What They Did
The researchers observed and filmed several hundred
grooming-based interactions among 42 male chimpanzees to examine the impact of
age, dominance, and closeness of relationship on turn-taking during grooming
sessions. Closeness of social relationships was quantified based on the
relative frequency with which pairs of individuals engaged in grooming together
or were in close physical proximity. To determine the dominance hierarchy, the
researchers analyzed the frequency with which individuals had clearly lost
fights with other chimpanzees and had engaged in submissive behaviors.
Turn-taking transitions were classified into four types:
action-action, in which A grooms B, then B grooms A; action-signal, in which A
grooms B, then B responds with a gesture; signal-action, in which A makes a
gesture and B responds by grooming; and signal-signal, in which A makes a
gesture and B responds with another gesture.
They found that turn-taking transitions were most likely
when the individual initiating the transition was older or when the responding
individual was younger or lower ranked. Younger chimpanzees were more likely to
respond to grooming by grooming the other chimpanzee in return. Transitions in
which grooming by one chimpanzee was followed by a gesture from another were
most likely to occur when the individual that initiated grooming was younger.
Lower ranked individuals were the most likely to respond to a gesture by
grooming the initiator; this interaction pattern was also more likely between
individuals with weak social bonds. Finally, older chimpanzees were the most
likely to respond to gestures with gestures of their own.
Further Exploration
I was surprised that the chimpanzees were more likely to
respond to a gesture requesting grooming when they had a weaker social bond with
the initiator. I’m more likely to respond to requests from people I’m close to
than from people I don’t know, and I don’t think I’m terribly unusual in that
regard. On the other hand, the chimpanzee groups only consist of 100 to 200
individuals. In a group that small, no one is exactly a stranger.
The researchers also suggest that the process of requesting
and receiving grooming may strengthen the social bonds, allowing for future
benefits, and that reciprocal grooming may occur more naturally in strongly
bonded pairs, without the need for a gesture of request. Both of these potential
explanations make sense to me. Trying to expand my social circle with new friends
involves more tentative bids for attention than starting up a conversation with
someone I’m already close to.
It’s interesting, though, that the data didn’t show a higher
rate of action-action transitions (i.e. A grooms B, then B grooms A) for more
closely bonded pairs, since that would seem to reflect the more natural occurrence
of reciprocal grooming. One possibility is that the reciprocity in grooming
among closely bonded pairs is more spread out in time: perhaps A grooms B for a
while, then a few hours or a day later, B grooms A. I’m sure one could design a
study to explore that hypothesis, but that’s a rabbit hole for another day!
https://commons.wikimedia.org/wiki/File:Chimps_Grooming_(2394016065).jpg
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