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Chapter 120to apply sexual selection theory to the topic of attractiveness (Grammar et al., 2003; Rhodes, 2006). Such evolutionary theories of attractiveness rely on two main assumptions. First, attractiveness, or attractive characteristics should reflect an individual’s ability to provide benefits either directly or indirectly (Jones & Ratterman, 2009). For example, an attractive individual might offer direct benefitsto a partner (e.g., a good territory or defense against intruders). Alternatively, attractiveness might reflect indirect benefits, meaning that high levels of attractiveness correspond to genetic quality. Second, evolutionary theories assume that selecting attractive individuals as a partner is adaptive, because it might affect the number, health, or attractiveness of offspring (Grammer et al., 2003; Little, Jones, et al., 2011). Thus, attractiveness should reflect the quality of the partner, which in turn might affect the quality and/or quantity of offspring. Both of these assumptions can be relatively easily tested in non-human animal species. For instance, in non-human primates, several examples of potentially attractive traits can be found. First, symmetry of either the face or the whole body has been hypothesized to indicate stable development, thereby serving as an indicator of genetic quality (Gangestad et al., 1994; Little, Jones, et al., 2011). In line with this, facial symmetry positively correlated with health and body condition in chimpanzees (Pan troglodytes: Sefcek & King, 2007) and rhesus macaques (Macaca mulatta: Little et al., 2012). Second, many male primates are characterized by secondary sexual characteristics preferred by females or associated with more reproductive output. For example, male gorillas (Gorilla gorilla gorilla) with more prominent sagittal crests, larger bodies, and larger gluteal muscles tend to have more mates and more offspring, reflecting female preferences for extreme sexual dimorphism (Breuer et al., 2012). Third, in some species, males show dynamic changes in appearance. Such changes in appearance often covary with competitive ability or health (reviewed in Petersen & Higham, 2020), and can covary with female preferences (e.g., Setchell, 2005). Thus, in line with the first assumption of evolutionary theories of attractiveness, specific male characteristics can be predictive of male quality, and might be preferred by females.An overwhelming amount of evidence supports the second assumption, that selecting attractive individuals should result in healthier and/or more offspring. Most studies investigating this topic used a mate preference test to identify preferred mates. In this test, individuals are placed in a mate preference arena (e.g., Anderson et al., 2007; Pogány et al., 2014). These arenas allow an individual to Tom Roth.indd 20 08-01-2024 10:41