Red-backed or Central American squirrel monkey (Saimiri oerstedii)
provided by EOL authors
The binomial name Saimiri oerstedii was given by Johannes Theodor Reinhardt to honour his fellow Danish biologist Anders Sandøe Ørsted. The red-backed squirrel monkey occurs along the Pacific coast of Costa Rica and Panama inland to altitudes of up to 500 m asl. (28,31,32,38), being restricted to the northwest tip of Panama near the border with Costa Rica and the central and southern Pacific coast of Costa Rica, primarily in Manuel Antonio and Corcovado National Parks [8]. It is the only species of squirrel monkey occurring outside South America [5]. Some people think squirrel monkeys lived in Colombia in the late Miocene or Pliocene and migrated to Central America, becoming the ancestors of the red-backed species. Passage through the isthmus of Panama later closed due to rising oceans, and eventually opened up to another wave of migration @ 2 million years ago, when ancestors of modern populations of monkeys out-competed the earlier migrants, leading to the small range of the red-backed squirrel monkey [19]. The red-backed species was thought to be a a population of a South American species of squirrel monkey brought to Central America by humans. Evidence included the very small range of the red-backed species and the large gap from the range of other species. Nuclear and mitochondrial DNA showed tht the red-backed squirrel monkey is a separate species that apparently diverged from the South American species 260,000 to over 4 million years ago.[3] The squirrel lives mainly in lowland scrub forest, but also inhabits humid tropical forest, mature upland forest, river edge and mangrove forest (22). It prefers seasonally inundated forests, forest, floodplain, secondary forests and primary forests which have been partially logged [12]. It needs forests with abundant low and mid-level vegetation and has difficulty surviving in tall, mature, undisturbed forests that lack such vegetation [8,12,42]. In general, squirrel monkeys are arboreal and can be found in primary and secondary forests (38), thickets, and mangrove swamps (52). They are also found in cultivated areas, usually around streams (38). Saimiri oerstedii is known to inhabit humid Pacific slope forests (28). This small monkey has a slender body. Adults are 225-295 mm long, excluding the 362-465 mm tail, and weigh 500-1,100 g [5,8,28,39]; males weigh @ 750-950 g and females 600-800 g (5). The tail is not fully prehensile, but aids balance as the monkey leaps between branches like a squirrel [10,11]. The limbs are long and slender and the thighs are shorter in relation to the lower leg than in monkeys that clamber. This adaptation lets squirrel monkeys exert more force when jumping, so they can propel themselves further (24). The fur is short, thick and yellow brown, with the underside being white or pale yellow and with red, golden-red or orange fur on the back (25,28,40), olive shoulders, hips and tail, a black cap on top of the head [39], a white face with black rims around the eyes and black around the nose and mouth [7,8], orange hands and feet and a black tail tip. Males and females are similar, but males are 16% larger and generally have lighter caps than females [5]. In the breeding season males become 'fatted', becoming larger around the neck and shoulders (24). Squirrel monkeys have the largest brains of all primates relative to body size; the red-backed squirrel monkey's brain weighs about 25.7 g or @ 4% of its body weight [7,9]. The monkey is active by day and is arboreal (26). It usually walks and runs through the trees on all fours [7]. It lives in groups of 20-75 with several adult males, adult females and juveniles [5,12,43,46,47]. Groups above 100 are thought to be temporary mergers of two groups [2]. On average, groups contain about 60% more females than males [5]. The monkey has an egalitarian social structure; neither males nor females are dominant over each other [5]. Females do not form dominance hierarchies or coalitions [2,5]. There is no evidence of coalition formation in social interactions. Males in a group tend to be related to each other, so tend to form strong affiliations, only forming dominance hierarchies in the breeding season [5,15]. This is especially the case among males of the same age [12]. Groups have a home range of 35-63 hectares [12], which can overlap, especially in large, protected areas rather than more fragmented areas[12]. Groups can travel 2.5-4.2 km a day [13,26,29]. They do not split into separate foraging groups during the day. Individuals may separate from the main group to engage in different activities for periods of time, so the group may be dispersed over an area of up 1.2 hectares at any given time [14]. The group tends to sleep in the same trees every night for months at a time [14]. Groups generally do not compete or fight with each other [2]. The monkey is noisy. It makes many squeals, whistles and chirps[8]. It travels through the forest noisily, while disturbing vegetation [8]. It has four main calls: a "smooth chuck", "bent mask chuck", "peep" and "twitter".