Selective and prolonged attention to emotional scenes in humans and bonobos.

Perceiving emotions in others is at the foundation of higher-order social cognition. The importance of emotions is evidenced by the fact that they receive prioritized attention at early stages of processing the environment in humans and some other primates. Nevertheless, we do not fully understand how emotion modulates attention over longer durations in primates, particularly in great apes. Bonobos, one of our closest relatives, stand out in emotion processing and regulation among great apes. This makes them an interesting comparison species and a valuable model for studying the evolution of emotion perception in hominids. We investigated how bonobos and humans spontaneously attend to emotionally valent scenes in a preferential looking task using eye-tracking. With Bayesian mixed modelling, we found that bonobos and humans generally looked longer at emotional scenes, mainly of conspecifics. Moreover, while bonobos did not have a bias toward emotional human scenes, humans sustained their attention toward bonobos playing, grooming and having sex. Furthermore, when exploring an immediate bias for emotions, humans showed a bias toward affiliative human scenes, and bonobos showed a bias away from bonobos-in-distress scenes. These findings suggest that emotions modulate attention at early and later attentional stages in bonobos, similar to humans.

Attention toward emotions is modulated by familiarity with the expressor: A comparison between bonobos and humans.

Why can humans be intolerant of, yet also be empathic toward strangers? This cardinal question can be tackled by studying emotions in our closest living relatives, bonobos. Their striking xenophilic tendencies make them an interesting model for reconstructing the socioemotional capacities of the last common ancestor of hominids. Within two dot-probe studies, we compared bonobos' and humans' attention toward scenes depicting familiar (kith and kin) or unfamiliar individuals with emotional or neutral expressions. Results show that the attention of bonobos is biased toward emotional scenes depicting unfamiliar bonobos, but not toward emotional groupmates (Study 1). In contrast, Study 2 shows that human attention is biased toward emotional rather than neutral expressions of family and friends, but not toward unfamiliar others. On the one hand, our results show that an attentional bias toward emotions is a shared phenomenon between humans and bonobos, but on the other, both species have their own unique evolutionarily informed biases. These findings support previously proposed adaptive explanations for xenophilia in bonobos that potentially biases them toward emotional expressions of unfamiliar conspecifics, and parochialism in humans, which makes them sensitive to the emotional expressions of close others. (PsycInfo Database Record (c) 2023 APA, all rights reserved).

Attentional Bias in Humans Toward Human and Bonobo Expressions of Emotion.

Correctly recognizing and efficiently attending to emotional situations are highly valuable skills for social species such as humans and bonobos, humans' closest living relatives. In the current study, we investigated whether humans perceive a range of emotional situations differently when these involved other humans compared to bonobos. A large group of children and adults participated in an emotion perception task and rated scenes showing either bonobos or humans in situations depicting distressed or aggressive behavior, yawning, scratching, grooming, playing, sex scenes or neutral situations. A new group of people performed a dot-probe task to assess attentional biases toward these materials. The main finding is that humans perceive emotional scenes showing people similarly as emotional scenes of bonobos, a result reflecting a shared evolutionary origin of emotional expressions. Other results show that children interpreted bonobos' bared teeth displays as a positive signal. This signal is related to the human smile, but is frequently seen in distressed situations, as was the case in the current experiment. Children may still need to learn to use contextual cues when judging an ambiguous expression as positive or negative. Further, the sex scenes were rated very positively, especially by male participants. Even though they rated these more positively than women, their attention was captured similarly, surpassing all other emotion categories. Finally, humans' attention was captured more by human yawns than by bonobo yawns, which may be related to the highly contagious nature of yawns, especially when shown by close others. The current research adds to earlier work showing morphological, behavioral and genetic parallels between humans and bonobos by showing that their emotional expressions have a common origin too.

Experimental evidence for yawn contagion in orangutans (Pongo pygmaeus).

Yawning is highly contagious, yet both its proximate mechanism(s) and its ultimate causation remain poorly understood. Scholars have suggested a link between contagious yawning (CY) and sociality due to its appearance in mostly social species. Nevertheless, as findings are inconsistent, CY's function and evolution remains heavily debated. One way to understand the evolution of CY is by studying it in hominids. Although CY has been found in chimpanzees and bonobos, but is absent in gorillas, data on orangutans are missing despite them being the least social hominid. Orangutans are thus interesting for understanding CY's phylogeny. Here, we experimentally tested whether orangutans yawn contagiously in response to videos of conspecifics yawning. Furthermore, we investigated whether CY was affected by familiarity with the yawning individual (i.e. a familiar or unfamiliar conspecific and a 3D orangutan avatar). In 700 trials across 8 individuals, we found that orangutans are more likely to yawn in response to yawn videos compared to control videos of conspecifics, but not to yawn videos of the avatar. Interestingly, CY occurred regardless of whether a conspecific was familiar or unfamiliar. We conclude that CY was likely already present in the last common ancestor of humans and great apes, though more converging evidence is needed.

Low relationship quality predicts scratch contagion during tense situations in orangutans (Pongo pygmaeus).

Primates show various forms of behavioral contagion that are stronger between kin and friends. As a result, behavioral contagion is thought to promote group coordination, social cohesion, and possibly state matching. Aside from contagious yawning, little is known about the contagious effect of other behaviors. Scratching is commonly observed during arousal and as such may play a role within group dynamics. While the Bornean orangutan (Pongo pygmaeus) is commonly considered the least social great ape, orangutans do engage in social interactions. Therefore, their social organization makes them a suitable case for studying the social function of behavioral contagion. Through behavioral observations of captive orangutans, we recorded all yawn and scratch events together with the corresponding behavior of all bystander group-members. As yawning was rarely observed, no conclusions could be drawn regarding this behavior. Scratching was contagious and occurred within 90 s after the triggering scratch. Specifically, orangutans showed increased scratch contagion when they had seen a weakly bonded individual scratch during tense contexts. When the orangutan had not seen the triggering scratch, the contagiousness of scratching was not affected by context or relationship quality. Our results indicate that behavioral contagion is not simply higher between individuals with stronger social relationships, but that the contagiousness of behaviors may vary based on the context and on social factors. We discuss these findings in light of an adaptive function that may reduce aggression.