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Chapter 7156In the last two decades, an increasing number of studies have investigated sustained attention and its relation to sexual selection in primates, mostly using a preferential looking paradigm (Winters et al., 2015). In this paradigm, individuals are confronted with different types of pictures presented simultaneously during each trial, and the relative attention to each stimulus is used as a proxy for interest. Many of the studies on this topic have been performed with rhesus macaques. Seminal work by Waitt et al. (2003, 2006) employing a preferential looking paradigm established that macaque females had an attentional bias towards bright red male faces when they were paired with paler faces, while males had an attentional bias towards bright red female hindquarters, but not faces. More recently, researchers have elaborated on these studies by testing free-roaming rhesus macaques. For instance, Dubuc et al. (2016) found that macaques had a bias for red male faces, whereas Rosenfield et al. (2019) identified an attentional bias for more masculine male faces. Thus, the preferential looking paradigm has been successfully used to study the interaction between sexual selection and attentional processes in primates.The above-mentioned studies mostly relied on video recordings of participants combined with frame-by-frame analysis to determine gaze direction. Partly because this is an extremely time- and labour-intensive method, many recent studies on captive primates have used eye tracking. An eye tracker has infrared cameras specialized for the automatic and accurate detection of eye movements and gaze patterns to study primate social cognition (Hopper et al., 2021; Lewis & Krupenye, 2022). Recently, some studies have employed eye tracking to study the interaction between visual attention and sexual selection in primates. For example, Damon et al. (2019) used eye tracking to establish that rhesus macaques show an own-species bias for attractive faces similar to humans (Damon et al., 2019). Additionally, Lonsdorf et al. (2019) showed that brown-tufted capuchin monkeys (Sapajus apella), both male and female, were especially attentive towards pictures of same-sex individuals instead of opposite-sex individuals, suggesting that capuchin monkeys were more interested in potential competitors than potential mates. Thus, previous studies in primates have established that eye tracking can successfully elucidate attentional biases in the context of sexual selection.To date, most studies have focused on macaques. However, to fully understand the evolutionary underpinnings of such attentional biases, it is important to test a wide range of species (Smith et al., 2018), ideally with different mating systems Tom Roth.indd 156 08-01-2024 10:41