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Saimiri Natural History

boliviensis boliviensis (male) (female)
boliviensis peruviensis
( male) (female)
orstedii citrinellus

orstedii orstedii (female)
sciureus albigena
sciureus cassiquiarensis
sciureus macrodon
sciureus sciureus
(male) (female)
ustus
~ Herskovitz, 1984

Taxonomy

Squirrel monkeys are small neotropical primates. There are up to five generally recognized subspecies which are found throughout the Amazon Basin with isolated populations found along the western Panama and Costa Rica border, and in eastern Costa Rica, which indicates that this species had a much larger distribution in the past (Cooper, 1968; Hershkovitz, 1984; Thorington, 1985). There are two generally recognized groups of squirrel monkeys within the genus Saimiri. They are commonly called the Roman arch type and the Gothic arch type as determined by the shape and color of the pelage of the upper eyebrow.

The first group contains one species Saimiri boliviensis with two subspecies Saimiri boliviensis boliviensis and Saimiri boliviensis peruviensis. Saimiri boliviensis peruviensis is found in northwestern Peru south to about 108 S. Altitudinally they are found between 90 and 800 meters above sea level (Hershkovitz, 1984). Saimiri boliviensis boliviensis are found in parts of extreme western Brazil between the Ríos Juruá and Purus. In Bolivia, they inhabit the upper Rio Madeira river basin occupying approximately the upper two thirds of the country. This subspecies also inhabits the lower one third of Peru in the Río Ucayali basin and is found altitudinally in its range between 50 and 500 meters above sea level (Hershkovitz, 1984).

The second or Gothic arch group contains three species: Saimiri sciureus with four distinct subspecies, Saimiri ustus and Saimiri oerstedi, which has two recognized subspecies. (Hershkovitz, 1984) S. sciureus subspecies are: Saimiri sciureus sciureus, Saimiri sciureus macrodon, Saimiri sciureus cassiquiarensis and Saimiri sciureus albigena (Hershkovitz, 1984). Saimiri oerstedi subspecies are Saimiri oerstedi oerstedi and Saimiri oerstedi citrinellus.

Geographically, S. sciureus sciureus is found in most of Guyana, Suriname, French Guiana and in extreme northeastern Brazil around the Río Amazonas south to about 68 S. Altitudinally they are found between sea level and 100 meters above sea level. S. sciureus macrodon is found in Brazil between the Río Juruá and Río Japurá westward, in extreme southeastern Colombia, in Ecuador, in northeastern Peru with some overlap in range with Saimiri boliviensis peruviensis. S. sciureus cassiquiarensis inhabits the Amazon basin in extreme northwestern Brazil, extreme southern Venezuela and parts of eastern Colombia. S. sciureus albigena occupies the eastern two-thirds of Colombia from the Cordillera Oriental eastward, and altitudinally is found between 150 meters to approximately 1,000 meters above sea level (Hershkovitz, 1984).

Saimiri ustus is geographically distributed in eastern central Brazil between the Río Purus and Río Xingú south of the Río Solimões. Altitudinally they range from sea level to 100 meters above (Hershkovitz, 1984). Saimiri oerstedi oerstedi is found on the Pacific coast lowlands of northern Costa Rica and in much of northern Panama. S. oerstedi citrinellus also inhabits the Pacific coast of Costa Rica south of the range of S. o. oerstedi. The ranges of these two subspecies are believed to be entirely separate (Hershkovitz, 1984).

Ecology

In the wild, these monkeys inhabit most types of tropical forest including both wet and dry forest, continuous and secondary forest, mangrove swamps, riparian habitat and forest fragments (Hernández-Camacho and Cooper, 1976; Terborgh, 1983; Baldwin, 1985; Boinski, 1987b). They appear highly flexible in their ability to adapt to different environments and in some geographic areas appear to prefer disturbed habitats (Konstant and Mittermeier, 1982; Boinski, 1987b). They are omnivores which eat insects when they are available but include fruit, flowers and occasionally vertebrates in their diet when necessary or readily available (Thorington, 1967; Baldwin and Baldwin, 1972; Jones, 1973; Izawa, 1975; Mittermeier and van Roosmalen, 1981; Scollay and Judge, 1981; Boinski, 1988; Mitchell et al., 1991; Rowe, 1996).

