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. 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. 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. 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. 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. 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. Communication 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. Commonly used sounds include ‘chucks’, 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’. Infant Black-capped squirrel monkeys tend to vocally communicate much more than adults. 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. After maturation, the most commonly used call for adult females is a