Table 1.

Definitions of great ape gesture actions together with newly identified mountain gorilla units. The repertoire of great ape gesture actions is taken from Grund et al. (2023), based on systematic studies in the existing literature (e.g., Byrne et al. 2017), and combined with the new gesture actions described in this study (newly identified gesture actions are indicated with asterisks). Gesture actions considered as part of the mountain gorilla repertoire (observed on at least three occasions) are in bold and listed in alphabetical order in the upper part of the table. Gesture actions that were observed on fewer than three occasions are in italic font in the middle part of the table. Gesture actions seen in at least one other ape species but not seen in mountain gorillas are indicated as struck out in the last section of the table. Italic font in the gesture definition column indicates additional information not necessarily part of the gesture action definition (comments relating to categorisation). More detailed definitions of newly identified gesture actions, including MAU start and stops are presented in Table S10. For MAU definitions of all other gesture actions listed see Grund et al. (2023)

Gesture action	Gesture definition
Beckon	A scooping movement from one or more of the joints (e.g., fingers, wrist, elbow), movement extends towards recipient and then back to the signaller in one active motion.
Bite	Signaller's mouth/teeth close on the recipient’s body - this may be very brief or could be held in place.
Bite: threat	Signaller opens mouth rapidly as if preparing for a bite (often with a movement towards recipient) but moves past or away quickly before making contact.
Body cross*	Signaller is holding arm across their own body (i.e., if the right arm is used it is held across to the left side of the body), either in the air or clutched to the signaller’s chest/front.
Bump into*	Signaller approaches (running or slowly walking) recipient and bumps into recipient’s body (full body movement). It is different from “Lean in” as the action is a fast (dynamic) movement and the contact with the recipient very short.
Chest beat	Signaller’s hands make (alternating) short audible contact with signaller’s chest (always produced while bipedal, rhythmic, long-distance audible chest beat).
Chest beat informal	Signaller’s hands make (alternating) short audible contact with signaller’s chest (rhythm or beats can be irregular, slower, or less formal etc.). Produced bipedally or sitting, lying etc. Has to include at least 3 hits, otherwise treated as ‘Hit Self’. In Byrne et al. 2017 no distinction made between Chest beat and Chest beat Informal (but see ‘Body drum’).
Dangle	Signaller hangs from one or both hands (or feet) from a branch with at least one limb not in contact with branch, some movement of body as a result of hanging. Includes ‘Dangle with shake’ where the signaller produces an additional shake of arm(s) or leg(s). Dangle was not coded as often as a gesture action because of difficulties to discern it from play actions. It also only ever was performed by immatures, and almost exclusively to initiate play.
Embrace	Signaller wraps one or both arms/legs around recipient (not necessarily only affiliative).
Fling	Rapid movement of hand (from wrist) or arm (from elbow or shoulder; rarely with foot or head) away from the signaller’s body, typically towards recipient. Energy of stroke is focused on way out away from signaller, more relaxed on way back.
Gaze stance*	Signaller positions body and head towards the recipient’s face, stares at them and ‘holds’ their gaze. The intensive ‘looking’ lasts at least 2 seconds or longer (unlike e.g., a quick attention checking). The gaze is accompanied by a stiff (motionless) body posture (standing or crouching in front of the recipient), and so is different from the Gaze hold. See discussion.
Grab	The hand or foot is firmly closed over part of the recipient’s body or a handful of hair.
Grab hold	Same as "Grab" but hand(s)/foot of signaller stays closed around recipient’s body for > 2 seconds.
Head avert*	Signaller’s body is positioned towards the recipient’s body but the head is tilted away from the recipient in an exaggerated almost 90 degree angle. May be interrupted by short monitoring of the recipient’s (re)action (at times this attention checking can resemble the description of a (horizontal) head shake but the movements in the head avert are stiff, whereas the head shake is a relaxed throwing the head in different directions)
Head stand	Signaller bends forward and places head on or very near to ground and pauses, at least briefly, in this position. Contact may be made with recipient’s body.
