The origin of great ape gestural forms

Abstract

Two views claim to account for the origins of great ape gestural forms. On the Leipzig view, gestural forms are ontogenetically ritualised from action sequences between pairs of individuals. On the St Andrews view, gestures are the product of natural selection for shared gestural forms. The Leipzig view predicts within- and between-group differences between gestural forms that arise as a product of learning in ontogeny. The St Andrews view predicts universal gestural forms comprehensible within and between species that arise because gestural forms were a target of natural selection. We reject both accounts and propose an alternative "recruitment view" of the origins of great ape gestures. According to the recruitment view, great ape gestures recruit features of their existing behavioural repertoire for communicative purposes. Their gestures inherit their communicative functions from visual (and sometimes tactile) presentations of familiar and easily recognisable action schemas and states and parts of the body. To the extent that great ape species possess similar bodies, this predicts mutual comprehensibility within and between species - but without supposing that gestural forms were themselves targets of natural selection. Additionally, we locate great ape gestural communication within a pragmatic framework that is continuous with human communication, and make testable predications for adjudicating between the three alternative views. We propose that the recruitment view best explains existing data, and does so within a mechanistic framework that emphasises continuity between human and non-human great ape communication.

Keywords: adaptation; communication; exaptation; great ape gestures; language evolution; meaning; pragmatics.

Fig. 1

A selection of chimpanzee and bonobo gestures and the outcomes that they achieve, illustrating visual resemblance between the gesture action and its outcome. In this and the following legends, percentages indicate the proportion of times a gesture is used with a particular function. Many (although not all) great ape gestures are used with multiple communicative functions. (A) In the “Mouth stroke” gesture, a “signaller's palm and fingers are repeatedly run over the mouth area of the recipient” (Hobaiter & Byrne, , Table 1, p. 755). Bonobos and chimpanzees use the “Mouth stroke” to request “Acquire object/food” (100% and 87%, respectively) (Graham et al., 2018). (B) The “Reach/Climb on” gesture is performed by extending an arm “to the recipient with hand in an open, palm upwards, [downwards, or sideways] position” (Hobaiter & Byrne, , Table 1, p. 755). In bonobos and chimpanzees, the “Reach” gesture is the most commonly used gesture to achieve the outcome “Climb on you” (Hobaiter & Byrne, 2014). (C) To perform the “Rocking” gesture, sitting bonobos “rock forward and back or side to side, repeated[ly]” (Graham et al., , supplementary material 1, p. 1). This is done to request “Initiate copulation” (54%) or “Initiate genito‐genital (GG) rubbing” (46%) (Graham et al., 2018). (D) The “Big loud scratch” gesture is performed by a “loud exaggerated scratching movement on the signaller's own body” (Hobaiter & Byrne, , Table 1, p. 753). It is used by bonobos and chimpanzees as a request to “Initiate grooming” (100% and 82%, respectively) (Graham et al., 2018).

Fig. 2

A selection of gestures that show the part of the body with which the recipient should interact. (A) The “Arm up” gesture deploys extended “straight arm(s) out to side and away from body” (Graham et al., , supplementary material 1, p. 1). It is used by bonobos to request “Initiate contact” (80%) and “Climb on me” (20%) (Graham et al., 2018). (B) In the “Bipedal stance” gesture, apes stand bipedally, arms out to the side, and with their back arched, to present their exposed genitalia (Graham et al., 2017). It is used by bonobos to request “Initiate copulation” (50%) and “Initiate genito‐genital (GG) rubbing” (50%) (Graham et al., 2018). (C) In the “Present (climb on)” gesture, an “arm or leg is extended to young recipient in order to facilitate them climbing onto the signaller's body (normally mother to infant)” (Hobaiter & Byrne, , Table 1, p. 756). Both chimpanzees and bonobos use this gesture to request “Climb on me” (100% for both species) (Graham et al., 2018). (D) In the “Present (grooming)” gesture, the body is moved to “deliberately expose an area to the recipient's attention which is immediately followed by grooming of the area” (Hobaiter & Byrne, , Table 1, p. 756). Bonobos and chimpanzees both use this to request “Initiate grooming” (100% for both species) (Graham et al., 2018). (E) In the “Present (sexual)” gesture, the signaller “sits and spreads their limbs displaying their genital swelling or erect penis” (Graham et al., , supplementary material 1, p. 4). This gesture is used by bonobos to request “Initiate genito‐genital (GG) rubbing” (64%) and “Initiate copulation” (36%), and by chimpanzees to request “Initiate copulation” (74%) (Graham et al., 2018).

Fig. 3

Two gestures that direct the recipient towards a certain location. (A) The “Directed push” gesture is a “light short non‐effective push that indicates a direction of desired movement, immediately followed by the recipient moving as indicated” (Hobaiter & Byrne, , Table 1, p. 753). It is used by bonobos to request “Climb on me” (100%), and by chimpanzees to request “Reposition” (50%), “Move closer” (21%), and “Climb on me” (15%) (Graham et al., 2018). (B) In the “Grab‐pull” gesture, a “closed hand contact is maintained and a force exerted to move the recipient from their current position” (Hobaiter & Byrne, , Table 1, p. 754). The gesture is used by bonobos to request “Follow me” (58%), “Reposition (21%)” and “Climb on me” (10%) and by chimpanzees to request a peer to “Move closer” (31%), “Climb on me” (25%) and “Contact” (13%) (Graham et al., 2018).