Idiosyncratic gesture use in a mother-infant dyad in chimpanzees (Pan troglodytes schweinfurthii) in the wild

Abstract

One promising method to tackle the question, "In which modality did language evolve?" is by studying the ontogenetic trajectory of signals in human's closest living relatives, including chimpanzees (Pan troglodytes). Concerning gestures, current debates centre on four different hypotheses: "phylogenetic ritualization", "social transmission through imitation", "ontogenetic ritualization", and "social negotiation". These differ in their predictions regarding idiosyncratic gestures, making such occurrences a crucial area of investigation. Here, we describe a novel and potential idiosyncratic behaviour - 'hand-on-eye' - which was initially observed in one mother-infant dyad in a community of chimpanzees living in the wild. We systematically investigated the form, sequential organisation, intentionality, usage, function, and distribution of the behaviour over a five-year period. The results showed that 'hand-on-eye' was nearly exclusively deployed in a single mother-infant dyad, was accompanied by hallmarks of intentionality, and served to initiate or resume joint dorsal travel. Although the behaviour was observed once in each of three other mother-infant dyads, these lacked the same frequency and hallmarks of intentionality. 'Hand-on-eye' thus qualifies as an idiosyncratic gesture. The proposed developmental pathway gives support to both the "ontogenetic ritualization" and "social negotiation" hypotheses. It also stresses the crucial need for longitudinal approaches to tackle developmental processes that are triggered by unique circumstances and unfold over relatively long time windows.

Keywords: Chimpanzees; Evolution of language; Gesture acquisition; Gestures; Idiosyncratic gestures; Mother-infant interactions.

Fig. 1

Beryl

Fig. 2

Screenshots of key moments from exemplar sequence of use of ‘hand-on-eye’ during a joint-travel initiation between Lindsay and Beryl. Complete description found in Results, and the full video clip is included in the Supplemental Materials. A – C: Two instances of ‘hand-on-eye’ in rapid succession from Lindsay as Beryl gets up after resting. D: Beryl turns head away from Lindsay’s right hand. Lindsay persists with minor elaboration by reaching both hands to cover both of Beryl’s eyes. E: Beryl turns further away from Lindsay’s hands. Lindsay persists yet again with a very extended reach to cover Beryl’s right eye. F. Lindsay reaching to cover Beryl’s right eye upon re-approaching after walking away briefly from Beryl. G: Lindsay walks away from Beryl again, with a POUT FACE, while emitting quiet WHIMPER vocalizations. H: Lindsay EXTENDS HAND towards Beryl, while WHIMPERING with a POUT FACE from ~ 5 m away. I: Lindsay mounted dorsally on Beryl after joint-travel begins