Squirrel monkey
Saimiri

TAXONOMY

Suborder: Haplorrhini
Infraorder: Simiiformes
Family: Cebidae
Subfamily: Saimiriinae
Genus: Saimiri
Species: S. boliviensis, S. oerstedti, S. sciureus, S. ustus, S. vanzolinii
Subspecies: S. b. boliviensis, S. b. peruviensis, S. o. citrinellus, S. o. oerstedti, S. s. albigena, S. s. cassiquiarensis, S. s. macrodon, S. s. sciureus

Other names: S. boliviensis: black-headed squirrel monkey or Bolivian squirrel monkey; macaco de cheiro (Spanish); boliviansk dödskalleapa, boliviansk ekorrapa, or svarthövdad dödskalleapa (Swedish); S. oerstedti: S. oerstedtii or S. oerstedii; black crowned Central American squirrel monkey, Central American squirrel monkey, or red-backed squirrel monkey; panamavsaimiri (Finnish); saimiri à dos roux, singe écureuil à dos rouge, or singe écureuil à dos roux (French); geel doodshoofdaapje (Dutch); gelbes totenkopfäffchen (German); testina di morto (Italian); barizo dorsirrojo, mono ardilla, mono tití, or saimiri dorsirrojo (Spanish); nordlig dödskalleapa, rödryggad dödskalleapa, or rödryggad ekorrapa (Swedish); S. sciureus: common squirrel monkey or South American squirrel monkey; saimiri ecureuil (French); macaco de cheiro (Spanish); ekorrapa gråhövdad dödskalleapa, gråhövdad ekorrapa (Swedish); S. ustus: bare-eared squirrel monkey or golden-backed squirrel monkey; barörad dödskalleapa, nakenörad dödskalleapa, or nakenörad ekorrapa (Swedish); S. vanzolinii: black squirrel monkey, black-headed squirrel monkey, or blackish squirrel monkey; mörk dödskalleapa (Swedish)

Saimiri sciureus
Photo: Luiz Claudio Marigo

Until recently, there were only two species of squirrel monkeys, a South American species, S. sciureus and a Central American one, S. oerstedti, but with genetic tools increasingly available, the genus has been reclassified into five species based on genetic, physical, and behavioral characteristics (Groves 2001). Squirrel monkeys are now divided into two groups, the Saimiri sciureus group, containing S. oerstedti subspecies, S. sciureus subspecies, and S. ustus and the Saimiri boliviensis group, containing S. boliviensis subspecies and S. vanzolinii. Most of the literature and information available on squirrel monkeys does not make these newer distinctions and focuses on the traditional taxonomy with only two species (Groves 2001).

MORPHOLOGY

Even though there are slight morphological differences among squirrel monkey species, all have the same general facial and body colorations and are easily distinguished as part of the genus. Squirrel monkeys have white masks of fur around their eyes and dark brown or black coloration around the mouth and chin. Species are separated by the shape of the arch of white fur over their eyes, and are either characterized as having a "roman" or "gothic" arch (Rowe 1996; Groves 2001). The species in the Saimiri sciureus group have a "gothic" arch in which the white fur is dramatically high and the darker fur on their heads forms a deep "V" shape between their eyes. S. boliviensis and S. vanzolinii are categorized in the Saimiri boliviensis group and have a "roman" arch of fur which is more rounded than the "gothic" type and does not extend as far up onto the forehead. The gray or black fur on their head makes a very shallow "V" pattern between their eyes (Groves 2001).

Saimiri boliviensis peruviensis
Photo: Rosie Bolen

S. oerstedti subspecies are predominantly orange to golden-orange over their backs, hands, feet, and forearms while the crown fur is blackish in females or blackish-brown in males. Their hips and shoulders are grayish-brown and the base of the tail is also this color but is tipped in black (Rowe 1996). They have tufts of fur on their ears and their heads appear very round. S. o. citrinellus are sexually dichromatic with males having gray crown fur and females exhibiting black crowns (Groves 2001). The average height for both male and female S. oerstedti is 270 mm (10.6 in), but males weigh more than females. Average weights in males range from 750 to 950 g (1.65 to 2.1 lb), depending on the time of year, while females weigh between 600 and 790 g (1.32 to 1.74 lb) (Groves 1996; Boinski et al. 2002). The common squirrel monkey, S. sciureus subspecies, has brownish-gray crowns and have less red-orange pelage than S. oerstedti. They are mainly greenish-gray to auburn but S. s. sciureus and S. c. macrodon have orange or yellow hands, feet, and forearms. Differing in the coloration of the forearms and hands from the other subspecies, S. s. albigena exhibits grayish-brown coloration with a very pale orange tinge (Groves 2001). Average weights of S. sciureus range between 554 and 1150 g (1.22 to 2.53 lb) for males and 651 to 1250 g (1.43 to 2.76 lb) for females, with the males measuring only slightly longer than the females at 318 mm (12.5 in) compared to 316 mm (12.4 in) (Rowe 1996). The bare-eared squirrel monkey, S. ustus, stands apart from the other species because of its lack of ear tufts. It is also orange or yellowish on its forearms, hands, and feet with brownish-gray on the head (Groves 2001).

