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Saimiri boliviensisBolivian squirrel monkey

By Levent Sipahi

Geographic Range

Saimiri boliviensis, Bolivian squirrel monkeys, can be found in the tropical rain forests of South America. They are found from the Andes in the east, north to the Caribbean Sea, and south and east into Brazil. (Ankel-Simons, 2000)

Habitat

Bolivian squirrel monkeys are most commonly found in gallery forests, but will also inhabit forest edges (Ankel-Simons, 2000). Within these tropical rain forests, Saimiri boliviensis are typically arboreal, residing in the canopy among the small branches. However, they will occasionally leave the canopy to the shrub layer or the forest floor to scavenge (Napier and Napier, 1967; Nowak, 2000). They occur at elevations from sea level to 1500 meters (Napier and Napier, 1967). (Ankel-Simons, 2000; Napier and Napier, 1967; Nowak, 2000)

  • Range elevation
    Sea Level to 1500 m
    to 4921.26 ft

Physical Description

Squirrel monkeys (Saimiri) are all fairly similar in appearance. The head is elongated and egg-shaped (Napier and Napier, 1967). It has been suggested that this characteristic cranial morphology is a compromise between a small facial skeleton and a relatively large braincase (Ankel-Simons, 2000). The fur of Bolivian squirrel monkeys is dense and short, and is generally a yellowish tan color, mottled with black hair tips (Ankel-Simons, 2000). The fur on the undersides of the limbs is yellow, white, or orange (Napier and Napier, 1967). Males and females are very similar in appearance, with sexual dimorphism occurring in size and color of crown fur (gray in males and black in females). The sizes of male Bolivian squirrel monkeys range in length, weight, and tail length, from 250-370mm, 550-1135g, and 370-465mm respectively. Females are smaller than males, from 225-295mm in length, weighing 365-750g, and having tail lengths of 370-445mm (Chiarelli, 1972). The face has white areas on the cheeks and around the eyes that appear “mask-like” (Ankel-Simons, 2000). One key identifying feature of S. boliviensis that differs from other squirrel monkeys is the arched eyebrows (Gibson, Kuehl, and Ruiz, 2005). The snout of S. boliviensis is similar to other squirrel monkeys. It is short and blunt in shape and dark in color. The ears are white and tufted and are large in comparison to the monkey’s head. The tail , which has a black tip, is almost twice the length of the body but is not prehensile. However, the tail is not prehensile. They have pseudoopposable thumbs (Ankel-Simons, 2000). (Ankel-Simons, 2000; Chiarelli, 1972; Gibson, et al., 2005; Napier and Napier, 1967)

  • Sexual Dimorphism
  • male larger
  • sexes colored or patterned differently
  • Range mass
    365 to 1135 g
    12.86 to 40.00 oz
  • Range length
    225 to 370 mm
    8.86 to 14.57 in

Reproduction

Within a troop of Bolivian squirrel monkeys, mature males live in a subgroup, generally separate from a female/young subgroup (Hinde, 1983). This level of segregation between males and females is unique among Bolivian squirrel monkeys. Theories as to what causes this separation include social dynamics between the males, and female initiated active exclusion (Gibson,Kuehl, and Ruiz, 2005). During the mating season, males, who have a well-developed dominance hierarchy, will interact and mate with the females. The dominance hierarchy in males is based on testosterone levels and copulatory frequency (Gibson,Kuehl, and Ruiz, 2005) as well as fierce fighting (Nowak, 2000). Among males, the more dominant male is allowed to interact with the females. Females will often mate with several males during their short mating season. When a male wishes to mate with a female, he will use an aggressive behavior while making his penis erect. This behavior is also used when approaching an inferior male (Bourne, 1971). When the female becomes submissive, the male mounts her from behind. This mounting behavior has been observed in infants and juveniles who apparently develop the behavior without penetration before maturity (Schrier, 1977). During mating season, males have been known to gain considerable amounts of weight. This "fattening" helps the males by increasing their sperm production. Saimiri boliviensis are polygynandrous creatures, meaning that both males and females in the troop may mate with multiple partners in a given breeding season. Males that do mate with multiple mates are usually the more dominant males (Ruiz et al., 2005). Following the mating season, males and females once again segregate into different groups (Hinde, 1983). Squirrel monkeys also display cooperative breeding behavior, meaning that the mother has help from other females in raising her young (Carpenter, 1973). (Bourne, 1971; Carpenter, 1973; Gibson, et al., 2005; Hinde, 1983; Nowak, 2000; Schrier, 1977)