Hit fake*	The signaller is positioned close to the recipient and both individuals are stationary. The signaller raises the arm/hand and lashes out as if to hit the recipient but does not make contact. The outward and backward movement have the same speed and often the arm/hand is still held up after the first ‘faked’ hit for a while (and the gesture action is then either repeated or the arm slowly put back into a neutral position). If contact is made this gesture action is classified as Hit tap.
Hit non-recipient*	Signaller approaches/positions towards recipient with a young offspring (or a ‘stolen’ infant) clinging to the signaller’s front (belly/chest) and (very) softly hits on the infants back similar to a ‘chest beat’ (so alternating rhythmical hand movements on the clinging infant) while gaze and body posture clearly indicate that the behaviour is directed at the other individual (recipient). Note that very soft (rhythmical) hitting on young infants is also observed in mother-offspring interaction.
Hit object/ground	Signaller makes a short hard contact with the ground/object; energy in movement is focused on the way out, contact can be brief or remain in place for longer. Typically occurs in front of or to the side of the signaller’s body (rather than straight up and down as in Stomp). Produced with a variety of body parts - typically hand, fingers, knuckles, fist, foot.
Hit object/ground with object	An object is brought into short hard contact with another object or the ground.
Hit recipient	See “Hit object/ground”, but signaller makes deliberate contact with the recipient as part of action.
Hit recipient (soft)	See “Hit object/ground (soft)” but contact is deliberately made with the recipient’s body. There is clear variation in hit intensity for the gesture action ‘Hit recipient’ in the mountain gorillas. This variation was not observed in the two other very common ‘hit’ gesture actions (hit self and hit object ground).
Hit self	See “Hit object/ground”, but signaller makes deliberate contact with own body as part of action.
Hit tap*	The signaller is positioned close to the recipient and both individuals are stationary. The signaller raises the hand, lashes out and makes (very) short contact with the recipient’s body. The outward and backward movement have the same speed and often the arm/hand is still held up after the first ‘tap’ for a while (and the gesture action is then either repeated or the arm slowly put back into a neutral position). In movement form similar to ‘Hit fake’ but there is contact made.
Jab*	Signaller makes a short hard contact with the recipient’s body, typically with foot (occasionally with fist), unlike in “Hit recipient” the energy is rapid on both the way to and from the recipient.
Jump	While bipedal both feet leave the ground simultaneously, while quadrupedal all four limbs free of ground simultaneously; accompanied by horizontal displacement.
Kiss	Gentle contact with the mouth that doesn't hold the recipient’s body (see "Bite"). Includes the ‘share air’ kiss type where they breath on each other while almost touching.
Lay on*	The signaller lifts their leg to the side (like a dog wanting to urinate) and lays it on the recipient’s body. Depending on the size difference between signaller and recipient this can look quite acrobatic.
Lean in*	The signaller approaches the recipient and leans into the recipient’s body (usually the signaller’s shoulder or head to the recipient’s chest). It is a contact gesture but if it happens during locomotion, contact may not be stable throughout the duration of the gesture.
Locomote bipedal	Signaller stands bipedally and takes at least one step with each foot.
Locomote gallop	An exaggerated running movement where the limbs movements are typically stiff (straightened) and rhythmic.
Locomote recipient*	Signaller finishes a locomote gesture (bipedal, gallop, stiff walk) by moving over the body of the recipient; includes some contact.
Locomote stiff run	An exaggerated walking movement with stiff limbs and forelimbs held in a straightened position (running)
Locomote stiff walk	An exaggerated walking movement with stiff limbs and forelimbs held in a straightened position (slow locomotion)
Lunge	Signaller’s body is rapidly thrust towards recipient. No contact is made.
Object drop(*)	The signaller takes an object (e.g., a stick or moss) and drops it close to (or onto) the recipient. Potentially the same gesture action as leaf drop in bonobos – not included as new
Object in mouth	Signaller holds an object (e.g., a small branch) in mouth. Hands should not normally be involved. In the mountain gorillas this includes attached objects.
