Humans and chimps adjust pupils during interaction with same species
Not only the pupils of humans, but also those of chimpanzees adjust during contact with one another. This pupil-mimicry or pupil-synchronisaton effect is most pronounced during an interaction with an individual from the same species. This is the conclusion of a study conducted by University of Amsterdam (UvA) psychologist Mariska Kret in collaboration with researchers from Kyoto University (Japan). Their findings were published on Wednesday, 20 August in the academic journal PLOS ONE.
Pupillary synchronisation
The researchers assessed whether the mirroring of pupil size – pupillary synchronisation, i.e. the phenomenon whereby the pupil size of one individual adjusts to that of another individual – only occurs in humans, or also in our closest genetic relatives: the chimpanzee. They also assessed whether pupillary synchronisation is more prominent in interactions between two individuals of the same species as opposed to those between two individuals from different species.
Pupillary synchronisation helps members of the same group to understand one another and empathise more effectively. This can yield greater mutual trust and more effective cooperation. The phenomenon is mainly of interest because - unlike other forms of synchronisation or behavioural mirroring studied in biology and psychology - it is fully automatic, unconscious and involuntary, thus representing an ‘honest’ signal.
Kret and her colleagues used eye-tracking systems to study the pupil size of both human and chimpanzee subjects while they were shown short video clips of the dilating and contracting pupils of other humans, but not with chimpanzees. ‘We found that humans synchronise their pupils with other humans, and do not show the same behaviour with chimpanzees. In the chimpanzees, an opposite effect was observed, and was more pronounced in the mothers than in the other chimps,’ Kret explains.
The pupil as a means of communication
Little is currently known about pupillary synchronisation. It is already known that pupils reflect a broad range of emotions (such as excitement, fear or interest) rather than simply responding to changes in light. This is relevant information to social animals living in groups. We know that humans are capable of identifying pupillary signals in others and unconsciously adjusting their behaviour in response.
Humans are the only species on earth with a large amount of visible sclera. Sclera, commonly known as the white of the eye, was developed over the course of our evolution as a means of facilitating non-verbal communication such as the tracking of eye movements. The white of the eye and colourful iris draw the viewer's attention to the centre of the eye, the pupil. Although chimpanzees do not have visible sclera similar to that found in humans, they do mirror each other's behaviour (including emotional facial expressions, yawning, vocalisations). Previous studies have shown that the phenomenon of mirroring strengthens the bond between two individuals and is stronger in closer genetic relatives. It was previously unknown whether this also applied to pupillary synchronisation and interactions between two different species.
Future research
Kret: ‘The next step will be to study the extent to which pupillary synchronisation affects day-to-day behaviour and to identify whether such synchronisation might have practical applications. We'll be focusing on the clinical practice (locked-in-patients, autism, social anxiety, care robots), and the business sector (negotiations, selection procedures). I hope to gain more insight into these aspect by using my recently received Veni grant.’
Publication details
Kret ME, Tomonaga M, Matsuzawa T (2014) 'Chimpanzees and Humans Mimic Pupil-Size of Conspecifics', in: PLOS ONE 9(8): e104886. doi:10.1371/journal.pone.0104886.