Comparing Human and Nonhuman Primate

Introduction

Gestural behaviour can be defined as spontaneous movements of the hands and arms which convey a meaning that is semantically and temporally in close synchrony with the flow of speech (McNeil, 1992). Humans and non-human primates such as bonobos and chimpanzees (hereafter referred to as primates) share approximately 95% of the same DNA as humans (Pika, 2008). Thus, investigating the gestures of humans and primates may provide insight into the evolution of human communication. This paper provides an overview of the similarities and differences between the gestural behaviour of humans and primates, focusing on the brain regions employed; the pointing gesture and the degree to which gestures can be learned from conspecifics.

Gestures activate the mirror neuron system (MNS) in the brain (Corballis, 2009).That is, the same network of neurons is activated when performing a particular action or observing the same action being performed by another. For instance in primates, the MNS is activated when either a monkey is cracking nuts open or observing another monkey cracking nuts (Kohler et al., 2002). In humans, the mirror system is activated with actions and speech (Corballis, 2009).

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Willems, Ozyurek & Hagoort (2007) recruited humans in a functional Magnetic Resonance Imaging study to identify the neural networks involved in gesturing behaviour. Participants underwent a series of matched and mismatched trials where semantic information was conveyed through speech and iconic gestures. A matched trial is when gestures exhibit images expressed concurrently in speech and a mismatched trial is when gestures are incongruent with speech. Results revealed that Broca’s area had significantly greater activation in the matched than in the mismatched trials.Thus, Broca’s area increases in activation when information is simultaneously presented from the speech and action domains, compared to the speech domain alone.

Furthermore, Taglialatela, Russell, Schaeffer and Hopkins (2008) recruited primates in a Positron Emission Tomography study to identify neural networks in gesturing behaviour. Primates took part in two tasks; one which induced vocal behaviour in the primates and another which induced vocal and gestural behaviour. Results revealed more activation in the left inferior frontal gyrus and the prefrontal cortex during the vocal and gestural behaviour task than the vocal behaviour task. This area is homologous to Broca’s area in humans (Corballis, 2009). Taken together, these studies suggest similarities between humans and primates in terms of the shared neural region, Broca’s area; however, there are some differences with innate gestural acquisition.

Pointing is a deictic gesture that is typically used as a communicative body movement which projects a vector from a body part to draw another person’s attention to an object or entity (Kita, 2003).Canonical pointing at nearby targets begins in humans at approximately 11 months and at more distant targets at around 12 months (Morissette, Ricard, & Gouin-Decarie, 1995). Additionally, by 12 months, infants will typically check to establish whether they have the awareness of another person to share the focus of attention with prior to pointing (Cochet & Vauclair, 2010). Thus, pointing has been observed during early development with joint attention between the infant and the recipient, highlighting the deictic quality of the gesture.

In wild primates, pointing has been rarely observed between conspecifics (Vea & Sabater-Pi, 1998). Instead, to share attention, wild primates typically orientate their whole body towards the desired object or entity (Menzel, 1974). Nevertheless, successful pointing behaviour has been found in captive primates. This was found serendipitously in a primate named Clint during testing, when on one occasion he was observed repetitively pointing with his index finger through the cage at a food reward, while alternating his gaze between the experimenter and the food (Leavens, Hopkins & Bard, 1996). This observation led to further experimental testing and vast reports of pointing in primates for food or objects (Meguerditchian & Vauclair, 2009; Zimmermann, Zemke, Call & Gomez, 2009).

Liszkowski, Schafer, Carpenter and Tomasello (2009) compared 12 month old infants and primates, in pointing to a desired absent object. The experimenter placed a highly desirable reward (e.g. a toy or food) in the target location and an undesirable reward (e.g. paper towels or bedding) in the alternative location for the participants to associate the target location with obtaining the highly desirable reward. In the test condition, the experimenter waited for the participant to make a request. If there were no request, the experimenter looked expectantly at the participant and touched both locations simultaneously. The results revealed that 12 month old infants pointed to the absent target object location repeatedly and often before the trial started, but primates failed to point at all.

Bullinger, Zimmermann, Kaminski & Tomasello, (2011) compared the intention of the pointing gesture between primates and 25 month old children. Participants undertook two tasks in which they could see the reward but the experimenter could not. In one task, pointing would directly benefit the participant (e.g. to gain food or a toy reward) and in the other task, pointing would benefit the experimenter.Results revealed that while children would point to benefit themselves and another person (i.e. experimenter), primates only pointed when it would benefit them. This suggests that unlike 25 month old children, primates do not have cooperative motivation. Taken together, this evidence suggests that while primates are able to point, this behaviour is significantly different from pointing behaviour in infants and young children, who are able to point at a present object, in addtion to an absent object or to help another person retrieve a reward.

Another difference between humans and primates lies with the acquisition of gestures through social learning. Social learning theory postulates that an individual learns by observing and imitating the actions of others (Franzoi, 2000). Developmental research has shown that the mechanism for social imitation of manual gestures, such as tongue protrusions and sequential finger movement begins in infants between 12 to 21 days of age (Meltzoff & Moore, 1977). Meltzoff and Moore (1997) propose that imitation in infants is accomplished without any cognitive understanding using a mechanism which directly maps between perceptual input and motor output.

Carpenter, Nagell and Tomasello, (1998) investigated infants aged between 9 and 15 months to find out when they understand the intention of their action. Before being requested to turn a light on, infants received demonstrations of how to perform the task by depressing a switch with either their hands or head. Results revealed that while all the infants copied the action, only those from 13 months checked to ensure the light came on and were producing the same goal as the adults. This suggests that social learning in older infants is guided by goal directed behaviour.

There is less evidence for the social learning of gestures within primates, with individual differences in the execution of gestures being found in primate communities (Pika, Liebal, Call & Tomasello, 2005). For instance, the ‘grooming handclasp’, which is where two primates hold hands overhead during mutual grooming has been found to vary between two different communities within the same subspecies of wild primates. That is, intergroup differences have been observed in different communities with palm to palm touching and non-palm to palm touching, which indicates social learning amongst conspecifics within each community (McGrew, Marchant, Scott & Tutin, 2001). Nevertheless, in both communities, some primates failed to use the gesture at all.

Laidre (2011) investigated the same subspecies of captive primates in 19 various communities worldwide. Findings revealed that only primates in Colchester, England had a unique ‘eye covering’ gesture, where they would cover their eyes for extended periods of time, conveying a ‘do not disturb sign’ to the other primates. This gesture was observed for over a decade despite births, deaths and removals. Nevertheless, there was some individual variation in the performance of the gesture, with 32% of the primates raising their elbow in addition to covering their eyes. Taken together, this evidence suggests that infants have an innate ability to learn gestures by means of a social learning mechanism, yet in primates, this is not the major learning process of gestures.

In conclusion, humans and primates share the same neural region for processing gestural behaviour (i.e. Broca’s area). However, there are some differences between the two species. Infant’s deictically point towards a present or absent desired object or entity, whereas primates rarely point unless they have acquired the behaviour from humans in order to achieve a variety of ends. Additionally, primates, unlike children, only point when they benefit from a reward and fail to point to inform others. This suggests that pointing is a unique, innate, human attribute which primates acquire whilst interacting with humans.

Furthermore, infants from approximately 16 days old have shown social imitation gestures and from the age of 13 months, can use these gestures to guide specific behaviour towards a goal. In primates however, there is less evidence to support the acquisition of gestures by means of social learning, with large variability of individual gestures existing in the same community. Future research could investigate whether inter-zoo transfers translocate the unique eye covering gesture to a new community.

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