COLLINS’ SQUIRREL MONKEY
Geographic Distribution and Habitat
Collins’ squirrel monkeys (Saimiri collinsi) are endemic to eastern Amazonia, south of the Amazon River. They are found in the Amazon and in the transition areas between the Amazon and the Cerrado biome, a wooded grassland, neighboring the Amazon rainforest. They prefer secondary forest habitats situated close to rivers or streams but can also be found in primary forests. This variation in habitat may be associated with seasonal fluctuations in the availability of food resources. Considered a resilient species, they can tolerate living close to human settlements and have been found in rural areas and urban forest fragments in large cities in eastern Amazonia.
The number of species included within the genus Saimiri has been widely investigated. Historically, the genus was divided into only two species based on geographic distribution: the Central American squirrel monkey, Saimiri oerstedii, and the common squirrel monkey, Saimiri sciureus. However, genetic analysis allowed for reclassification of the genus into seven species. Until recently, the Collins’ squirrel monkey was considered to either be a subspecies or synonymous with the common squirrel monkey. In 2003, genetic analysis of samples collected from the north and south sides of the Amazon River indicated differences in the taxa. Numerous studies, notably in 2010 and more recently in 2015, found similar supportive evidence, and the Collins’ squirrel monkey was proposed as a full species, more closely related to the golden-backed squirrel monkey (Saimiri ustus) than to the common squirrel monkey.
Surprisingly, there are instances in which recent literature still fails to list the Collins’ squirrel monkey as a species under the genus Saimiri. Additionally, due to its prior identity as the common squirrel monkey, researchers must be wary when collecting information regarding the Collins’ squirrel monkey since sources may list them as the common squirrel monkey, making it difficult to gather data specific to the former.
Size, Weight, and Lifespan
Squirrel monkeys are sexually dimorphic with the adult males on average being approximately 25–30% heavier than adult females. Male Collins’ squirrel monkeys weigh approximately 1.63 pounds (740 g) and have a head and body length of about 15 inches (38 cm) with a 14.8 inch (37.6 cm) tail. Females weight around 1.4 pounds (635 g) and have a head to body length of about 11 inches (27 cm) with a 16.3 inch (41.3 cm) tail.
There are no known records for how long Collins’ squirrel monkeys live. However, in general, members of the genus Saimiri may live to about 15 years old in the wild, and over 20 years in captivity.
Living in close association in a way that allows one species to benefit without harming the other.
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Similar to its fellow Saimiri species, the Collins’ squirrel monkey has a white face, with black around the muzzle and white hair around the eyes. The ears have white tufts of hair. The forearms, hands, feet, and upper back are covered with dark rich tawny colored hair, whereas the underside is white, and the head is grayish. The tip of the tail is covered with black hair, and the rest of the tail is brown gray.
Some researchers distinguish one squirrel monkey species from another on the basis of the pattern created by the white patch of hair above the eyes. Based on this, they divide squirrel monkeys into two groups. Those possessing a pointed arch of whitish hair are classified as “gothic arch” and those possessing a more shallow, semicircular pattern are referred to as “roman arch.” The monkeys found in the south and west of the Amazon River basin such as Collin’s squirrel monkeys have been observed to have roman arch.
Similar to the other species of their genus, Collins’ squirrel monkeys are frugivorous-insectivorous with fruits, flowers and insects making up their diet. They prefer insects over the other food resources, with insects accounting for 80% of their feeding and foraging time. Some studies report that squirrel monkeys have developed specialized foraging techniques for invertebrate prey. For example, insects such as caterpillars which are covered with stings, are picked up using the end of the tail as a cushion and then rubbed against branches. Once the stings are dealt with, the caterpillars are consumed. Research has indicated that females forage for more fruits and flowers during the birth and lactation months, as well as consume a greater variety of fruit types. This may be because of the high energy costs related to reproductive childbearing and rearing.
Behavior and Lifestyle
Squirrel monkeys are arboreal, spending most of their time in trees actively foraging. They dedicate up to 75% of their time foraging and less time resting. However, this may change based on food availability. For example, periods of high food availability reduce foraging times and free up time to engage in social interactions.
While foraging, males descend closer to the ground more frequently than females who tend to forage in the understory foliage and barely descend to the ground. This may be because the foliage offers greater concealment from predators, which is especially important when carrying infants. In each troop, adult females, particularly those carrying infants, and juveniles get feeding priority and feed from a fruit-bearing tree first, whereas adult males eat after the females and juveniles have moved away from the tree.
Around mating season, males undergo dramatic physical changes, wherein they fatten up and become big, to attract more females.
Squirrel monkeys form mixed-species relationships with capuchins to receive predator protection and with birds who catch the insects escaping foraging.