Saimiri oerstedti oerstedti
Photo: Sue Boinski

In addition to having a "roman" arch pattern over their eyes, S. boliviensis and S. vanzolinii have remarkably thin tails compared to members of the Saimiri sciureus group (Groves 2001). S. boliviensis females have gray bodies while males have black bodies. Both sexes have yellow fur on the forearms, hands, and feet (Rowe 1996). Males and females are similar heights, 310 mm (12.2 in) on average, but males are heavier than females, weighing between 963 and 1088 g (2.12 to 2.4 lb), depending on the season while females weigh between 700 and 900 g (1.54 to 1.98) (Rowe 1996). The black squirrel monkey, S. vanzolinii, gets its name from the black band from the crown to the tail. The feet, hands, and forearms are a light yellow (Groves 2001). Height ranges from 275 to 295 mm (10.8 to 11.6 in) in females and 278 to 320 mm (10.9 to 12.6 in) in males, while females weigh significantly less than males, 650 g (1.43 lb) and 950 g (2.09 lb), respectively (Rowe 1996).

Male and female squirrel monkeys are seasonally sexually dimorphic. Male squirrel monkeys have widely fluctuating weights throughout the year and gain weight during the two months prior to the breeding season, taking on a "fatted" appearance from water and fat stored between the muscle and skin on their head, shoulders, upper arms, and ribs. During this period of growth they can gain up to 20% of their body weight and the larger they appear, the more attractive they are to females (Boinski 1987a; Kinzey 1997).

Squirrel monkeys are quadrupedal and move through the forest preferentially traveling on branches between one and two centimeters in diameter (.394 and .787 in). They rarely come to the ground and when they do, it is to forage or play and they are rarely seen leaping horizontal distances greater than two meters (6.56 ft) (Boinski et al. 19981998; Kauffman pers. comm.).

RANGE

Squirrel monkeys are found widely throughout Central and South America. S. boliviensis is distributed throughout Brazil, Bolivia, Colombia, Peru, and Venezuela. S. b. boliviensis is found in Brazil, Peru, and Bolivia while S. b. peruviensis is found in only in Amazonian Peru (Groves 2001; Gold 2004). The Central American squirrel monkey, S. oerstedti, is found in Costa Rica (both subspecies) and Panama (S. o. oerstedti only) and is completely geographically separated from the other species of squirrel monkeys found in South America (Groves 2001). Once long thought to be separated from the other South American species because of introduction into Central America by early humans, molecular data prove that S. oerstedti is, in fact, endemic to this region (Cropp & Boinski 2000). S. sciureus is distributed across several countries: Brazil, Colombia, French Guiana, Guyana, Suriname, and Venezuela. The most restricted in range of the squirrel monkeys is S. vanzolinii which can be found only on the left bank of Lago Mamirauá and at the mouth of the Rio Japura in Brazil (Groves 2001). S. ustus ranges in Brazil south of the Amazon (Groves 2001).

Since 1981, Sue Boinski has studied squirrel monkeys in the wild, focusing her long-term research on S. o. oerstedti in Costa Rica at Corcovado and Manual Antonio National Parks. She has also done comparative research on S. b. boliviensis at Manu National Park in Peru (Kinzey 1997). More recently, Boinski has continued working with squirrel monkeys in a long-term study of S. sciureus in the Central Suriname Nature Preserve in Raleighvallen, Suriname. Most of the literature from field work comes from studies done at one of these four parks, but captive research has supplemented information about wild squirrel monkeys. Saimiri are the second-most used primates in captive research after rhesus macaques (Macaca mulatta) and have been studied widely across a variety of institutions and by a number of researchers (Kinzey 1997).

HABITAT

Squirrel monkeys are found primarily in tropical lowland rainforest throughout the Amazon basin from Paraguay to Guyana and in Costa Rica and Panama. Squirrel monkeys are habitat generalists and have few restrictive requirements in regard to forest type compared to other neotropical primate; they can survive in a myriad of habitat types, including disturbed and edge forests (Kinzey 1997; Kauffman pers. comm.). Across their range, squirrel monkeys are found in similar habitat types ranging from undisturbed tropical and evergreen primary forests, selectively logged tropical forests, secondary growth tropical forests, and disturbed or edge forests (Boinski 1987b; Rowe 1996; Kinzey 1997; Boinski 1999; Boinski et al. 2002). There is marked seasonality in most of these ecosystems, with the dry season lasting from approximately January to March and the rainy season from April to December with coinciding periods of fruit and flower abundance (Boinski 1987b).

