SOCIAL ORGANIZATION AND BEHAVIOR
Though Saimiri do not exhibit species differences in ecology,
the social structure and behavior of each species varies widely (Boinski
1999). 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 (Boinski 1999). 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 (Boinski 1994; Boinski et al. 2005). 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 (Boinski
1994; Boinski et al. 2005).
Photo: Roy Fontaine
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 (Mitchell 1994). 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 (Mitchell 1994). 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 (Mitchell 1994; Boinski
1999; Boinski et al. 2005). 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 (Mitchell 1994).
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 (Boinski 1999; Boinski et al. 2002). 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 (Boinski
1999; Boinski et al. 2002). Males are affiliative and form close bonds
while females do not appear to have coalitions or other types of social
bonds (Boinski 1999). Both male and female S. sciureus emigrate from their natal groups (Boinski et al. 2005). 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 (Boinski et al. 2005).
Squirrel monkeys have a polygamous
mating system, but usually one or two males copulate the most frequently
of any in the group (Boinski 1987a). Females reach sexual maturity
around 2.5 years of age and males are sexually mature at 3.5 years
Photo: Roy Fontaine
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 (Boinski 1987a). 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
(Boinski 1987a). 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 (Boinski 1987a). 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 (Boinski 1987c).
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
(Boinski 1987c; 1999). 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 (Boinski 1987c). 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
(Boinski 1999). 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 (Kinzey 1997; Sussman 2000). 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 (Sussman 2000). 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 (Boinski 1999).
Photo: Roy Fontaine
Squirrel monkeys exhibit complex communication behaviors including
distinct vocalizations and postural displays. They also use olfactory
cues in communication with conspecifics (Kinzey 1997; Sussman 2000).
Squirrel monkeys have at least 25 to 30 calls, divided into six groups:
"peeps," "twitters," "chucks," "cackles," "pulsed calls," and "noisy
calls" (Newman 1985). "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 (Newman
1985). "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 (Newman 1985). 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 (Newman 1985).
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 (Baldwin
& Baldwin 1981; Boinski 1992). 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 (Boinski 1992). 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 (Boinski 1992). Males use olfactory
cues to determine the reproductive status of a female by physically
restraining her and inspecting her genitals (Boinski 1987a; 1992).
Content last modified: March 16, 2006
Written by Kristina Cawthon Lang. Reviewed by Laurie Kauffman.
Cite this page as:
Cawthon Lang KA. 2006 March 16. Primate Factsheets: Squirrel monkey (Saimiri) Behavior . <http://pin.primate.wisc.edu/factsheets/entry/squirrel_monkey/behav>. Accessed 2015 February 1.