Object move	Object is displaced, contact is maintained through movement. While movement may not be in a single direction it does not include the rapid back and forth of ‘Object move: shake’. Object is free to move and not attached to environment (e.g., a fallen branch). Note that in rare cases a third-party may be used as the ‘object’ - communication should still be clearly directed to the recipient (so this should not be a triadic interaction).
Object move: fiddle*	Signaller grabs or touches vegetation in front of the recipient and ‘fiddles’ with it. Object (usually the leaves of a sapling, or the sapling itself) movements look uncoordinated (i.e., going into various directions with no clear pattern) but are calm and slow. Movements usually come out of the hand/wrist or fingers.
Object move: shake	Repeated back and forth movement of an object (typically one still attached in the environment, e.g., sapling). Object movement must be controlled by the signaller’s hand/foot actively shaking the object and not a biproduct of the flexibility of the object.
Object stance*	Signaller grabs a tree trunk or liana usually above shoulder height and leans into it (almost as if stretching). The body is stiff and the posture held in place while there is only little movement of the object that supports it (if any at all, unlike object move).
Over stance	Signaller pauses while standing with at least one limb that has been moved into position and held over the recipient’s body. In full form signaller’s body forms a bridge over the recipient.
Pivot(*)	Signaller grabs an object (e.g., a small tree trunk or a firm branch) and swings his body in a semicircle around the object’s axis.
Place on object/ground*	Arm is extended in front of the body and hand (or fingers, knuckles) placed on an object or the ground in a location between the signaller and recipient (its usually the ground, but there may be thick vegetation/ or a log in that place). The movement of the arm/hand towards the object/ground is coordinated, the contact may be relatively short or last longer than 2 seconds. Note that this is not the same gesture action as Touch object, a gesture action that is reported for chimpanzees and bonobos in the context of food sharing (where signallers touch the desired object).
Present	Body or body part moved to deliberately expose an area to the recipient’s attention (excludes present fingers/hand/toes/foot in a ‘reach’ form). Contact is sometimes made during this gesture.
Pull	Same as “Push” but the force is exerted away from the recipient’s body (not effective in achieving the goal). Usually involves a “Grab” from the hand/foot that is not coded as a separate gesture. “Pull” vs. “Pull (directed)”: the direction of the movement afterwards is not consistent with the direction of the pull.
Push	Contact with recipient’s body (typically hand or foot) and the force is exerted into the recipient’s body (not effective in achieving the goal). “Push” vs. “Push (directed)”: the direction of the movement afterwards is not consistent with the direction of the push.
Raise	Raise body part (typically hand or arm) in a generally vertical movement, often with a brief pause near the top of the movement.
Rake object/ground	Signaller holds hands with fingers stiff and spread in order to displace objects towards their body, e.g., dry leaves. In the mountain gorillas performed before hit ground.
Reach	Body part (typically arm/hand) extended towards the recipient (or in their direction) with no contact.
Roll over	Signaller rolls or rocks so that their back is on the object/ground exposing their stomach and holds position.
Rub	Signaller pushes/rubs body part up and down against body of recipient (typically with hands or genitals).
Shake	Signaller moves part of their body quickly and repeatedly back and forth (typically hand/arm or head). Movement is loose and typical form has several repetitions.
Spin: pirouette	Signaller stands and turns around their body’s vertical axis while (at times) also displacing along the ground. All spin gesture actions need to include a full 360 degrees of turn (movement is more than would be required for locomotion).
Spin: side roulade	Signaller is lying down and turns around their body’s vertical axis while also displacing along the ground. All spin gesture actions need to include a full 360 degrees of turn (movement is more than would be required for locomotion).
Stance bipedal	Signaller moves into bipedal stance and holds position (must not be in order to achieve other action, e.g., standing to see something arriving). Observed over 3 times in this study; however, only few cases and only immatures – unless more data in the future, maybe exclude.
Stiff stance	Signaller holds stiff quadrupedal body position with stiff limbs and forelimbs (often accompanied by ‘tight lips’ facial expression).