ECOLOGY

Though they are geographically widespread, squirrel monkeys exhibit very little difference in ecological behavior because the habitats in which they are found are quite similar (Boinski 1999; Boinski et al. 2002). Squirrel monkeys are insectivores-frugivores, consuming insects and fruit in their diet, depending on seasonal abundance of each resource, and supplementing their diets with small vertebrates, nectar, flowers, buds, seeds, leaves, and gum (Boinski & Timm 1985; Kinzey 1997; Boinski 1999). Squirrel monkeys rarely go after insects that are in motion and prefer to capture stationary insects on plant surfaces. They hunt for insects on the surface of live leaves or by unfolding leaf curls of dead foliage and prefer caterpillars and grasshoppers over other insects (Boinski 1988; Janson & Boinski 1992). Saimiri preferentially ingest small, soft, berry-like fruits less than one centimeter in diameter found in the lower and middle canopies of the forest (Janson & Boinski 1992; Kinzey 1997). The small vertebrates consumed include bats, which they systematically search for in large stands of trees, small birds, and bird eggs (Janson & Boinski 1992). During the year, the period of greatest food abundance is between April and June (Boinski 1988).

Saimiri vanzolinii
Photo: Luiz Claudio Marigo

Differences in distribution of available resources across study sites and species leads to different levels of intragroup feeding competition. In Suriname, very small, dense patches of fruit characterize the feeding sites of S. sciureus while in Costa Rica, S. oerstedti forages in fruit patches which are larger but which have few ripe fruits available in each patch. S. boliviensis in Peru forages in the largest fruit patches with the highest densities of fruit in any squirrel monkey study site (Boinski 1999). In Costa Rica, where each fruit tree is depleted quickly by foraging squirrel monkeys, there is little within-group competition for resources because individuals simply move on to another patch after such a short time. In Peru, though, where there is a higher density of available fruit, within-group competition at feeding sites is high as group members have incentive to protect the particular area of the patch in which they are foraging. Among common squirrel monkeys, S. sciureus, within-group feeding competition is intense because fruit is found in small, dense patches easily monopolized by one monkey. The incentive to protect this patch is high and the result is direct feeding competition between members of the group. Between-group aggression is also seen in S. sciureus but not in S. oerstedti or S. boliviensis (Boinski et al. 2002).

Home range sizes and population densities are available for S. oerstedti at Corcovado National Park and for S. boliviensis at Manu National Park in Peru. At Corcovado, squirrel monkeys have home range sizes of about 2 km² (.77 mi²) and the population density is 36 individuals per km² (22 per mi²) compared to Manu where they have home range sizes about 2.5 to 5 km² (.97 to 1.93 mi²) and live in densities of 60 per km² (37 per mi²) (Mitchell et al. 1991). S. sciureus have home ranges between 2.5 and 3 km² (.97 and 1.16 mi²) and live in much lower densities than other species with only 13 individuals per km² (8 per mi²) (Boinski et al. 2002). Squirrel monkeys exhibit similar daily activity patterns, regardless of species, and spend more than half of their day traveling and foraging for insects, while about 11% of their day is spent feeding on fruit and nectar, another 10% of their day is spent resting, and the remainder of their day is spent doing miscellaneous activities such as social behavior and self-grooming (Mitchell et al. 1991).

Their small body size makes squirrel monkeys susceptible to a variety of predators across their range including raptors, snakes, and felids (Mitchell et al. 1991). Raptors are responsible for the highest number of observed predations on squirrel monkeys (Boinski et al. 2002). One predator avoidance pattern seen in squirrel monkeys in Peru is prolonged associations with other primate species. Saimiri and Cebus are seen forming mixed-species groups and associating for many days. These interactions are peaceable and squirrel monkeys benefit from the extensive alarm calling system of Cebus, which serves to alert group members of potential predators (Sussman 2000). Squirrel monkeys may also benefit from maintaining proximity to Cebus in terms of gaining access to previously unexploited resources. Cebus use similar fruit sources and small ranges and squirrel monkeys can exploit Cebus' knowledge of these ranges to utilize new fruit patches (Kinzey 1997; Sussman 2000).

Content last modified: March 16, 2006

Written by Kristina Cawthon Lang. Reviewed by Laurie Kauffman.

Cite this page as:
Cawthon Lang KA. 2006 March 16. Primate Factsheets: Squirrel monkey (Saimiri) Taxonomy, Morphology, & Ecology . <http://pin.primate.wisc.edu/factsheets/entry/squirrel_monkey/>. Accessed 2013 June 19.