[7] There is little competition or agonistic interactions between groups. The squirrel monkey rarely associates with the white-headed capuchin. Its food occurs in smaller, more dispersed patches than with South American species, so associating with capuchins would impose higher foraging costs. While male white-headed capuchins are alert to predators, they devote more attention to detecting rival males than to detecting predators, and relatively less time to detecting predators than their South American counterparts. Associating with capuchins provides less predator detection benefits and imposes higher foraging costs on the red-backed squirrel monkey than on South American species [5,11,16,17]. Some bird species associate with the red-backed squirrel monkey. They follow monkeys to try and prey on insects and small vertebrates that the monkeys flush out, especially in the wet season, when arthropods are harder to find [12]. The monkey is omnivorous and spends most of the morning and afternoon foraging through the lower and middle levels of the forest (mainly the lower canopy and understory), typically at 4.5-9 m high.[12,14], but may travel at other levels. It eats berries and other fruits, nuts, seeds, leaves, buds, flower nectar, gums and other plant materials, invertebrates (especially grasshoppers and caterpillars) and small vertebrates [4,27,28,38,41,43,44]. Nearly half of the diet is made up of fruit [4]. It has difficulty finding its desired food late in the wet season, when fewer arthropods are available [8]. It is said to recognise the folded leaf-tents made by some fruit-eating bats, attacking them to extract roosting bats (28). When it finds a bat it climbs to a higher level and jumps onto the tent from above, trying to dislodge the bat. If the fallen bat doesn't fly away in time, the monkey pounces on it on the ground and eats it [12]. The fruits the monkey typically exploits occur in small and very scarce patches; feeding competition is very low. In the dry season shortages of appropriate fruiting trees means it may depend entirely on animal prey (45). The monkey is an important seed disperser and a pollinator of passion flowers and other flowers [12]. It is not a significant agricultural pest, but may eat corn, coffee, bananas, mangos and other fruits[12,14]. When monkeys find food in a tree, they often do not completely use up the resources available and may return to it later (6). Predators include birds of prey, cats and constricting and venomous snakes [5]. The oldest males show high levels of vigilance for predators and bear most responsibility for detecting them [2,12]. When a monkey detects a raptor, it gives a high-pitched alarm peep and dives for cover, as do other monkeys that hear the alarm call. The monkeys are very cautious about raptors and give alarms when they detect any raptor-like object, including small airplanes, falling branches and large leaves [14]. Raptors spend much more time near monkey troops when infants are born and prey on many of the newborn infants. Other predators include toucans, tayras, opossums, coatis and spider monkeys [14]. Reproduction is seasonal, seldom exceeding two months in duration in the dry season (51). Mating occurs in September [12] or January and February. All females enter oestrus at about the same time. A month or two before the breeding season begins, males become larger, due to altered water balance within the body. This is caused by converting the male hormone testosterone into oestrogen; the more testosterone he produces, the more he grows before the breeding season. Reproductively mature males collaborate in mobbing females in the mating season. As males in a group have not been seen fighting over access to females in the breeding season, nor trying to force females to copulate with them, it is thought that female choice determines which males get to breed with females. Females tend to prefer males that expand the most in advance of breeding season. This may be because the most enlarged males are generally the oldest and most effective at detecting predators or it may be due to runaway intersexual selection [14]. Males may leave their group for short periods of time in the breeding season to try to mate with females from neighboring groups. Females