In the distant, distant past, there lived a species of ape that would become the ancestor of three species of ape that still exist today: the bonobo, the chimpanzee, and man. Scientists like to study the behavior and other characteristics of bonobos and chimpanzees to gain more insight into us as humans and our evolution.
It was also what drove Evy van Berlo when, in her PhD research, she decided to look at the evolutionary basis of emotions in humans and closely related great apes. Van Berlo now works as a postdoctoral researcher at the University of Amsterdam, but did his PhD research at Leiden University. In one of her studies, conducted together with lead researcher Mariska Kret from Leiden University and the zookeepers at Apenheul, she looked at how bonobos and humans respond to images of conspecifics with different types of expressions. The results of that study have now been published in the journal Emotion from the American Psychological Association.
Tap on a screen
Van Berlo trained bonobos in Apenheul to press a dot on a screen. After touching the dot, two images appeared very briefly. The presented images showed bonobos from the same group as well as unknown conspecifics. Some were neutral, while others showed an expression or activity that clearly had an emotion associated with it, such as fear, play, or sex.
A dot appeared behind one of these two pictures, which the monkeys had to touch as quickly as possible. Touching the dot was eventually followed by a reward in the form of a piece of apple. The system kept track of how quickly the bonobos, once they figured out the trick, pressed the dot that appeared after seeing the different images. The idea behind this study was that monkeys are faster to touch the dot that appears behind the picture and that immediately catches their attention.
“Do you react faster than a bonobo?”
Van Berlo and her colleagues also conducted a variation of this study with human visitors to Apenheul. People were presented with the same screen and dot, with pictures of strangers or someone they visited the zoo with that day. There were different facial expressions in the pictures: neutral, happy, scared, angry and so on. The challenge for the human visitors was: “Are you faster than a bonobo?” Reaction time was also examined here, where a shorter reaction time was interpreted as drawing attention to the image that was being shown at that moment.
“We saw in these studies that both humans and bonobos respond more quickly to images of conspecifics with an emotional charge than to neutral images,” says Van Berlo. “It was also what we expected: It fits with the fact that we are both social animals. Yet there was also a notable difference. We humans are primarily focused on emotional images of people we know, while the bonobos’ attention is precisely to study emotional photographs of bonobos unknown to them.”
This finding fits with previous studies showing that bonobos are a so-called xenophilic species: they are more attracted to unknown than to known conspecifics. For example, unlike us humans, they will share food more quickly with bonobos who are strangers to them than with familiar congeners.
Van Berlo: “We suspected that this is an evolutionary difference that has arisen due to differences in living environment. Bonobos live in a relatively stable ecological environment in the Congo, where there is enough food available. Under these circumstances, peaceful interaction with strangers probably beneficial. for the preservation of the species. Early humans, on the other hand, lived in itinerant groups that had to compete with each other for food. Under such circumstances, it is probably evolutionarily more advantageous to favor individuals from one’s own group over strangers.”
According to Van Berlo, it would be interesting to repeat this study with other great apes. Bonobos are known as the most peaceful monkey. Bonobos are very similar to chimpanzees in appearance – they were formerly known as pygmy chimpanzees – but their social organization and behavior are very different. For example, chimpanzees are much more competitive, and chimpanzees are males who lead a group, while females are dominant and in charge of bonobos.
Van Berlo: “It would be nice to test this idea about the importance of the living environment for the evolutionary development of the attitude towards strangers in other apes through a study like this. With chimpanzees, for example, I would expect that they would be more more interested in feelings of people they know than feelings of strangers.”
Photo: DBeaune, Wikimedia Commons (main photo: bonobos at a Congo research site); Hugh Jansman, Apenheul