Social Structure

In the wild, squirrel monkeys are found in large multi-male/multi-female groups numbering up to at least 50 animals with possible reports of up to 300 (Thorington, 1967; Baldwin and Baldwin, 1971; Hernández-Camacho and Cooper, 1976; Baldwin, 1985; Boinski, 1987b; Mendoza et al., 1991, Rowe, 1996). Sex ratio within wild groups tends to be approximately 50:50 males and females made up of adult females and their offspring, subadult males and fully adult males. Group size may vary somewhat depending on habitat type (Baldwin and Baldwin, 1971; Scollay and Judge, 1981). It was thought that these groups tended to break up during the day for foraging in smaller groups, rejoin for a time during the day for a rest period and then continue foraging separately until coming back together for the night (Thorington, 1967). More recent reports find that these groups tend to forage as cohesive groups and explain the earlier reports as documenting the use of several large fruiting trees at one time by the same troop (Mitchell, 1990). Boinski (1987b) found that in Costa Rica squirrel monkeys tend to forage more widely and rest less when food is scarce during the peak wet season and travel least during the birth season.

In captivity, age/sex ratio differs depending on the type of social grouping that can be maintained in the available housing. Generally, social groups are maintained in captivity with only one or two males per group as problems have developed with aggression among the males if more than two males were maintained within a social group. Groups which have access to a more semi-natural living situation with a larger available living space, such as the Monkey Jungle housing area in Florida, are able to house more males within the same groupings due to the increased complexity of the environment and the possibility of immigration to an adjacent group and increased provision of hide areas. In captivity, social group size can range from a single pair to up to 35-50 animals per group depending on available housing and the project for which the animals are being maintained. Squirrel monkeys are extremely social animals and in captivity are maintained in species-typical social groupings as much as possible.

Female squirrel monkeys tend to reach maturity and begin breeding at around the age of 2.5 to 3 years of age. Frequency of female intertroop transfer differs among groups from differing geographical regions with a high rate of female transfer occurring in Saimiri oerstedi, with males remaining in their natal groups in these species, to a low rate of female transfer in Saimiri sciureus and boliviensis spps., which demonstrate relatively high rates of male transfer (Boinski, 1987c). Males reach sub-adult age by the time they are 2.5 to 3 years old and generally transfer from the natal group at that time depending on species. They may then join an all male group of juveniles and sub-adults for a time until they become fully adult at about the age of 5 and are able to work their way into the male dominance hierarchy of an established group and begin breeding.

Social Organization

Squirrel monkey society is generally unified by the adult females because all age-sex classes except fully adult males have been shown to be more attracted to adult females than to any other age-sex class (Baldwin, 1969; Strayer and Harris, 1979; Scollay and Judge, 1981; Baldwin, 1985). Sexual segregation present on a seasonal basis is a unique feature of social organization in this species. Group males remain near the periphery of the group in the non-breeding season, and the majority of social interactions between the sexes take place during the breeding season (Dumond, 1967, 1968; Baldwin, 1968, 1969, 1971; Mason and Epple, 1969; Baldwin and Baldwin, 1972; Candland et al., 1973; Coe and Rosenblum, 1974, 1978; Alverez, 1975; Strayer et al., 1975; Kaplan, 1977; Vaitl, 1977, 1978; Hopf, 1978; Mendoza et al., 1978b; Strayer and Harris, 1979; Leger et al., 1981; Scollay and Judge, 1981; Baldwin, 1985; Mendoza et al., 1991; Lyons et al., 1992; Boinski and Mitchell, 1994).

There are, however, differences in the expression of this trait in the social behavior repertoire reported for populations from different geographical regions (Mendoza et al., 1978a; Gonzalez et al., 1981; Mitchell et al., 1991). Saimiri boliviensis demonstrate strong sexual segregation whereas Saimiri sciureus spps. and Saimiri oerstedi spps. societies are more sexually integrated (Mendoza et al., 1978a; Boinski, 1987b). Some studies have found evidence that this sexual segregation may be female initiated through active exclusion of the males (Baldwin, 1968, 1969; Dumond, 1968; Baldwin and Baldwin, 1972; Coe and Rosenblum, 1974; Fairbanks, 1975; Vaitl, 1977; Mendoza et al., 1978b). Other studies have found that the segregation may be due to intermale social dynamics instead of female agonism (Strayer and Harris, 1979; Lyons et al., 1992).

