets/links/ateles

Written by K. Clare Quinlan, Aug 2020