Page 42 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Chapter 2
We aimed to replicate and extend previous findings by Kret et al. (2016) and tested for a possible interaction between familiarity and emotional attention in bonobos. When comparing the AIC statistics of a normal and gamma distribution (AICnormal = 18949, AICgamma = 18995), the model with a normal distribution was found to be a better fit. We found a significant interaction effect between Familiarity and Congruency (c2(1) = 4.14, p = .042); bonobos responded faster on probes replacing emotional (M = 521.11, SD = 131.50) rather than neutral scenes (M = 529.84, SD = 127.13) in the Unfamiliar condition (b = -10.48, SE = 5.12, t(1641) = -2.05, p = .041) but not in the Familiar condition (b = 4.59, SE = 5.34, t(1641) = .86, p = .391, see Figure 4 and Table S4.1 and S4.2 for individual averages and further model results). In short, familiarity with the expressor of an emotion significantly modulated attentional bias towards emotions, with responses to emotional scenes being faster than neutral scenes when they involved unfamiliar, but not familiar conspecifics.
Discussion
Previous research has shown that bonobos have heightened attention to the emotional expressions of unfamiliar conspecifics, especially when these were rated as emotionally intense by their keepers (Kret et al., 2016). The current study builds on this research. Specifically, by adding photographs of group mates to the stimulus materials, Experiment 1 showed that familiarity with the expressor has a moderating effect on an attentional bias towards emotions; early attention appears to be modulated mostly by emotional expressions of unfamiliar individuals, but not familiar individuals. From a human perspective, this finding may appear counter-intuitive. However, this novel finding largely confirms our à priori predictions which were based on previously conducted behavioral studies in bonobos highlighting their strong xenophilic tendencies and other-regarding preferences (Fruth & Hohmann, 2018; Tan et al., 2017; Tan & Hare, 2013; Tokuyama et al., 2021). Attention can be driven by the biological relevance of the emotional signal to the observer, for instance by the presence of dangerous animals such as snakes (Öhman et al., 2001a). It is thought that for bonobos, the benefits of bonding with new individuals outweigh the costs, thereby making socializing with unfamiliar conspecifics beneficial (Hare et al., 2012). In turn, these interactions may enhance survival by promoting cooperation among individuals (Tan et al., 2017). Though we have to be careful with generalizations given our sample size, our results appear to support this notion and suggest that the brains
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