Black-capped squirrel monkey

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.[13]

Originally all squirrel monkeys were considered to be of the same species;[14] they were first divided into two different 'types' – 'Roman' and 'Gothic' – in Paul D. Maclean's article 'Mirror Display in the Squirrel Monkey, Saimiri sciureus’, published in 1964.[15] Many different opinions on the taxonomic classification of Saimiri boliviensis as a separate species have been published,[14] however various studies conducted by several researchers have concluded that Saimiri boliviensis is one of at most seven different species of Saimiri.[16][14] 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.[9][17] Based on cranial measurements and coat colouring, Thorington proposed Saimiri sciureus boliviensis as a sub-species of Saimiri sciureus.[2][17] Another prominent report published in 1993 supports Thorington's classification,[16] 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.[18] 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,[19] and has the potential to cause a differentiation in its susceptibility to certain pathogens and infections.[20]

Alternative or previously proposed taxonomic names include:[2]

• Saimiri sciureus boliviensis
• Callithrix boliviensis (monkeys from Bolivia)
• Callithrix entomophagus  (monkeys from Tefé, Brazil)
• Saimiri boliviensis nigriceps (monkeys from Cosnipata, Peru)
• Saimiri boliviensis pluvialis (monkeys from the Rio Jurua in Brazil)

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.[12]  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.[21]  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.[7] Black-capped squirrel monkey troops display high levels of aggression between females.[12] Female monkeys will often compete with other female members of the troop to determine access to resources,[7] 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.[21]

A family of Saimiri boliviensis

A Black-capped squirrel monkey will typically reach sexual maturation at around 3 years of age for females and 5 years of age for males.[4][6] The yearly reproductive cycle of mature Saimiri has been observed by several researchers to be affected by a number of environmental factors, including the cycles of rainfall and levels of illumination in their habitat. The mating season coincides with the dry season, and will typically result in a single infant being born to each mother.[8] It has been suggested that adult female monkeys are more receptive to environmental cues for the mating season to begin, and the response in males is in part attributed to behavioural and scent cues from the females.[10]  During the mating season, males of the species will gain a large amount of subcutaneous upper body fat, leading to what is known as a 'fatted' appearance. It has also been observed that the testosterone levels of monkeys of the Bolivian and Peruvian species are noticeably higher during the mating season than those of other species of male Saimiri from Guayana.[22] Males will become more irritable and aggressive, fight more frequently for the purpose of achieving dominance within the troop, and engage in genital display towards less dominant males.[10] It has been observed in both natural and laboratory settings that the hierarchy of the troop may change up to as often as three times in a month, and this will often result in highly aggressive fights which may lead to the complete exclusion of younger adult males from the troop.[10] The scent and behavioural cues of a female monkey assist a male in his judgement of whether or not she will be receptive to his approach and attempt at mating with her. If she is not receptive, the female, sometimes with the aid of other nearby females, will usually chase the male away.[10] Consorting and copulation between a male and female monkey may last between one minute and over an hour depending on the presence of other monkeys and the environment in which it takes place. The gestation period of the monkey has been estimated to last between 160 and 170 days.[10] During the first week following its birth, an infant monkey will cling to its mother's back and will seldom move or be attended to by the mother unless it is in some way in need of assistance.[10]

Squirrel monkeys have been found to be some of the most vocal primates, with a large range of different types of calls documented throughout their lifespan.[4] Commonly used sounds include 'chucks',[23] a variety of purrs and squawks elicited during the birthing and mating seasons, chirps and peeps used for alarm or attention, as well as aggressive screaming and 'barking'.[4] Infant Black-capped squirrel monkeys tend to vocally communicate much more than adults.[23] The most common form of infant communication is a number of different 'peeps', which begin to occur most frequently at around 2 months of age when the infant starts to spend more time away from its mother.[23] After maturation, the most commonly used call for adult females is a variety of 'chucks', used to maintain contact in dense vegetation where visual identification is not possible.[24]

Black-capped squirrel monkeys are omnivorous. A typical squirrel monkey diet includes fruits, insects, eggs, small vertebrates, arachnids, leaves, flowers, nuts, seeds, and rarely fungi;[4][6][25] however it has been observed that they prefer insects to fruit.[10] Mature Saimiri spend most of the day foraging. They will begin foraging at around 60 to 40 minutes before sunrise and will spend the first part of the day actively feeding on fruits and any insects they are able to hunt while foraging for fruit.[10] They will then adopt more sedate feeding behaviours and spend the rest of the day resting and hunting for more insects.  Often, when food is not scarce, they will stop and rest for an hour or two in the middle of the day when it is too hot to continue.[10] When fruits are scarce, squirrel monkeys have been reported to consume Ascopolyporus, a fungus that is parasitic on scale insects which are parasitic to local bamboo species. Researchers formulate that this is done as an alternative food source during dry seasons.[25]

Black-capped squirrel monkeys are mostly found in trees and will often leap 1–2 metres between branches. They are capable of moving swiftly through dense vegetation at a four-legged walk or run with diagonal sequence footfalls (wherein the hind leg on one side will touch down followed by the foreleg of the opposite side).[26] They will occasionally adopt a stationary bipedal stance at ground level while foraging. The monkey's tail is usually used for balance, or by infants to secure them to their mother's tail or abdomen.[10]