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This dissertation is the result of a close collaboration between Leiden University, Apenheul Primate Park, and the European Endagered Species Programme (EEP) for orang-utans. In 2017, Thomas Bionda, Warner Jens (both Apenheul), and Mariska Kret (Leiden University) decided to team up with the goal of refining the orang-utan breeding programme by taking individual mate preferences into account. One promising approach to do this, was application of computerized tasks within the orang-utan breeding programme to identify such mate preferences in zoo-housed orang-utans. Until then, computerized tasks had mainly been used to study fundamental scientific questions. For instance, the bonobos (Pan paniscus) in Apenheul had been participating in multiple noninvasive experiments to study emotion perception (Kret et al., 2016; van Berlo et al., 2023). This research programme inspired the idea of finding a more practical application for such methods, in addition to fundamental questions.One particular species for which such a practical application was considered useful was the orang-utan (Pongo spp.). Zoo-housed orang-utans have relatively low breeding success compared to other great ape species (Kaumanns et al., 2004), and their birth rate may have been too low to maintain a stable population size (Bemment, 2018). One potential reason for this is the solitary nature of orangutans: they live a semi-solitary lifestyle (Roth et al., 2020), and group life may actually be a stressor for them (Amrein et al., 2014; Weingrill et al., 2011). At the same time, orang-utan females are known to be selective in their mate choice (Knott et al., 2009). This led to the idea of developing tasks that can assess orangutan mate preferences, ultimately providing them with more control over their social environment, which could also lead to increased reproductive success. With this goal in mind, I started my PhD project in 2019, eventually resulting in this dissertation.Currently, most captive breeding programmes do not take mate preferences into account, although a large body of literature shows that animals have higher reproductive success when allowed to choose their own mate (reviewed in Martin-Wintle et al., 2019). Instead, breeding recommendations are heavily based on genetic data and practical factors such as housing space. Admittedly, it has not been easy to incorporate mate preferences into breeding programmes: studies that find beneficial effects of incorporating mate choice often rely on behavioural measures during inter-individual interactions to identify mate preferences. Such methods are not feasible in zoo populations, where individuals are spread across multiple locations. This has led some authors to argue for long-distance Preface12Tom Roth.indd 12 10-01-2024 08:13