Contagio di sbadiglio in un gruppo di bonobo (Pan paniscus): Approccio sperimentale e naturalistico

Contagious yawning occurs when a subject’s yawn is induced by another subject’s yawn. Depending on the species, this phenomenon has been linked to inter-individual coordination, physiological synchronization, and emotional contagion. To date, research on yawn contagion (YC) in bonobos has provided mixed results. Firstly, yawn contagion has not been found in all studied groups. Secondly, the effect of individual and social variables, such as sex, age, rank and familiarity, remains unclear. Nevertheless, such research has been carried out through either observational or experimental methods, with no study evaluating the impact of the approach in shaping yawn contagion in Pan paniscus. Here we aim to fill this gap by investigating yawn contagion in bonobos through both observational and experimental methods. To reach this goal, we performed naturalistic observations (May-September 2021) and experimental tests (October 2021) on 13 bonobos hosted at the Twycross Zoo (UK) and managed through a fission-fusion system (i.e., two groups of variable composition). During the naturalistic observations, we recorded yawns, affiliative behaviours (e.g., play or contact sitting, used to establish the relationship quality) by the All Occurrences sampling method. During the experimental tests, performed through a tablet, the individuals were randomly exposed to videos in which familiar or stranger subjects yawned (experimental condition) or performed mouth movements (control condition). The statistical models show that yawn contagion is present in both the observational and the experimental dataset. Nevertheless, the variables explaining YC in the statistical models differ depending on the methodology. More specifically, the experimental data show a decrease in YC along with age, whereas the observational data show an opposite trend. These results may be related to the fact that older subjects are generally less curious than younger ones and that they might not approach/observe the tablet and, by consequence, they perceive less yawning stimuli. Moreover, only the experimental data show an effect of familiarity in increasing the frequency of YC. These findings suggest that the effect of familiarity is easier to detect when the two "extremes" of the relationship quality (i.e., familiar vs stranger subject), are experimentally presented. As a whole, this study confirms that YC is present in bonobos at the population level and highlights the importance of taking into account the methodology used to interpret (possibly) contrasting results.