Abstract

In non-human primates, allo-grooming has a hygienic function and is the main behaviour used
to establish and maintain friendly relationships (Dunbar, 1991). Allo-grooming is also used as
a ‘currency’ that can be exchanged for grooming or other social services such as agonistic
support and tolerance over food (Noé & Hammerstein, 1994; Schino, 2007; Carne et al, 2011;
Berghänel et al, 2011). The biological market approach predicts that the availability of a
commodity and the resource-holding-potential of each animal both affect the payoffs that
each animal gain from the exchange of commodities (Noë et al, 2001). For example, when
food is scarce only high-ranking individuals have direct access to high-quality food sources
whereas low-ranking individuals may exchange a service given (e.g. grooming) against
tolerance over food. Allo-grooming can give various benefits (e.g. hygienic or agonistic
support) and its distribution is affected by the dominance position of each animal in a group.
Therefore, dominant animals can coerce allo-grooming from subordinates or prevent other
group members from accessing the social benefits associated with grooming exchanges. Here
we analysed whether grooming disruption is a tactic animals use to coerce grooming or to
take control over the social relationships other group members build or maintain.
Subjects of this study were 13 adult monkeys from a wild non-provisioned group of Barbary
macaques living in the Middle Atlas Mountains of Morocco. Data were collected from April
to December 2011, using a post-disruption (PD) – matched-control (MC) focal sessions
method. As soon as a monkey approached two grooming partners at a minimum distance of
1.5m and stopped the grooming interaction for at least 10 seconds, we ran a 10-minutes PD
focal session on the disrupting monkey (i.e. the disrupter). During the PDs, we recorded the
first grooming interaction and aggression (e.g. charge or chase) involving the disrupter, as
well as ID of the partners. At the same time, data were collected on whether the former
grooming partners resumed grooming within 10 minutes from the disruption. Ten minutes
MC focal sessions were run on the disrupter and using the same data collection method used
in the PD sessions. As potential benefits of the disruption for the disrupter, we considered 1)
any grooming between the disrupter and the targets of the disruption (i.e. any of the two
former grooming partners), and 2) whether the disruption was successful (i.e. no grooming
was observed between the former grooming partners in the PD session) or not. As potential
costs of the disruption for the disrupter, we considered aggression received by the disrupter
from one or both the former grooming partners.
All the disrupters were dominant over one or both former grooming partners. We found no
significant difference for the grooming rate exchanged between the disrupter and the former
grooming partners between PDs and MCs. However, the disrupters were significantly
successful at disrupting grooming in the PDs. Finally, we found that the disrupters were at
greater risk of receiving aggression from the former grooming partners in PDs than MCs, but
not significantly so.
Our results indicate that grooming disruption is costly for the disrupter (i.e. increased
aggression risk) but does not give grooming benefits for the disrupter. However, the disrupters
were successful at stopping the grooming between two grooming partners. We suggest that
monkeys disrupt grooming as a strategy to maintain their dominant position. By stopping
grooming, dominants prevent lower-ranking individuals to maintain or build social
relationships that these individuals could use to form alliances against dominant animals. This
spiteful behaviour may allow individuals to influence the social relationships of other group
members at their own advantage. The advantage of keeping a dominant position may outweight
the direct cost of the disruption. Such a strategy would be similar to the ‘Divide et
impera’ maxim that has been successfully employed in political, military and economic
contexts in human history.