Both genera of the subfamily Cebinae are diurnal and arboreal in habit. In the wild, squirrel monkeys are found in multimale-multifemale groups of 10-55 animals, with some groups as large as 300 individuals observed. Larger groups tend to break into smaller groups for foraging during the day, aggregating together at night. Sex ratio is close to 1:1, with some species having fewer adult males than adult females.
Society generally revolves around the adult females, as studies show that all age/sex classes including adult males are most attracted to the adult females. Females are responsible for determining spatial relations between adult animals through affiliative and agonistic interactions. In Saimiri boliviensis and to some extent in Saimiri oerstedii, males are peripheral to the group in the nonbreeding season and are actively chased away when they approach non-estrus adult females. Adult females of these species are dominant to adult males except during the breeding season. Adult males are more integrated into the groups in Saimiri sciureus, and all adult males are dominant to all females. Juvenile and adult animals will huddle together during times of rest, with adult animals huddling almost exclusively in same-sex groupings. The huddling behavior is distinct, with animals in lateral contact and each animal's head tucked against its chest and its tail curled over its head and body.
Capuchins live in multimale-multifemale groups of 8-30 animals, with some larger groups of up to 50 animals noted for Cebus oliveceus. Sex ratio is 1:1 in some species, with other species having more females than males. Troops generally have one male who is dominant to all other individuals, and who aggressively defends the group against other groups. Males typically emigrate from their natal groups at between 2 and 4 years of age. Cebus capucinus groups have been reported to have frequent turnover of adult male group membership. Some species are reported to have males that are occasionally solitary or nomadic. All capuchin species are territorial. Capuchin individuals are active throughout most of the day traveling and foraging within their range.
Communication is both vocal and visual. Both genera are vulnerable to predation and give alarm calls in response to large carnivorous mammals, boas, and birds of prey. It has been reported that Cebus apella males have an alarm call directed towards harpy eagles. This call is a distinctive barking that varies in frequency and loudness to indicate to other group members the relative proximity of the eagle. Squirrel monkeys are known to emit more than 24 different vocalizations including predator alarm calls and distancing calls that allow individuals to locate each other when out of sight while foraging. Unique visual signals include the so-called "genital displays" of the squirrel monkey. One leg is extended outward, presenting a view of the genitals to another animal. Both male and female individuals use this signal as a greeting when one animal flashes another its genitals from a distance (open genital display). The genital display is also used to establish and exert dominance when a dominant animal approaches another at very close proximity and exposes its genitals to the other while averting its gaze (closed genital display). The submissive animal will huddle quietly facing the display. Erections and occasionally the squirting of urine often accompany closed genital displays by males.
Both squirrel monkeys and capuchins practice urine washing of the fur with their hands; this may help an animal scent mark its surroundings and other olfactory communication. Capuchins are known to throw things towards potential predators in their aggressive displays. Boinski reports Cebus capucinus in Costa Rica throwing branches, fruit, and other objects at coatimundis, tayras, opossums, and humans. She describes one incident in which a capuchin threw a squirrel monkey at her when it had depleted its supply of readily detachable branches.
Capuchins are also noted for their manual dexterity and ability to manipulate objects. They have comparatively large and developed brains for their body size. Their high level of intelligence has made them one of the primates of choice for animal behavior and cognitive research. Young capuchins in captivity are easily trained, leading to their popularity in the pet market. Older animals become problematic as pets once reaching sexual maturity, and male capuchin pets are sometimes castrated or have their teeth pulled to try and control their aggressive tendencies. This trainability of young capuchins in the past led to their being used as organ grinder monkeys in many parts of the world. Now some capuchin females are being trained as helper animals for paraplegics and other wheelchair-bound humans. With a capuchin's ability to move in three-dimensional space and retrieve items desired by their human hosts, they have proven themselves much more helpful than canine companions. In captivity they are avid tool users, and in the wild have been observed using rocks to open oysters and one was seen using a branch as a club to kill a snake that had been caught under a fallen branch. In captivity they have been observed to use their prehensile tails to manipulate and play with objects.
Social and self-grooming is a common behavior in capuchin monkeys and social grooming helps to reinforce the group dynamics. Dominant animals are groomed more than less dominant animals. Squirrel monkeys have rarely been observed to engage in social grooming, with the exception of some mothers grooming their infants. They do engage in high frequencies of self-grooming using both fingers and toes to groom their fur. Capuchins also engage in self-anointing behavior, often rubbing fragrant items on their chests and other body parts. This behavior is also seen in captivity, with onions being of particular interest.
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