A coalition is formed when one animal intervenes in an ongoing conflict bet
ween two parties to support one side. Since support of one party is also an
act against the other party, coalitions are triadic interactions involving
a supporter, a recipient, and a target. The purpose of this study was to t
est which of three possible theories explains coalition formation among mal
e Barbary macaques: 1) Males support kin to enhance their indirect fitness
(kin selection). 2) Males support nonkin to receive future reciprocal suppo
rt (reciprocal altruism). 3) Males pursue self-interests and immediately be
nefit via nonkin support (cooperation). Coalition formation was investigate
d among 31 semi-free male Barbary macaques in the Salem Monkey Park, German
y during the mating season. The results show: 1) Males intervened more ofte
n in dyadic conflicts in which a related opponent was involved and supporte
d related opponents more than unrelated opponents. Close kin supported each
other more often than distant kin. 2) Some evidence for reciprocal support
was found. However, reciprocity was probably a by-product of targeting the
same individuals for dominance. 3) Coalition formation among nonkin is bes
t interpreted as cooperation, based on self-interests. Male Barbary macaque
s seem to intervene more often to stabilize and less often to improve their
rank. Although our data were limited, the results revealed that kin suppor
t, reciprocal support, and cooperative support were all involved in coaliti
on formation among male Barbary macaques. (C) 2000 Wiley-Liss, Inc.