Coalitionary aggression in white-faced capuchins.
Susan Perry, UCLA
Until quite recently, little was known about the dynamics of social relationships in
New World monkeys, and the vast majority of research on social complexity in primates
was conducted on Old World monkeys and apes. Because of their extraordinarily large
encephalization quotients (which can be calculated from data in Harvey et al. 1987),
proponents of the social intelligence hypothesis would certainly expect to find social
complexity and cooperation in the gregarious capuchin monkey, genus Cebus. Indeed,
capuchins, unlike most other New World monkeys, form frequent coalitions in a wide
variety of contexts, and it seems reasonable to assume that skill in forming and
maintaining alliances is critical for enhancing reproductive success. Here, I present data
from a wild population of white-faced capuchins, Cebus capucinus, studied at Lomas
Barbudal Biological Reserve, Costa Rica from 1990-2001.
Capuchins have an elaborate repertoire of gestures for coalitionary aggression
(see accompanying photo of the “overlord,” the most frequent coalitionary posture).
They form coalitions in a broad array of social contexts. About 25% of female-female
coalitions vs. conspecifics occur in the context of feeding competition (Perry 1995).
Even when the monkeys are foraging in trees large enough to hold the entire group plus a
few more individuals, the lowest-ranking male and female are typically barred from
entering the tree by coalitions.
Redirected aggression towards subordinates is common in capuchins, and many
coalitions occur in the wake of other fights or in other socially tense situations. Rank
reversals always entail extensive coalitionary involvement from multiple individuals.
Although the female dominance hierarchy is relatively stable and linear, changes occur
when adolescent females ascend the hierarchy, and when key members of social
networks die, leaving their former allies vulnerable to being challenged (Perry 1996a,
Manson et al. 1999). Unlike most Old World monkeys with despotic ranking systems,
capuchin females do not appear to support their own daughters preferentially in the rank
acquisition process (Manson et al. 1999). All adult males are dominant to adult females
in most capuchin groups (Fedigan 1993, Perry 1997). The alpha male has a very distinct
role from subordinate males, but the relative rankings of subordinate males are
sometimes difficult to discern (Perry 1998a). Both of the two overthrows of the alpha
male that were witnessed at Lomas Barbudal were bloody and involved large coalitions
of both males and females. The alpha male was killed in one of these reversals (GrosLouis, in prep), and in the other the alpha male was temporarily evicted but returned as a
subordinate male later (Perry 1998b).
One coalitionary context particularly noteworthy because of its parallel with
chimpanzee coalitionary strategies (de Waal 1978; Nishida and Hosaka 1996) is
separating interventions. Separating interventions are quite common in capuchins, and
although they are employed by both males and females, they are a particularly common
strategy used by alpha males (Perry 1998a,b). When the alpha male sees two subordinate
males affiliating or engaging in a sexual interaction, he threatens one of the two and
solicits aid from the other one, thus ending the interaction. The alpha male is remarkably
inconsistent with regard to which males he supports against which other males, even over
short time intervals, and this coalitionary fickleness, combined with the fact that the alpha
male rarely permits subordinates to associate, may account for the lack of clarity in
subordinate males’ dominance relationships (Perry 1998a,b).
According to the standard definition of coalitions (de Waal and Harcourt 1992:3),
a coalition involves aggressive cooperation between group-mates against conspecifics.
However, capuchins engage in the same motor patterns as found in true coalitions when
they cooperate in menacing heterospecifics, and these pseudocoalitions are far more
common than true coalitions. For example, 25% of all male-male coalitions are directed
at monkeys, and the rest are directed at heterospecifics, including both predators and
harmless creatures (see below). Although all group members mob predators to to some
extent, adult males are the most frequent participants (Perry 1995, Rose 1998). When
adult females form coalitions with adult males against potential predators, they do so
preferentially with the alpha male (Perry 1997).
Capuchins form pseudocoalitions against harmless animals such as frogs, deer,
anteaters, howlers & primatologists, and sometimes even inanimate objects. For
example, capuchins sometimes form vehement and prolonged coalitions directed at a pile
of monkey feces, some innocuous-looking insect, a dead or sleeping animal, an egg, or a
bare patch of dirt. In fact, these pseudocoalitions are far more common than coalitions
against conspecifics. Pseudocoalitions most likely serve to communicate something about
the relationship between the two participants. It is possible that they provide practice in
coordinating aggression for future coalitions against other monkeys and predators.
Because pseudocoalitions are often prolonged and can consume a fairly significant
portion of the activity budget, the opportunity costs entailed by engaging in them may
also make them useful signals of commitment to alliances. One of the most common
contexts of pseudocoalitions is when a subordinate monkey is foraging peacefully and a
dominant comes into view, heading his way. Then the subordinate will start threatening
the harmless target and solicit aid from the dominant, who almost invariably responds by
joining in the threats and perhaps forming an overlord. Psuedocoalitions may be an
effective tactic for subordinates to avoid displacements by dominants at feeding sites, as
the dominant often leaves the subordinate to feed peacefully following a pseudocoalition.
The last main context of cooperation is male-male coalitions vs. extragroup males
(Perry 1996b). During intergroup encounters, the females and juveniles flee while the
resident males of each group form coalitions with one another against the males of the
other group. Intergroup fighting sometimes results in severe wounding and is probably
very important in preventing foreign males from immigrating. Immigration of a group of
males generally results in the eviction of some or all of the resident males, and there are
sometimes infanticides associated with turnovers in male membership (Fedigan et al.
1996, unpublished data, Rose 1998). Two infanticides have been witnessed at Lomas
Barbudal (Julie Gros-Louis, pers. comm.), and several infanticides have also been
reported at the nearby Santa Rosa (Rose 1994, K. MacKinnon & K. Jack pers. comm.).
Partner choice in coalition formation follows a set of fairly rigid rules in
capuchins. When a monkey witnesses a fight between a female and a male and opts to
take sides, s/he sides with the female 85% of the time, despite the fact that males are
dominant to females. In the fights that did not conform to this pattern, the victim was a
low-ranking female and the male’s supporter was a female higher-ranking than the
victim. When fights break out between same-sexed partners, however, capuchins almost
always support the dominant opponent. This is true in about 89% of male-male fights,
and 98% of female-female fights, and most exceptions to this rule consist of an adult
supporting a juvenile against an adult. Capuchins also tend to preferentially support
monkeys with whom they have more affiliative relationships (as measured by “proportion
of interactions that are affiliative”); in 85% of those fights in which a witness chose to
join in the fight, s/he supported the monkey with which s/he had the more affiliative
relationship. All three of these “rules” (preference for females, dominants, and affiliative
partners) exert independent effects on partner choice.
Capuchins clearly have complex social relationships and social organizations, and
their coalitionary strategies parallel those of chimpanzees in many ways. The extent to
which cognitive complexity is involved in creating the observed patterns of behavior is
less clear, and further research in captive settings will probably be necessary to resolve
this issue.
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