Page 35 - Emotions through the eyes of our closest living relatives- Exploring attentional and behavioral mechanisms

Emotional attention is modulated by familiarity
Experiment 1: Bonobos’ attentional bias towards emotions of familiar and unfamiliar conspecifics
Method 2
Participants
Four female bonobos (Besede, 11 years old; Monyama, 6 years old; Kumbuka, 17 years old; Yahimba, 7 years old and daughter of Kumbuka)1 living in a social group of 12 individuals at Apenheul primate park in Apeldoorn, The Netherlands, took part in the study and were tested over a period of 4.5 months.
All participating females were born in captivity and had prior touchscreen experience through participating in the study by Kret et al. (2016). There were eight months of no testing between the two studies. At the time of testing, none of the individuals were pregnant nor on contraceptives. All individuals were housed in large in- and outdoor enclosures (2970 m2 in total) containing several climbing structures, trees, bushes and ropes, puzzles from which they could acquire food, and small streams of water. To mimic natural fission-fusion behavior, bonobos were always housed in two separated groups that varied in composition regularly. All participants in this study were exposed to humans since birth and interacted with them on a daily basis. Daily diet consisted of a variety of fruits, vegetables, branches and leaves, and pellets enriched with necessary nutrients. The bonobos were fed four to five times a day, and water was available ad libitum. Furthermore, bonobos were never deprived of water or food at any stage of the experiment.
Testing took place in the presence of non-participating group members and during winter when the park was closed for visitors. Bonobos were tested three to four times per week in one of the indoor enclosures, and one test session lasted ~15- 20 minutes per individual.
Tests with the bonobos were conducted adhering to the guidelines of the EAZA Ex situ Program (EEP), formulated by the European Association of Zoos and Aquaria (EAZA). Bonobos participated voluntarily and were never separated from their group
1 We acknowledge that our sample size is limited compared to studies with humans. Nevertheless, it is in line with touchscreen-based experiments involving apes, which have an average sample size of four (Egelkamp & Ross, 2019). Despite this limitation, we argue that comparative studies such as ours have scientific merit and provide crucial insights into the cognitive abilities of animals. This is especially true for bonobos, as they are a critically endangered species and rarely kept in zoos and sanctuaries (Fruth et al., 2016). Access to and testing of bonobos is very limited. To partly compensate for the low sample size, we maximize the number of trials per individual and make individuals’ data available for future work.
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