Stomp object/ground	The foot (or sometimes hand) is lifted vertically and brought into short hard contact with the ground (or object). Usually this is the surface that the signaller is standing/sitting on. Typically, with sole of foot/flat or hand/fist.
Stroke	Active gentle movements of the signaller’s palm and/or fingers (rarely other) on the recipient’s body. May include movements in more than one direction.
Swing	Smooth continuous motion of body part (normally arm of leg) back and forth (may be repeated).
Throw object	Object is moved and released so that there is displacement of the object through the air after moment of release.
Touch	Light contact (typically of the fingers, knuckles, hand, or foot, rarely other) on the body of the recipient, contact < 2 seconds.
Touch long	Same as ‘Touch’ but contact is held for 2 seconds or longer. Termed ‘Hand(s) On’ in Byrne et al. (2017). In some studies, ‘Hands on’ is only referring to touches on the head (e.g., Genty et al. 2009; Pika et al. 2003; Luef & Liebal 2013)
Turn*	Signaller gazes back at the recipient and then makes an exaggerated half-turn with his/her body and head (one fore- and/or hindlimb may be raised during the turn). Similar to Beckon, however unlike Beckon, the movement is around the signallers own body axis.
Big loud scratch	Loud exaggerated scratching movement on signaller’s own body (must not be followed by self-grooming). A single BLS might include a small change in movement angle; but if contact stops or body part changes code as new BLS. There is one possible occasion of the use of a big loud scratch gesture action in the mountain gorillas, but the intentional use was not clear.
Bounce	Rhythmic vertical up-down movement (or’bobbing’) of the signaller’s body – in chimpanzees all hands and feet remain on the ground (rarely hands may be free of ground). There is one clear example of a bouncing movement in the mountain gorillas seen performed bipedally before a chest beat (adult female).
Bow	Signaller bends forward from the waist while standing bipedally. Note that head should not make contact with ground, and in typical form body is held straight and only lowered a little (compare: Head stand). There is one example of a bow movement in the mountain gorillas seen performed by an immature individual.
Finger(s) in mouth	Signaller inserts finger(s) - usually palm down - in the mouth of recipient with contact. Signaller makes movement into recipient's mouth - recipient might open or close mouth a little, but not the same as actively moving towards the other individual's hand to bite it. There is only one example of this gesture action in the mountain gorillas.
Freeze*	Signaller is approached by the recipient (e.g., the signaller is positioned in the travel path of the recipient) and instead of e.g., moving away or continuing with their activity the signaller ‘freezes’ into a rather unusual posture (e.g., crouching on the belly exposing the back or lying on the back with limbs stretched to the side exposing the belly).
Hit recipient with object	An object is brought into short hard contact with the body of the recipient. There are two relatively clear observations in this study.
Individual move*	Signaller grabs an individual and drags it along the ground as if it was an object. If this was directed at the individual being dragged and not effective in achieving the goal, the action was coded as Pull. However, there were a few occasions where the signaller seemingly just ‘used’ the individual he/she dragged as an object, while directing gaze and other behavioural markers at a different individual. As this behaviour was only observed a few times and generally difficult to analyse we did not include it in the repertoire.
Jab at*	See Jab - same action but signaller deliberately avoids making contact with the recipient's body.
Object clamp*	Signaller takes an object and carries it by clamping it between body parts. There is only one example (here the signaller clamped a stick between the head and his chest).
Object on head	Signaller places detached object on their head and leaves it in place. Hand may remain in contact with object for balance. In mountain gorillas one occasion of this gesture action was observed but the object was attached.
Spin: somersault	Signaller’s body is curled into a compact position on the ground and rolled forward or backward so the feet are brought over the head and returned to sitting position. All spin gesture actions need to include a full 360 degrees of turn (movement is more than would be required for locomotion). Behaviour generally present but spin somersault was only used twice as a potentially communicative signal.