Squirrel monkeys live in large multi-male and multi-female social groups of 25 to 75 individuals. Groups are sexually segregated, with males generally remaining at the periphery of troops for most of the year and having weak associations with females. As a result of such segregation, females form relationships with other females and with juveniles, while males spend more time with other males or alone.
During the mating period, adult males move closer to associate with the females and drift towards the periphery once mating subsides. A partial hierarchical structure exists, where individuals of either sex can occupy dominant positions within groups simultaneously. Dominant males have a larger body size than dominant females and exhibit agonistic behavior towards other males during the breeding season.
Squirrel monkeys have been documented to form mixed-species relationships with capuchins and could receive predator protection due to the alarm calls given by the capuchins. These associations are almost always initiated and maintained by the squirrel monkeys with few benefits for the capuchins. In addition to capuchins, squirrel monkey troops are usually accompanied by at least one species of bird. This is a commensal relationship in which the monkeys flush out insects and prey during foraging, which are caught and eaten by the birds.
A majority of the studies on the vocal repertoire of squirrel monkeys have been performed on common squirrel monkeys (Saimiri sciureus). Research focused specifically on the Collins’ squirrel monkey is lacking; however, information on the common squirrel monkey may provide some insights.
Vocalizations are the most important signals in squirrel monkey communication. Vocalizations start to occur shortly after birth and contribute significantly to the interaction between mothers and infants during early development. In adults, “chuck” is uttered in more relaxed social situations, for instance when huddling animals get slightly restless, when mothers call their infants, or when group members contact each other over small distances to maintain troop cohesion. An “err” call is given by females, and it increases during the breeding season and thus might be a part of the squirrel monkey repertoire exclusive to the breeding season. “Twitters” are typically uttered when a food source has been detected, when a lost member regains contact with its group, or during other positive events. “Cackles” represent social mobbing calls and express displeasure or annoyance. Their function is to recruit fellow combatants in agonistic interactions.
Collins’ squirrel monkeys are highly seasonal breeders, with mating occurring during July and August and births occurring from December to January. After a gestation length of approximately 5 months, females produce only one infant per reproductive event. Infants are well-developed at birth, weighing almost 16–20 percent of the mother’s body weight and they grow quickly during the first three months. Lactation lasts from six to eight months, with the end of weaning coinciding with the start of the next mating season. During lactation, females increase their foraging time, decrease travel time, and increase time spent in social activities. Lactating females decrease travel time, perhaps to avoid carrying their relatively large infants over long distances. Females participate in allocare, transferring their infants to other females and to older juveniles for short periods of babysitting time.
While squirrel monkey males are approximately 25–30% heavier than adult females even during the non-breeding season, preceding and during the short mating season, males show even more increased weight gain, which produces a “fattened” appearance in the upper arms, shoulders, and torso. This occurs due to fat deposition and water retention and is not related to increased food consumption by the males. Females tend to approach fattened males more and become more interactive and reactive with them. Additionally, the fattening may protect the males from wounds caused by higher levels of intrasexual aggression.
By consuming fruits, squirrel monkeys help native tree species by performing seed dispersal and dissemination. Monkeys are efficient dispersers and seeds can be dispersed by up to several hundred feet (or meters) by squirrel monkeys. Moreover, the seed’s germinative power can still be maintained even after passing through the monkey’s digestive tract. Additionally, due to their insectivorous diet and foraging techniques, they support the local bird populations and keep the insect populations in check.
The International Union for Conservation of Nature lists the Collin’s squirrel monkey as Least Concern (IUCN, 2015), appearing on the IUCN Red List of Threatened Species.
Collins’ squirrel monkeys are considered to be an abundant and widespread species, able to survive in areas within the Amazon undergoing anthropogenic disturbances, that is disturbances created by human activities. Only some of the areas they are found in are protected. Even though they are a resilient species, both the protected and unprotected areas they call home are facing increased pressures from deforestation, agriculture, livestock, and mining. A decrease in squirrel monkey populations has been observed.
The Collins’ 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.
Collins’ squirrel monkeys are found in numerous protected areas such as biological reserves, national reserves, and environmental protection areas. However, even these so-called protected areas lack conservation plans and are subject to threats. Despite the persistence of Collins’ squirrel monkeys as a whole, certain populations have declined and will continue to unless conservation plans are put into place proactively. Population surveys of the species within the protected areas need to be performed to determine their population size. Establishment of monitoring programs is essential for the long-term conservation of this species.
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- Merces, M. P., & de Paula, W. S. (2018). New records of Saimiri collinsi Osgood, 1916 (Cebidae, Primates), with comments on habitat use and conservation. Mammalia, 82(5), 516-520.
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- Zimbler-DeLorenzo, H. S. (2009). Zoo ecology of a primate species: Squirrel monkey (Saimiri sp.). Auburn University.
Written by Divya Patar, November 2022