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| S.sciureus |
On one of the islands in the lake can be found our group of a very familiar, but little understood, small monkey, the South American Squirrel monkey Saimiri sciureus. Squirrel monkeys are widely spread throughout central and South American rainforests, but until recently it was thought that there were only two species, The South American S.sciureus and the Central American S.oerstedti. More recent work has elevated some subspecies to specific status, and identified other new forms, with the result that there are now considered to be at least five species, some of which themselves may contain several subspecies.
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| S.oerstedti |
This causes some problems in analysing past studies, as the different species differ in important aspects of their behaviour, especially group size and breeding patterns. In the captive population, several subspecies or even species may have been hybridised, which may account for some of the problems experienced in captive breeding. At Bristol our group is believed to be pure S.sciureus sciureus, the nominate subspecies. The various forms differ slightly in size, but the most obvious difference is in the shape of the ‘V’ between the eyes as the darker fur of the crown of the head points down. In S.sciureus this is a very distinct and narrow ‘V’ whereas in S.bolivensis it is much shallower.
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| S.bolivensis |
In the wild, squirrel monkeys are mostly found in lowland tropical forests, all though in some areas they range to an altitude of 2000 metres. They are habitat generalists however, and can live almost anywhere there is sufficient food sources, especially edge forests or abandoned farmland with regenerating woodland. In fact, untouched primary forest with a mature canopy often has lower densities than areas with at least some human disturbance. As long as an area is not heavily felled, the main problem is logging roads – squirrel monkeys are very reluctant to descend to the ground and will not leap gaps over 2m, so roads cut through the forest can seriously disrupt their habitat.
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| S.vanzolinii |
As with many monkeys, squirrel monkeys will eat almost anything, but the bulk of their diet is fruit – as with most monkeys, they are important seed disbursers. Their animal prey is mostly insects, especially grasshoppers and caterpillars, but they also eat birds eggs, frogs, lizards, and even bats, which they catch at their roosts in trees or under leaves. Because they feed on smaller fruits and can forage further out onto branches, they can associate with other monkey species without much competition, and consequently benefit from travelling in company with them for protection. Capuchin monkeys (to which squirrel monkeys are closely related) are favoured companions, and as capuchins are powerful, aggressive, and highly intelligent they provide much-needed protection. An animal the size of a squirrel monkey has many predators, especially birds of prey and snakes, so it needs all the help it can get.
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| S.sciureus |
The various species of Saimiri differ widely in their group structure, which is probably due to the different food types available to them. The most social species is probably S.bolivensis, which is found in groups of up to 75 or more animals (one troop of 300 was reported). By caontrast, S.sciureus lives in groups of 15-30 animals, and at much lower densities. The relations between males and females differ greatly as well, with males in S. bolivensis subordinate to the females and mostly on the periphery of the group as it moves, whereas S.sciureus has a fully integrated multi-male/multi-female group with a strong hierarchy incorporating both sexes. How young animals disperse also differs, with males remaining and females dispersing in S.oerstedti, females remaining and males dispersing in S.bolivensis, and both leaving in S.sciureus. This variation in social behaviour is another issue with management of captive Saimiri species, as the best way of integrating new individuals can be very different depending on the species involved.
Squirrel monkeys are seasonal breeders, with all the females in a troop giving birth within a few weeks of each other. Prior to the breeding season, males undergo a remarkable physiological change called ‘fatting up’ when they put on a lot of weight – up to 20% of their normal weight. As there is intense competition among males for mating rights, this weight gain is vital to their success, in the same way that bull seals put on a lot of weight prior to the breeding season at their rookeries. Squirrel monkeys reach maturity at 2-3 years and can live up to 20 in captivity, but probably nearer 10-15 in the wild.
As a result of their adaptability, most squirrel monkey species are less vulnerable to human modifications of their forest than some other species, but at least one species, S.ustus, is classed as near Threatened by the IUCN as a result of deforestation. S.ustus has the smallest range of any of the Saimiri species, and is consequently more at risk. It is alsoone of the least studied, and its own unique social structure is little understood. Other human threats to squirrel monkeys are capture for the pet trade, both internally and internationally, and use in medical research (it is one of the commonest laboratory primates).
Even though squirrel monkeys are widely kept, the captive breeding record is pretty poor. The main cause is probably the size groups they are kept in – they do not often breed successfully in groups of under 15 animals, even for S.sciureus, and for species with larger group size such as S.oerstdti and S.bolivensis the groups probably need to be at least twice that size. To compound the problem, smaller groups seem to be more likely to produce male offspring. These kind of effects occur in many other animal species, and is probably a result of stress hormones differentially affecting pregnancy success in male and female embryos. It is notable that the most successful groups, with the most female offspring, are large and long-established groups. From the lifespan of captive animals, the dietary requirements seem to be mostly met, but the social requirements are harder for zoos, especially small ones, to meet successfully owing to the large number of animals that must be maintained.
Next week, a much larger monkey – the Black and Gold Howler, one of the species we have been much more successful with.
(images from primateinfo.net)
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