Water splash	Signaller's limb (typically hand) is moved vigorously through the water so that there is audible displacement of the water. Bwindi mountain gorilla immatures often play with water/mud, however only observed once as a gesture action in a play invitation.
Butt	Signaller's head is briefly and firmly pushed into the body of the recipient (contact is typically short).
Circle	Signaller rotates hands around each other (in gorillas like the start of a chest beat but without any contact with chest). Circling movements are often observed in the build-up phase of (usually immature) chest beats (so before the actual contact hits, maybe as a sort of ‘finding the rhythm’). “Circle” has not been observed as a separate gesture action in this study. In Byrne et al. (2017) called “Disco arms shake” (includes “Circle hands”)
Clap	Signaller moves both palms towards each other which are brought together with audible contact (may be repeated).
Crouch	Signaller lowers body by bending knees and/or elbows, while maintaining at least three points of contact with ground. Not observed in this study but see the gesture action Freeze.
Flap	Flap movement of extremities, typically with one or both legs. Signaller sits with knees bent and opens/ closes one or both legs to side (can be repeated).
Gaze hold(*)	Signaller looks towards the recipient's face and'holds' their gaze (moving their head if needed as the recipient moves theirs), holds for > 2 seconds. Termed ‘Look’ in Byrne et al. (2017). Mountain gorillas employ a gesture action that would fit the label ‘Gaze hold’; however, in mountain gorillas the gaze (which is more an intense starring) is accompanied by a stiff body posture and other behavioural markers. It is a very distinctive behaviour and does not resemble ‘peering’ (or ‘Look’). We therefore classified it under the gesture action ‘Gaze stance’. While it is reported, there was not enough example data for ‘Gaze hold’ available from other species for a thorough comparison.
Hit object/ground (soft)	See “Hit object/ground” but contact is gentle (as in tap for example). No obvious variation in hit intensity observed in the mountain gorillas.
Hit self (soft)	See Hit soft object/ground but contact is deliberately made with signaller's body. No obvious variation in hit intensity observed in the mountain gorillas..
Knock object	Signaller's knuckles (or rarely heel) brought into short hard audible contact with an object like a tree buttress to make a clear sound.
Leaf clipping	Signaller tears strips from leaf (or leaves), or across leaves, held with the hand/teeth using the teeth or hands to tear; produces conspicuous sound.
Leaf clipping: leaf drop	Leaf or leaves are plucked off stem with signaller's fingers or mouth and quietly dropped. May be pulled/torn at petiole. Note that there is little tearing action on the leaves as seen in leaf clipping. Not observed in this study but see ‘Object drop’ gesture action.
Out	Typically, the signaller's arm is extended out from the shoulder to the side of the body, elbow and hand are held in line.
Poke	Short firm contact made with the signaller’s fingers held straight and ‘pushed’ briefly into the recipient’s body. Could be confused with ‘Hit tap’
Rake self	Signaller holds hands in distinctive position with fingers stiff and spread and pulls them over their body (e.g., over head, may be repeated; coded as a single gesture unless a clear break/change in movement). Rarely seen - often in association with temper'tantrums'.
Rocking: bipedal	Signaller moves body back and forth or side to side while standing bipedally.
Rocking: sitting	Signaller moves body back and forth, or side to side, while sitting (includes single/half'Rock' movement where the signaller moves back and holds).
Stomp recipient	See ‘Stomp object/ground’ but contact is deliberately made on recipient's body. Note that this typically occurs only when standing/sitting on top of someone, otherwise consider if Hit recipient a better fit. Not observed.
Throw threat	Object is lifted into position to throw it but is held in that position (typically raised above shoulder) without release. There is one possible observation of a throw threat in the mountain gorillas, while it was a communicative behaviour, it was unclear whether the initial plan was to throw the object.
Thrust	Groin is pushed forward towards recipient (may be repeated).
Touch object	Same as ‘Touch’ but contact is made with a specific object, usually food (begging/food sharing context). See comment for ‘Place on object/ground’.
Wave	Large loose back and forth movements of the signaller's arms while raised above the shoulder.