Also called “aunting”, cooperative breeding is where helpers provide help in raising young that is not their own. In Bolivian squirrel monkey troops, females will act as “aunts” to the infant of another female. Aunting behavior includes dorsal carriage (carrying the infant on their back), retrieval, and cleaning. “Aunts” can be any female in the group but primarily consist of females who the mother spent a lot of time with prior to birthing, females who the mother previously acted as an “aunt” to, or previously birthed females. As many as nine different subjects have been observed interacting with a given infant in the first week of life. This “aunting” behavior is unique to squirrel monkeys and is thought to create, maintain, and extend group cohesiveness over long periods of time (Carpenter, 1973). (Carpenter, 1973)

The breeding season for S. boliviensis is restricted to three months in length with estrus times averaging 7-8 days in length (Gibson,Kuehl, and Ruiz, 2005). Unique among squirrel monkeys is their highly synchronized mating seasons, where every female will come into estrus at approximately the same time. Heavy scent marking by females during the mating season may result in this level of synchronisity. The coordination of fertility among females of the troop may be influenced by these sexual pheromones (Ankel-Simons, 2000). In each breeding season a female will have one offspring (Gibson,Kuehl, and Ruiz, 2005). The gestation period is between 152 and 172 days and weaning occurs between four and six months old (Napier and Napier, 1967). Females reach sexual maturity generally around two and a half to three years of age. Males, on the other hand, will usually leave the female/young subgroup of the troop at two and a half to three years of age. From here the adolescent males will form their own subgroup in the troop, consisting of immature males that cannot compete with the older males for dominance. At around five years of age, the males will join the mature male subgroup and begin to compete for dominance (Gibson,Kuehl, and Ruiz, 2005). (Ankel-Simons, 2000; Gibson, et al., 2005; Napier and Napier, 1967)

  • Breeding interval
    Bolivian squirrel monkeys breed once yearly.
  • Breeding season
    Breeding is restricted to 3 months of the year, synchronized by females in a group.
  • Average number of offspring
    1
  • Range gestation period
    152 to 172 days
  • Range weaning age
    4 to 6 months
  • Average time to independence
    1 years
  • Range age at sexual or reproductive maturity (female)
    2.5 to 3 years
  • Average age at sexual or reproductive maturity (male)
    5 years

Parental investment by Bolivian squirrel monkeys is taken on entirely by the females. Upon birth of the infant, the mother protects her offspring and provides entirely for it. Responsibilities include dorsal carriage, cleaning, retrieval, and nursing. Mothers are protective of their offspring and don't let them wander far. (Carpenter, 1973)

  • Parental Investment
  • altricial
  • pre-fertilization
    • provisioning
    • protecting
      • female
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female
  • pre-weaning/fledging
    • provisioning
      • female
    • protecting
      • female
  • pre-independence
    • provisioning
      • female
    • protecting
      • female
  • post-independence association with parents
  • extended period of juvenile learning
  • inherits maternal/paternal territory

Lifespan/Longevity

Bolivian squirrel monkeys have lived up to 30 years in captivity.

