Data from: Bystanders intervene to impede grooming in Western chimpanzees and sooty mangabeys

Grooming interactions benefit groomers, but may have negative consequences
for bystanders. Grooming limits bystanders’ grooming access and ensuing
alliances could threaten the bystander’s hierarchy rank or their previous
investment in the groomers. To gain a competitive advantage, bystanders
could intervene into a grooming bout to increase their own grooming access
or to prevent the negative impact of others’ grooming. We test the impact
of dominance rank and social relationships on grooming intervention
likelihood and outcome in two sympatric primate species, Western
chimpanzees (Pan troglodytes verus) and sooty mangabeys (Cercocebus atys
atys). In both species, rather than increasing their own access to
preferred partners, bystanders intervened mainly when an alliance between
groomers could have a negative impact on them: when the lower-ranking
groomer was close to the bystander in rank, when either groomer was an
affiliation partner whose services they could lose, or the groomers were
not yet strongly affiliated with each other. Thus, bystanders in both
species appear to monitor grooming interactions and intervene based on
their own dominance rank and social relationships, as well as triadic
awareness of the relationship between groomers. While the motivation to
intervene did not differ between species, mangabeys appeared to be more
constrained by dominance rank than chimpanzees.