Black-capped squirrel monkey
The black-capped squirrel monkey is a species of New-World monkey native to the upper Amazon basin in Bolivia, western Brazil and eastern Peru. They weigh between and measure, from the head to the base of the tail, between. The black-capped squirrel monkey is primarily tree-dwelling and is found in both native and plantation forests as well as some farmed areas near running water. Its diet is omnivorous and mostly consists of flowers, fruit, leaves, nuts, seeds, insects, arachnids, eggs and small vertebrates. It mostly lives in female-dominated troops of around 40 to 75 monkeys, with males having been observed to disperse to live in all-male troops after reaching sexual maturation. Its current conservation status according to the International Union for Conservation of Nature is 'Least Concern'. The species belongs to the genus Saimiri and has two subspecies, S. b. boliviensis and S. b. peruviensis.
Description
The black-capped squirrel monkey displays sexual dimorphism, with males normally weighing between and females weighing between. Infants typically weigh between when they are born. Adults of the species measure in length between for males and for females. The coat of the monkey is short, soft and dense, and the majority of the fur covering the back of the monkey is a grey to olive-brown hue, while the undersides are typically white, yellow or ochre. The head is characteristically black with white arches over the eyes. The tail is the same colour as the body with a black tufted tip and is not prehensile; it usually measures around.Physically, the black-capped squirrel monkey is very similar to a number of other species of squirrel monkey, but is distinguishable from other species by a number of features. The most noticeable of these are the dark black cap and the white 'Roman type' arch pattern over the monkey's eyes, which is more narrow and rounded than the 'Gothic type' arch pattern over the eyes of the other species. The tail of the 'Roman type' species is also narrower than that of the 'Gothic type'.
Evolution
Saimiri boliviensis is thought to have emerged in the Saimiri genus approximately 1.5 million years ago. It has been hypothesised that this diversification occurred due to environmental changes in the Pleistocene period which allowed for thicker vegetation to appear in the Amazonian rainforest.Fossils
Several different fossils have been linked to the genus Saimiri through examinations of their dental and cranial morphology, including the Early Miocene fossil Dolichocebus which was discovered in Gaiman, Argentina and dated to around 20.5 million years ago, and the Middle Miocene fossil of the genus Neosaimiri discovered in La Venta, Colombia, which has been dated to between 12.1 and 12.5 million years ago.Taxonomic classification
Originally all squirrel monkeys were considered to be of the same species; they were first divided into two different 'types' – 'Roman' and 'Gothic' – in 1964. Many different opinions on the taxonomic classification of Saimiri boliviensis as a separate species have been published, however various studies conducted by several researchers have concluded that Saimiri boliviensis is one of at most seven different species of Saimiri. Based on the geographical distribution and the morphological and behavioural characteristics of the specimens studied, Hershkovitz proposed the existence of Saimiri boliviensis as a distinct species with two sub-species, Saimiri boliviensis boliviensis and Saimiri boliviensis peruviensis. Based on cranial measurements and coat colouring, Thorington proposed Saimiri sciureus boliviensis as a sub-species of Saimiri sciureus. Another prominent report published in 1993 supports Thorington's classification, however after further investigation into the genetic characteristics of the monkey a more recently published report concludes that Hershkovitz's description is the most accurate. The dispute over the taxonomic and genetic classification of the black-capped squirrel monkey has become increasingly relevant with regards to their use in biomedical research, due to the fact that hybridisation may have an effect on the reproductive capabilities of the monkey, and has the potential to cause a differentiation in its susceptibility to certain pathogens and infections.Alternative or previously proposed taxonomic names include:
- Saimiri sciureus boliviensis
- Callithrix boliviensis
- Callithrix entomophagus
- Saimiri boliviensis nigriceps
- ''Saimiri boliviensis pluvialis''
Distribution and habitat
Biology
Saimiri boliviensis are mostly arboreal but will occasionally also be found on the forest floor. They are diurnal and have been observed to be most active during the early to mid-morning, before resting for one or two hours in the afternoon, followed by another period of activity from the early afternoon to evening.Social systems
The black-capped squirrel monkey is found in female dominated troops of around 45 to 75 monkeys, unlike its relative the Common squirrel monkey which habitually lives in male dominated troops of around 15 to 50. Similar to most other species of monkeys, female Saimiri boliviensis will remain in the troop into which they are born, while males are more likely to be excluded by more dominant females. When they reach sexual maturity, male Black-capped squirrel monkeys will disperse from their natal troop into smaller all-male groups, and will eventually join a larger mixed-sex troop, often together with other males from the all-male group. The females use an egalitarian social structure instead of a dominant hierarchy and will actively exclude males to the periphery of the group. Black-capped squirrel monkey troops display high levels of aggression between females. Female monkeys will often compete with other female members of the troop to determine access to resources, however it has been observed that despite heavy competition for food they still prefer to live in large groups in order to reduce the likelihood of predation.Often black-capped squirrel monkeys will socialize with other species of monkeys: primarily Cebus and Saguinus monkeys for the benefits of their alerting behavior and access to resources. Cebus has a distinct and developed predator alarm system that is highly accurate. Saimiri boliviensis benefits from this because their alerts are more likely to be false alarms, and the extra surveillance during feeding increases their chance of survival. This is important because Saimiri boliviensis forage quickly and the males have little genetic stake in protecting group members; thus, both factors leave members more vulnerable to predators. Secondly, squirrel monkeys receive more access to resources by staying with Cebus and Saguinus. The other species can forage large fruits and nuts that are normally too hard for squirrel monkeys to access. Furthermore, the Cebus monkeys are better at finding ripe fruit crops. The relationships between these species are usually nonhostile, but when food is scarce or the foraging areas are small, there can be increased aggression towards Saimiri boliviensis by the other species due to feelings of possession over the discovered feeding site. In these instances, Saimiri boliviensis are only scared away temporarily and not fully excluded.