Though Saimiri do not exhibit species differences in ecology, the social structure and behavior of each species varies widely. S. oerstedti live in extremely large, multi-male/multi-female groups ranging in size from 40 to 65 animals where there are low levels of aggression and social interactions between males and females are characterized as egalitarian.
The males within these groups have intense affiliations with one another while the females have low levels of association and social bonding and disperse from their natal groups at sexual maturity.
Males remain in their natal groups and breed with immigrant females. Costa Rican females exhibit no dominance hierarchy within their groups and there is only a slight dominance hierarchy among males. Females may move in and out of several groups over their lifetimes
The social organization of S. boliviensis at Manu National Park, Peru is markedly different from that seen in S. oerstedti. In Peru, squirrel monkeys live in multi-male/multi-female groups of 45 to 75 individuals in which males emigrate from their natal groups at sexual maturity and females remain in their natal groups throughout their lives.
Both sexes have independent dominance hierarchies, but females within this species are behaviorally dominant over all males within the group. Females often spatially segregate males to the periphery of the group through aggressive interactions, and exhibit aggressive behavior to other females except when interacting with their relatives. Males within the group, on the other hand, are not related but are extremely aggressive towards one another, especially during the mating season when they compete for mates.
When males disperse from their natal groups they form all-male bands, or coalitions, in order to immigrate into a new group and begin breeding . Members of these bachelor groups are usually members of the same age class and they work together to take over the highest positions in their new groups' dominance hierarchy. These alliances remain strong if the males remain in the same group as is evidenced by cooperative aggression toward immigrating males. Males may move in and out of several groups over their lifetimes.
Among squirrel monkeys of Suriname, social organization is different from both the Peruvian and Costa Rican species. S. sciureus live in much smaller groups than other squirrel monkeys, with only 15 to 30 individuals per group.
These multi-male/multi-female groups are fully integrated, unlike groups of S. boliviensis in which the males are peripheralized by females, and there is a strong dominance hierarchy incorporating both sexes. Aggression is extremely common and Surinamese squirrel monkeys often have disfiguring scars from fighting with group members . Males are affiliative and form close bonds while females do not appear to have coalitions or other types of social bonds.
Both male and female S. sciureus emigrate from their natal groups, females transfer between groups several times over their lifetime while males spend much of their lives on the periphery or ranging solitarily. When mixed-sex troops are formed, high-ranking males fight fiercely to prevent solitary or peripheral males from joining the group.
Squirrel monkeys have a polygamous mating system, but usually one or two males copulate the most frequently of any in the group.
Females reach sexual maturity around 2.5 years of age and males are sexually mature at 3.5 years.
Females prefer the sexually mature males that gain the most weight during the two months prior to the breeding season, which lasts from early August to early October. The first signs of "fatting" among males are seen in June.
While the largest males monopolize the majority of copulations, young females in their first or second breeding seasons are not as selective as experienced females and will mate with other sexually mature males who may not be as large.
In order to determine sexual receptivity, groups of males, usually related because of dispersal patterns among S. oerstedti, chase and grab a female, holding her down to inspect her genitalia. Presumably the males are using olfactory cues to determine her reproductive state because they remain nearby until the female shows interest in the largest males.
Gestation lasts 145 days and the birth season lasts from February to early April. This is the dry season and period of highest arthropod abundance All squirrel monkeys exhibit birth synchrony to decrease chances of predation, though S. oerstedti and S. sciureus exhibit more intense synchrony than S. boliviensis.
All of the pregnant females in S. oerstedti groups give birth within two weeks of each other while pregnant S. sciureus females give birth within less than a week. This concentration is much higher than S. boliviensis, who give birth within the same two months.
Most S. oerstedti females give birth every year while S. boliviensis give birth every other year.
Female squirrel monkeys are responsible for almost all infant care. Females within a group exhibit birth synchrony and increased vigilance during the birth season because neonates are particularly susceptible to predation.
In S. oerstedti and S. sciureus groups, males also vigorously protect infants from predators, but in S. boliviensis groups, where males are on the periphery of the social group, they exhibit no infant protection.
For the first month of life, infants remain in constant physical contact with their mother, usually being carried on her back because they are too large and impede movement if they are carried ventrally.
During weeks five to seven, infants begin to leave the mother to explore the surrounding environment and interactions between mother and infant become increasingly fewer.
Infants become independent during the second through fourth months of life, spending more time with peers or play groups. S. oerstedti are weaned by four months of age, but S. boliviensis are not fully weaned until 18 months of age.
Play is the most common behavior seen in squirrel monkeys younger than one year of age. Associations and relationships made with peers in play groups, especially among males, are thought to carry through to adulthood alliances.
Squirrel monkeys exhibit complex communication behaviors including distinct vocalizations and postural displays. They also use olfactory cues in communication with conspecifics.
Squirrel monkeys have at least 25 to 30 calls, divided into six groups: "peeps," "twitters," "chucks," "cackles," "pulsed calls," and "noisy calls". "Peeps" are heard in a variety of contexts including when isolated, during play, while exploring a new environment, during displays, and when disturbed or frustrated. They are tonal vocalizations that do not differ significantly between callers.
"Twitters" are given during feeding, partial isolation, during exploration, and greeting. They are also tonal and differ based on the individual caller. "Chucks" are the most widely used calls among squirrel monkeys and are calls consisting of notes of rapidly descending frequency heard in mother-infant interactions, sexual behavior, and as alarm calls. "Pulsed calls" are used in similar circumstances as "chucks," but sound like rapidly repeated pulses.
Low in frequency, "cackles" are heard during mild disturbance or agonistic encounters while "noisy calls" are both low and high pitched, nonharmonic shrieks heard during extreme distress.
One of the most widely recognized postural displays used by squirrel monkeys is also a method of olfactory or chemical communication. In a "urine-washing" display the monkey, male or female of any age, urinates on its hands and feet and then wipes its hands and feet on its shoulders, arms, and legs, spreading the urine over its body.
Some functions of "urine-washing" may include marking trails for other members of the group to follow, self-cleaning, displays of dominance, enhanced grasping of branches during locomotion, controlling body temperature through evaporative cooling, or communicating reproductive hormone levels.
Other typical chemical communication behaviors seen among squirrel monkeys include rubbing their scent glands of the chest or anogenital area on a substrate or conspecific, rubbing their nose along substrate and subsequently sneezing into their hands, and back rubbing either on surfaces or other squirrel monkeys.
Males use olfactory cues to determine the reproductive status of a female by physically restraining her and inspecting her genitals.