  • Range lifespan
    Status: captivity
    30 (high) years
  • Typical lifespan
    Status: captivity
    15 to 20 years

Behavior

Saimiri boliviensis are diurnal and live in social groups ranging from 10 to 550 individuals (Nowak, 2000) with an average size of 40-50 individuals (Ankel-Simons, 2000). They are very social creatures which establish hierarchies of dominance. (Ankel-Simons, 2000; Nowak, 2000)

Males establish their dominance through fierce fighting and assert their dominance through urine-washing and forced subjugation of an inferior. Urine-washing is a behavior by which the individual urinates on its hands, feet, and body, thus ensuring that wherever it travels it will leave its trace. Through penile display, males assert their dominance over subservient males. In this behavior, the dominant male will display its penis to the other male, often urinating on him (Nowak, 2000). (Nowak, 2000)

Along with the hierarchical structure of dominance among individuals of a troop, Saimiri boliviensis is one of the few primate species, outside of humans, who exhibit a wide range of play behavior. Play is most common between mother and offspring and between two immature individuals. However, adults also play with other adults outside of the mother/offspring relationship. This is a behavior that is rarely seen in nature. (Smith, 1978)

Another unique aspect of Bolivian squirrel monkey social behavior is that the amount of social interactions between individuals correlates to the type, quantity, and dispersion of available food. When it is more difficult to find food, social interaction is reduced. (Hinde, 1983)

Home Range

The home range of a troop of Bolivian squirrel monkeys varies greatly, often dependent on the size of the troop. The average movement of S. boliviensis between 0.6 and 1.1 kilometers per day. This could roughly be translated into the size of a territory. However, individuals have been observed with a nomadic range of up to three square kilometers. Bolivian squirrel monkeys are not territorial. (Nowak, 2000)

Communication and Perception

Saimiri boliviensis is one of the most vocal squirrel monkeys. There are 26 identifiable calls, consisting of chirps and peeps (used when alarmed), squawks and purrs (used during mating and birthing seasons), barks of aggression, and screams of pain (Nowak, 2000). Bolivian squirrel monkeys also communicates with other individuals using chemical signaling. Examples of this include urine-washing (Nowak, 2000) and release of sexual pheromones by females during mating season (Ankel-Simons, 2000). Concerning perception, S. boliviensis has been shown to be polymorphic for cone pigment and color vision, meaning that, like humans, they can see in color (Ankel-Simons, 2000). (Ankel-Simons, 2000; Nowak, 2000)

Food Habits

The diet of S. boliviensis consists primarily of insects and fruits. Other foods eaten include berries, nuts, flowers, seeds, leaves, arachnids, and small vertebrates such as bats, birds, and eggs (Ankel-Simons, 2000). (Ankel-Simons, 2000)

Saimiri boliviensis prefers to forage on terminal branches. Often, they will forage in large groups, possibly enhancing their ability to disturb insects and increase capture rates (Rodman and Cant, 1984). (Rodman and Cant, 1984)

  • Animal Foods
  • birds
  • mammals
  • amphibians
  • reptiles
  • eggs
  • insects
  • terrestrial non-insect arthropods
  • mollusks
  • Plant Foods
  • leaves
  • seeds, grains, and nuts
  • fruit
  • flowers
  • sap or other plant fluids

Predation

Predators of S. boliviensis include harpy eagles (Harpia harpyja) and humans. Eagles are avoided by staying in large groups and remaining vigilant. ("Rainforest Alliance", 2005)

Ecosystem Roles

Having a primary diet of insects and fruits, S. boliviensis would play several important roles in the ecosystem. First, by eating insects, the insect population is kept in check. Second, by consuming fruit, Bolivian squirrel monkeys act as an agent for seed dispersal. Many seeds cannot germinate or disperse properly without the help of animal digestion.

  • Ecosystem Impact
  • disperses seeds

Economic Importance for Humans: Positive

Bolivian squirrel monkeys are sometimes captured for food or for the pet trade. There was once a large trade in squirrel monkeys in the United States for both biomedical research and as entertainment (zoos and pet markets). Between 1968 and 1972, more than 173,000 squirrel monkeys were used for medical research. Regulations were then established that reduced trade in squirrel monkeys for non-research reasons (Nowak, 2000). (Nowak, 2000)

  • Positive Impacts
  • pet trade
  • food
  • body parts are source of valuable material
  • research and education

Economic Importance for Humans: Negative

Bolivian squirrel monkeys have no adverse effects on humans.

