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)