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Chapter 242Most primate studies on mate choice and visual attention have focused on macaques (Supplementary Table 1). Seminal work by Waitt and colleagues showed that rhesus macaque (Macaca mulatta) females have an attentional bias towards bright red male faces when they were paired with paler male faces (Waitt et al., 2003), while males seemed to have a preference for bright red female hindquarters, but not faces (Waitt et al., 2006). Similarly, macaques seem to be biased towards symmetrical faces (Waitt & Little, 2006). Such biases might be adaptive during mate choice, because factors such as symmetry or coloration are thought to reflect the mate quality or fertility of the bearer.More recent work has been based on these previous studies. For instance, Higham et al. (2011) showed that free-ranging rhesus macaque males on the island of Cayo Santiago showed a bias towards the faces of ovulating females when they were paired with faces of the same female from a non-ovulatory period. Interestingly, this bias became especially apparent when males were presented with pictures of familiar females, suggesting that familiarity could affect the perception of mate-relevant information. Other studies in Cayo Santiago revealed a bias for red faces (Dubuc et al., 2016) and masculine faces (Rosenfield et al., 2019). In addition, Pflüger et al. (2014) found that Japanese macaque (Macaca fuscata) males show a bias towards bright red female faces when paired with paler versions of the same face. Furthermore, recent macaque studies have investigated bias for averageness, with mixed results (Damon, Méary, et al., 2017; Tomeo et al., 2017), and it has been established that macaques show an own-species bias for attractive faces, similar to humans (Damon et al., 2019). Thus, many studies have successfully utilized the preferential looking paradigm to study selective attention toward secondary sexual characteristics.Only a handful of studies have applied this paradigm to primate species other than macaques. Cooper & Hosey (2003) used a sequential paradigm with different lemur species and found that in common brown lemurs (Eulemur fulvus), which are characterized by sexual dichromatism (i.e., the difference in colour between the sexes), females showed an attentional bias towards more brightly coloured males. No such bias was found in the control group, which consisted of lemurs that did not experience sexual dichromatism. This suggests that the coloration of common brown lemurs might be sexually selected through female choice. In addition, two studies investigated attentional biases in capuchin monkeys. Paukner et al. (2017) studied whether male and female capuchin monkeys (Cebus apella) had an attentional bias for symmetrical male and female faces. They only found a symmetry bias for males seeing male faces, suggesting that males Tom Roth.indd 42 08-01-2024 10:41