Communication and cooperation in wild chimpanzees (Pan troglodytes schweinfurthii)

Comparisons between animal and human communication are invaluable for
understanding the evolution of language and, as our closest living relatives,
chimpanzees can provide particularly important insights into this. Here I examined
unimodal (UM) and multimodal (MM) communication in wild chimpanzees, in an
integrated manner. I found that although MM signals were produced infrequently, and
at lower rates compared to captivity, the vast majority of adult and sub-adult
individuals did freely combine vocal, gestural and facial signals to produce MM signals.
A total of 48 free MM signal combinations were observed, incorporating a wide range
of different signal types from different modalities. Focusing on one specific vocalgestural
MM signal, I found that MM combinations and UM gestural signals were more
successful in eliciting responses compared to UM vocal signals.
To investigate signal function more systematically I focused on one common grooming
gesture, the big loud scratch (BLS), and tested several competing functional
hypotheses. I found little evidence to support the hypotheses that this signal operates
as an attention-getter, or as a referential signal. In contrast, my data suggested that in
this community of chimpanzees, the BLS facilitates the negotiation of roles within a
grooming bout. Groomers used BLSs to request grooming during grooming bouts and
the BLS seemed to show willingness to groom, both to initiate a grooming bout, and
potentially during a bout when groomees intend to start grooming their partner.
Finally, to explore the theoretical link between the evolution of communication and
cooperation I tested whether, on an individual level, there was a positive relationship
between communicativeness and cooperativeness in chimpanzees. In contrast to
theoretical predictions, I found a significantly negative relationship between these two
domains, indicating that more communicative chimpanzees were less cooperative. I
explore several potential explanations for this highly unexpected finding.