Call combinations in chimpanzees: a social tool?

A growing body of evidence suggests the capacity for animals to combine
calls into larger communicative structures is more common than previously
assumed. Despite its cross-taxa prevalence, little is known regarding the
evolutionary pressures driving such combinatorial abilities. One dominant
hypothesis posits that social complexity and vocal complexity are linked,
with changes in social structuring (e.g. group size) driving the emergence
of ever-more complex vocal abilities, such as call sequencing. In this
paper, we tested this hypothesis through investigating combinatoriality in
the vocal system of the highly social chimpanzee. Specifically, we
predicted combinatoriality to be more common in socially-driven contexts
and in females and lower-ranked males (socially challenging contexts and
socially challenged individuals respectively). Firstly, through applying
methods from computational linguistics (i.e. collocation analyses), we
built an objective repertoire of combinatorial structures in this species.
Second, we investigated what potential factors influenced call combination
production. We show that combinatoriality is predominant in i) social
contexts vs. non-social contexts, ii) females vs. males and iii)
negatively correlates with male rank. Together, these results suggest one
function of combinatoriality in chimpanzees may be to help individuals
navigate their dynamic social world. More generally, we argue these
findings provide support for the hypothesised link between social and
vocal complexity and can provide insight into the evolution of our own
highly combinatorial communication system, language.