(a) Introduction
Sexual armaments are usually structures or color patches displayed during male contests that are either used as weapons (Okada et al. 2006), and/or to signal the hierarchical status or fighting ability (Stapley & Whiting 2006). The latter reduces conflicts with subordinate, less competent males, who avoid the costs associated with fights, but may also increase conflicts with males of similar status (Olsson 1994; Abalos et al. 2016). In either case, these structures afford superior males more frequent access to reproduction (Baird et al. 2013; Morina et al. 2018). Thus, sexual armaments are expected to primarily evolve due to intrasexual selection through male competition, and secondarily due to intersexual selection by female choice preferences (Berglund et al. 1996). This is because these armaments often have a dual role acting simultaneously as sexual ornaments that attract the attention of potential mates (Berglund et al. 1996), although if they evolved first as ornaments or armaments is not always evident (e.g. Morris et al. 2007).
The good-genes hypothesis argues that a females’ preference for mating with high quality males exhibits positive selection because it increases the frequency of adaptive genes in the population (Arnold 1993). According to good-genes theory sexual ornaments evolve under intersexual selection for parasite resistance (Hamilton & Zuk 1982). Parasites draw resources from their hosts and undermine their energy budget, which alters the resource allocation to reproduction (Sorci et al. 1996; Webb & Hurd 1999). This may also affect allocation of pigments to sexual coloration, which usually have anti-oxidant activity, and are thus reallocated to the immune system to fight the infection (Hõrak et al. 2006; Megía-Palma et al. 2018a). This produces a phenotypic correlation between the severity of the immune insult and the expression of the color ornament which is interpreted as an honest signal of parasite infection or resistance (Bortolotti et al. 2009; Megía-Palma et al. 2018b). In this context, it is expected that females will evolve a preference towards mating with more colorful males, which would favor the transmission of adaptive genes for parasite resistance (Hamilton & Zuk 1982). Thus, the role of parasites in the evolution of ornaments is clear and multiple studies support it in different taxonomic groups (Houde & Torio 1992; Worden et al. 2000; Del Cerro et al. 2010; but also see Balenger & Zuk 2014). However, the role of parasites in the evolution of secondary sexual characters with dual function is poorly understood.
Male Western fence lizards, Sceloporus occidentalis (Squamata: Phrynosomatidae), have complex display behaviors that include the exhibition of their blue and yellow patches. The blue coloration in sceloporine lizards is based on light scattered in micro-structures and underlying (eu-) melanin in the dermis (Quinn & Hews 2003). Blue patches are ventral and are displayed when lizards compress their thorax laterally. Yellow patches are located in the forelimbs in S. occidentalis and may be more conspicuously displayed when the lizards perform push-ups. The yellow coloration in this group of lizards is pigment-based (Weiss 2006). Both are displayed during agonistic interactions with other males, and also to court females, which suggests a dual role for these color patches (Carpenter & Ferguson 1977). The behavioral displays of the males are ritualized because their alternative display may depend on the level of escalation in the social conflict or the distance between conspecifics (Sheldahl & Martins 2000). Males displaying these behavioral traits with higher intensity may get more access to females and, hence, more opportunities for reproduction (Schall & Dearing 1987; Schall & Houle 1992). Therefore, any conditions that reduce the intensity of these color displays may result in decreased fitness. Previous studies have shown multiple parasites infecting lizards in a coastal population of California and males infected by intestinal coccidians had blue ventral patches with less spectral chroma (Megía-Palma et al. 2018b). This suggests that the infection limits expression of the male ventral blue patches, which may affect social interactions (Megía-Palma et al. 2018b). We hypothesize that parasites affect male agonistic interactions in this species. To study the potential influence of parasites on male interactions we staged pairwise male contests in the laboratory and recorded their behavioral traits. We predict that infected lizards will perform poorly compared to healthy lizards and that blue patches, used as armaments, will be important predictors that explain the intensity of aggression by rival lizards (Morris et al. 1995). If these predictions are confirmed, our study will demonstrate an influence of parasites on male agonistic interactions and, therefore, define a role for parasites that influence intrasexual selection in S. occidentalis .