Bonobos, unlike humans, are more interested in the emotions of strangers than acquaintances
Humans and bonobos show striking similarities as well as differences when they see pictures of conspecifics. Both are more interested in photos of conspecifics that show emotion. But while our human attention is more easily drawn to photos of family members and friends that express certain emotions, the attention of bonobos is drawn to the emotion of individuals they don’t know. This is based on research conducted by psychologists Evy van Berlo and Mariska Kret in collaboration with Apenheul.
The results of that study have now appeared in the American journal Emotion. Bonobos, along with chimpanzees, are the monkey species most closely related to humans because of a common ancestor. About five to seven million years ago, humans and chimpanzee bonobos split off from this ancestor. There are a lot of similarities in our social behaviour, but also some remarkable differences.
These differences and similarities provide more insight into human evolution. Evy van Berlo studied the evolutionary basis of emotions in humans and closely related great apes for her doctoral research at Leiden University. She received her PhD in May 2022 and is now a postdoc at the University of Amsterdam. For one of her Leiden studies, she looked at how bonobos and humans respond to images of conspecifics with different types of expressions. She conducted the research with her supervisor Mariska Kret, professor of Cognitive Psychology in Leiden.
Bonobos point out photos in Apenheul
Van Berlo trained bonobos in the Apenheul, a zoo fully focused on primates, to press a dot on a screen. After touching the dot, two images appeared very briefly. The images showed bonobos from the same group as well as unfamiliar conspecifics. Some images were neutral, while others showed an expression or activity clearly associated with a type of emotion, such as fear, play, or sex. A dot then again appeared behind one of these two images, and the apes had to touch it as quickly as possible. Pressing the dot was then followed by a reward in the form of a piece of an apple. The system kept track of how quickly the bonobos pressed the dot that appeared after seeing the different photos. The idea behind this study was that apes would be faster at touching the dot that appears behind the photo that immediately grabs attention.
'By comparing with great apes, this research provides an evolutionary perspective on this important, always current topic and shows how deeply rooted certain traits are.’
Visitors Apenheul get the same assignment
Van Berlo and her colleagues also conducted a similar study with human visitors of the Apenheul. The people were presented with the same task, with photos of strangers or a person with whom they visited the zoo that day, and with varying facial expressions on the photos (neutral, happy, scared, angry, etc.). The challenge for the human visitors was, "Can you react faster than a bonobo?". Reaction time was also examined here. A a shorter reaction time meant the photos shown at that moment, drew the most attention.
'In these studies, we saw that both humans and bonobos react more quickly to photos of conspecifics with an emotional charge than neutral photos,' says Van Berlo. ‘That is what we expected: it makes sense given that we are both social animals. Yet there was also a remarkable difference. We humans are mainly focused on emotionally charged photos of people we know, while the attention of bonobos is focused on studying emotionally charged photos of unfamiliar others.'
Kret notes, ‘It had long been known that humans respond differently to the emotions of strangers than emotions expressed by acquaintances. But by comparing with great apes, this research provides an evolutionary perspective on this important, always current topic and shows how deeply rooted certain traits are.' Research with great apes is rare and thus research with bonobos is also almost never done, Kret said. 'However, this is important because only research with a close relative, the chimpanzee, gives a distorted picture. Chimpanzees, like humans, are focused on their 'ingroup' and tend to be less positive toward strangers, in stark contrast to the bonobo.
Bonobos are a xenophilic animal species
This finding fits with previous studies that show that bonobos are a so-called xenophilic animal species: they are more attracted to unfamiliar than familiar conspecifics. For example, unlike humans, they will share food more quickly with bonobos that are strangers to them than with individuals they know. Van Berlo: 'Scientists have suggested that this is an evolutionary difference that has arisen due to differences in the living environment. Bonobos live in a relatively stable ecological environment in Congo, where there is enough food available. Under those circumstances, there is less need for competition with other groups and peaceful interaction with strangers might even be beneficial for the conservation of the species. Early humans, on the other hand, lived in nomadic groups that had to compete with other groups of humans for food. Under such circumstances, it is probably evolutionarily more beneficial to favor individuals from your own group over strangers.'
Evy van Berlo, Thomas Bionda and Mariska E. Kret (2023). Attention Toward Emotions Is Modulated by Familiarity With the Expressor: A Comparison Between Bonobos and Humans. Emotion, 23(1).