By Jan Vermeer
Apenheul Primate Park, P.0. Box 97, 7300 AB Apeldoorn, The Netherlands
Squirrel monkeys (Saimiri spp.) have always been a common sight in zoos, but this seems to have changed in the past few years. Breeding is often poor, and zoos have discovered that this genus is not as easy to keep as has always been assumed. Several zoos have shown an interest ill establishing new squirrel monkey groups, and a good start can have a positive effect on future results. Therefore, it is important to understand the social behaviour of this species.
The social behaviour of squirrel monkeys
Although squirrel monkeys are among the most popular laboratory and zoo animals, relatively little is known about their social behaviour in the wild. There have been some field studies (Baldwin, 1985; Baldwin and Baldwin, 1971; Boinski, 1987a, 1987b; Boinski and Mitchell, 1994), which have shown that the social behaviour of Saimiri oerstedi of Central America differs significantly from that of S. boliviensis and S. sciureus of South America. As only the latter two species are currently kept in captivity, I will focus on their social behaviour. In addition to the information available from field studies, a considerable amount of data on the squirrel monkey's social behaviour has been collected in the semi- natural environment of Monkey Jungle in Miami (Baldwin and Baldwin, 1981; DuMond, 1969; DuMond and Hutchinson, 1967).
Group size in the wild varies significantly in different geographic areas. The most common size is from 20 to 40 animals; however, groups exceeding 200 animals have been reported, though it is not clear whether such large groups consist of a single unit, or are in fact composed of several smaller groups traveling together during periods when food is abundant.
Group composition has only been determined from a few feral groups of about 20 animals. These groups included about three adult males and at least twice as many adult females. The adult females function as the core of a Saimiri group, and all other group members are attracted to them. There are clear sub-groups of adult females, juvenile females, adult males and juvenile males. Within the group, the males are sexually segregated (although this is less pronounced in S. sciureus than in S. boliviensis), tending to live at the periphery of the group, except during the mating season, when they are spermatogenic and more socially active. Outside the breeding season, the females can react very aggressively towards approaching males. Saimiri groups are matrilinear. Young males leave their natal group in small troops of three to five animals at the age of three years. Adult males stay in a breeding group for two or three seasons (Mitchell et at., 1991), until other males take over the group.
Formation of a captive heterosexual Saimiri group
As squirrel monkeys are matrilinear, it is important that all females of a new group are related to each other, that is, that they come from the same natal group. The introduction of unfamiliar females to a small group with several adult females can result in much aggression with severe injuries. Sometimes such unfamiliar females are tolerated by other group members, but they will hardly ever become fully a part of the group, and there can be stress for long periods.
The minimum number of breeding females in a group should be five to seven. Smaller groups tend to have less reproductive success, probably because the onset of the reproductive cycle is stimulated by social behaviour between adult females (Mendoza and Mason, 1991). The presence of several reproducing females will also enhance the development of social behaviour of the infants, as they can interact with peers.
A maximum of two adult males should be added to these females. Additional adult males will not be accepted by the others in the group. Two adult males may tolerate each other, but preferably these two should be related animals, e.g. brothers. Castration does not seem to be successful as a means of controlling aggression (Salzen, 1984).
Experiences in several European zoos have shown that female squirrel monkeys stop breeding if the same adult males are in a group for more than four years (Vermeer, 1996). Therefore, and also to prevent inbreeding, adult males must be replaced every four years by other, unrelated adult males. These new males will not accept the presence of other males older than two years, which therefore have to be removed from the group.
As Saimiri groups tend to form sub-groups, inside quarters should consist of at least two cages, but preferably more, all connected with each other.
Formation of a captive all-male Saimiri group
As in social groups of squirrel monkeys females outnumber males, there is a surplus of males in captivity. This problem can be relatively easily solved by the formation of all-male groups. Groups of up to ten males can be formed without many problems. Preferably, these males should come from the same natal group, where they were used to living together in one of the sub-groups.
Most zoos prefer breeding groups, but an all-male group of squirrel monkeys can be very attractive for the public, and females may be difficult to obtain, as they are rarely surplus in zoos.
Baldwin, J.D. (1985): The behavior of squirrel monkeys (Saimiri) in natural environments. In: Handbook of Squirrel Monkey Research (eds. L.A. Rosenblum and C.L. Coe), pp. 35-53. Plenum Press, New York.
Baldwin, J.D., and Baldwin, J.I. (1971): Squirrel monkeys (Saimiri) in natural habitats in Panama, Colombia, Brazil, and Peru. Primates 12: 45-61.
Baldwin, J.D., and Baldwin, J.I. (1973): The role of play in social organization: comparative observations on squirrel monkeys (Saimiri). Primates 14 (4): 369-381.
Baldwin, J.D., and Baldwin, J.I. (1981): The squirrel monkeys, genus Saimiri. In: Ecology and Behavior of Neotropical Primates (eds. A.F . Coimbra-Filho and R.A. Mittermeier), pp. 277-330. Academia Brasileira de Ciencias, Rio de Janeiro.
Boinski, S. (1987a): Mating patterns in squirrel monkeys (Saimiri oerstedi). Behavioral Ecology and Sociobiology 21: 13-21.
Boinski, S. (1987b): Birth synchrony in squirrel monkeys (Saimiri oerstedi). Behavioral Ecology and Sociobiology 21: 393-400.
Boinski, S., and Mitchell, C.L. (1994): Male residence and association patterns in Costa Rican squirrel monkeys (Saimiri oerstedi). American Journal of Primatology 34: 157-169.
DuMond, F.V. (1969): A further note on the behaviour of semi-free ranging squirrel monkeys Saimiri sciureus. International Zoo Year- book 9: 149.
DuMond, F.V., and Hutchinson, T.C. (1967): Squirrel monkey reproduction: the 'fatted' male phenomenon and seasonal spermatogenesis. Science 158: 1067-1070.
Mendoza, S.P., and Mason, W.A. (1991): Breeding readiness in squirrel monkeys: female-primed females are triggered by males. Physiology and Behavior 49: 471-479.
Mitchell, C.L., Boinski, S., and van Schaik, P. (1991): Competitive regimes and female bonding in two species of squirrel monkeys (Saimiri oerstedi and S. sciureus). Behavioral Ecology and Sociobiology 28: 55-60.
Salzen, E.A. (1989): A closed colony of squirrel monkeys for laboratory studies. In: Housing, Care and Psychological Wellbeing of Captive and Laboratory Primates (ed. E.F. Segal), pp.115-134. Noyes Publications, Park Ridge, New Jersey.
Vermeer, J. (1996): EEP Survey of Saimiri. Apenheul Primate Park.
Reproduced with permission of International Zoo News.