Variable expression of linguistic laws in ape gesture: a case study from chimpanzee sexual solicitation

Code and data for research paper submitted to RSOS exploring linguistic laws in sequences of ape solicitation gestures

Two language laws have recently been identified as consistent patterns shaping animal behaviour, both acting on the organisational level of vocal and behavioural communicative systems. Zipf’s law of brevity describes a negative relationship between behavioural length and frequency of behaviour. Menzerath’s law defines a negative correlation between the number of behaviours in a sequence and the average length of the behaviour composing it. Both laws have been linked with the information-theoretic principle of compression, which tends to minimise code length. We investigate their presence in a case study of male chimpanzee sexual solicitation gesture. While we fail to find evidence supporting Zipf’s law of brevity, chimpanzee sexual solicitation gestures follow Menzerath’s law, with longer sequences of gestures having shorter average gesture duration. Our results extend previous findings suggesting gesturing can be shaped by individual energetic constraints; however, chimpanzee gesture does not appear to manifest a consistent principle of compression or pressure for efficiency described for most other close-range communication. Importantly, the same signallers and signal set adhered to these laws in subsets of the repertoire when used in play; highlighting that the expression of ape gestures appears shaped by factors such as the immediate socio-ecological context of the interaction.