Conservation Status

This species has not currently been evaluated by international conservation databases. They rely on intact rainforests, so are vulnerable to deforestation.

Other Comments

Bolivian squirrel monkeys have the ability to move by bipedal walking. This is generally only used by a mother when carrying an infant that doesn't have the ability to grasp the dorsal fur of its mother. Also, squirrel monkeys use their tails as an accessory, both to balance and to use as a third leg when bipedally walking (Napier and Napier, 1967) (Napier and Napier, 1967)

Contributors

Tanya Dewey (editor), Animal Diversity Web.

Levent Sipahi (author), Kalamazoo College, Ann Fraser (editor, instructor), Kalamazoo College.

Glossary

Neotropical

living in the southern part of the New World. In other words, Central and South America.

World Map

acoustic

uses sound to communicate

altricial

young are born in a relatively underdeveloped state; they are unable to feed or care for themselves or locomote independently for a period of time after birth/hatching. In birds, naked and helpless after hatching.

arboreal

Referring to an animal that lives in trees; tree-climbing.

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

chemical

uses smells or other chemicals to communicate

cooperative breeder

helpers provide assistance in raising young that are not their own

diurnal
  1. active during the day, 2. lasting for one day.
dominance hierarchies

ranking system or pecking order among members of a long-term social group, where dominance status affects access to resources or mates

endothermic

animals that use metabolically generated heat to regulate body temperature independently of ambient temperature. Endothermy is a synapomorphy of the Mammalia, although it may have arisen in a (now extinct) synapsid ancestor; the fossil record does not distinguish these possibilities. Convergent in birds.

fertilization

union of egg and spermatozoan

food

A substance that provides both nutrients and energy to a living thing.

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

omnivore

an animal that mainly eats all kinds of things, including plants and animals

pet trade

the business of buying and selling animals for people to keep in their homes as pets.

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

polygynandrous

the kind of polygamy in which a female pairs with several males, each of which also pairs with several different females.

rainforest

rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

riparian

Referring to something living or located adjacent to a waterbody (usually, but not always, a river or stream).

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

social

associates with others of its species; forms social groups.

tactile

uses touch to communicate

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

visual

uses sight to communicate

viviparous

reproduction in which fertilization and development take place within the female body and the developing embryo derives nourishment from the female.

References

2005. "Rainforest Alliance" (On-line). Accessed November 23, 2005 at http://www.rainforest-alliance.org/resources/forest-facts/species-profiles/squirrelmonkey.html.

Ankel-Simons, F. 2000. Primate Anatomy: An Introduction, Second Edition. San Diego: Academic Press.

Bourne, G. 1971. The Ape People. New York: Academic Press.

Carpenter, C. 1973. Behavioral Regulators of Behavior in Primates. Lewisburg, PA: Bucknell University Press.

Chiarelli, A. 1972. Taxonomic Atlas of Living Primates. London: Academic Press.

Gibson, S., T. Kuehl, J. Ruiz. 2005. "Squirrel Monkey Breeding and Research Resource" (On-line). Accessed November 23, 2005 at http://www.smbrr.org/.

Hinde, R. 1983. Primate Social Relationships: An Integrated Approach. Oxford: Blackwell Science Ltd..

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.

Rodman, P., J. Cant. 1984. Adaptations for Foraging in Nonhuman Primates: Contributions to an Organismal Biology of Prosimians, Monkeys, and Apes. New York: Columbia University Press.

Schrier, A. 1977. Behavioral Primatology. Hillsdale, NJ: Lawrence Erlbaum Associates, Inc..

Smith, E. 1978. Social Play in Primates. New York: Academic Press.

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