Dominance hierarchies within squirrel monkey groups differ in form and degree depending on species. Saimiri boliviensis boliviensis, which show a high degree of sexual segregation within their social groups, demonstrate distinct linear dominance hierarchies among males with dominance associated with higher testosterone levels and generally, copulatory frequency. A separate but less distinct dominance hierarchy is seen among females of this species. (Mendoza et al., 1978b) Saimiri sciureus spp., which tend towards sexual integration within a social group demonstrate a linear dominance hierarchy including both sexes with all males being dominant over all females. (Mendoza et al., 1978b, Mitchell et al., 1991). Saimiri oerstedi spp. in the wild do not demonstrate a dominance hierarchy among either sex and males even participate in cooperative mobbing of females for olfactory examination of estrous state during the breeding season (Boinski, 1987c; Mitchell et al., 1991).

Allomaternal care, or care of an infant in a maternal way given by someone other than the birth mother, has been documented in squirrel monkeys in studies from the field (Ploog, 1967; Dumond, 1968; Baldwin, 1969; Hunt et al., 1978) and the laboratory (Williams et al., 1988, 1994). Infant squirrel monkeys may spend as much as 30% of their time on allomothers during the first six months of their lives (Baldwin, 1969; Williams et al., 1988, 1994). Allomothering usually begins during the first two weeks of life in this species (Williams et al., 1994). In the wild, allomothers are reported to usually be juvenile females (Dumond, 1968). In captivity, reports have shown that the majority (53%) of the allomothering is done by young adult females age 4 to 6 while adult females age 7 to 9 provided about 20% of the allomaternal care (Williams et al., 1994). Females who had experienced a reproductive failure during that year (Williams et al., 1988) performed almost all of the allonursing.

Breeding and Reproduction

A yearly reproductive cycle is seen in these monkeys including a distinct three-month breeding season followed about six months later by a birth season (Dumond and Hutchinson, 1967; Dumond, 1968; Goss et al., 1968; Rosenblum, 1968; Baldwin, 1969; Michael and Zumpe, 1971; Coe and Rosenblum, 1978; Boinski, 1987a, Rowe, 1996). A unique feature of this yearly cycle is reproductive seasonality occurring in the males as well as the females in this species (Dumond and Hutchinson, 1967; Baldwin, 1969; Kaplan, 1977; Coe and Rosenblum, 1978; Mendoza et al., 1978a; Williams et al., 1986; Boinski, 1987c). This is demonstrated in what is termed the "fatted male" condition. Both sexes gain weight throughout the pre-breeding season attaining peak weights prior to breeding. Weight gain in males is associated with increased spermatogenesis in preparation for breeding. (Dumond and Hutchinson, 1967; Coe and Rosenblum, 1978; Williams et al., 1986)

The yearly cycle has been shown in some field studies to be related to annual rainfall cycles and thus seasonal food availability with the birth season occurring at the period of greatest food availability insuring the female adequate nutrition post-parturition (Dumond and Hutchinson, 1967; Baldwin, 1968; Dumond, 1968; Thorington, 1968; Baldwin and Baldwin, 1981; Boinski, 1987a). Changes in light cycles have also been shown to be related to the timing of the reproductive cycle (Rosenblum, 1968; Follett and Follett, 1981) as has humidity level (Dumond, 1968). Boinski (1987a) in her field studies in Costa Rica found a strong tendency towards birth synchrony in this species which she suggested might be an anti-predator adaptation.

Estrous cycles have been estimated to be around seven to eight days in length (Rosenblum, 1968; Wolf et al., 1975; Kaplan, 1977). This cycle length can be affected by social conditions (Hutchinson, 1970; Wolf et al., 1975) and light cycles (Rosenblum, 1968).

The breeding season in squirrel monkeys shows a shift when animals are moved into the northern hemisphere (Lehner et al., 1967; Dumond, 1968; Rosenblum, 1968; Kaplan, 1977). Several studies have linked this breeding season shift to humidity factors (Dumond, 1968; Baldwin and Baldwin, 1971; Harrison and Dukelow, 1973). It has been linked also to the photoperiod by other authors (Rosenblum, 1968; Coe and Levine, 1981). The breeding season in captive squirrel monkeys is probably controlled by a combination of several different factors that are all related to the monkeys natural habitat (Dukelow, 1985).

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