are receptive to males from other groups, but resident males try to repel intruders. Mating usually occurs during the dry season. In S. oerstedii, sexual receptivity in females is synchronized, lasting perhaps 12-36 hours a season [50]. Single births occur after 6-7 months at night and during wet season, the period of greatest food availability, within a single week in February and March [5,6,12,14,28], so there is enough food for mothers and their young and less time is spent foraging (30). The infant has a fully prehensile tail [10,11]. It depends on its mother for about one year [12]. The mother takes care of the young, which rides on her back and nurses, with little attention paid to it by the group members (6), but other females ('aunts') may help (6). During its third and fourth weeks of life, the infant begins to move around more and between weeks five and ten, it occasionally disembarks from its mother's back, explores the nearby area, and starts to eat solid foods (6). Over the next few months, contacts with the mother become less frequent (6). Social play probably first occurs @ 2 months (6) and helps separate the infant from its mother (52). In the first year of life, the young monkeys engage in social play with each other, usually in the form of fighting games (6). Females give birth every 12 months [48,53], so the prior infant becomes independent at about the same time the new infant is born. Only 50% of infants survive over 6 months, largely due to predation by birds [5]. Females do not resume cycling until their infant either dies or is weaned (4). Females become adult at 12-13 months and sexually mature at 1-2.5 years old; males become sexually mature at 4-6 years old [5,6]. Females leave their natal group on reaching sexual maturity; males usually stay with their group for their entire lives [5]. Males of the same age tend to associate with each other in age cohorts. On reaching sexual maturity, an age cohort may choose to leave the group and try to oust the males from another group to attain increased reproductive opportunities [5]. Captive can live over 15-18 years [12,55]; other species may live over 20 years.[5] The monkey was classified as Endangered on the IUCN Red List 2003 (2,22,38) and is listed on Appendix I of CITES (23). The population declined steeply after the 1970s, probably due to deforestation for agriculture and logging, hunting and capture to be kept as pets; its fragmented range encompasses 8,000 square km [18,38]. In 2008 the International Union for Conservation of Nature (IUCN) upgraded its conservation status from "endangered" to "vulnerable"; it is abundant in the areas it inhabits [4]. Its survival is entwined with the future of the forests (22). A survey in 2003 showed that the total population size (1300-1780) is significantly larger than previous estimates (26,28). The population density is estimated at 36 monkeys per square kilometre in Costa Rica and 130 in Panama.[13] It has been estimated that the population of the Central American squirrel monkey has been reduced from about 200,000 in the 1970s to less than 5000 [18]. There are significant efforts within Costa Rica to try to preserve this monkey from extinction.[20] A reforestation project within Panama tries to preserve the vanishing population of the Chiriqui Province.[21] There are two subspecies [1]. The black-crowned Central American squirrel monkey (Saimiri oerstedii oerstedii) lives in the Chiriqui and Veraguas provinces, on the Pacific coast of northwest Panama and the coast of the Puntarenas province and Osa Peninsula area of south-west Costa Rica (including Corcovado National Park) (28,29,31-34). It lives at 0-500 m above sea level (31). It has a black crown and more yellowish limbs and underparts [3]. In Panama, it has suffered habitat losses of 76% and now occurs in fragmented forest areas throughout its range (1,166km²) (22). It is Endangered. The grey-crowned Central American squirrel monkey (Saimiri oerstedii citrinellus) lives in the Quepos forests in the Central Pacific portion of south-west Costa Rica [22,26,35], up to 500 m above sea level. In 2003, the remaining wild population was estimated to be 1300-1800 individuals [2] in a restricted, severely fragmented range of 210 km² [22,36,37]. The male has an agouti crown; the crown is blackish-grey in the female (24). It is Critically Endangered and has lost 89% of its original habitat, due to widespread logging and clearing for cattle ranches which started during the 1950s. Large areas were planted with African oil palms and rice. The largest single population occurs in the Manuel Antonio National Park in Panama (22). The total number is up to 1300-1800 individuals (26,32,37).
