(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 .