Infection mimicry & Could-Be Mates: Costs to Attractiveness
Though deterring same-sex rivals via appearing infected could be beneficial, the difficulty posed by infection mimicry is that the benefits of deterring one class of conspecifics (same-sex rivals) must be weighed against the costs of deterring others, such as potential mates. Indeed, many theories of sexual selection posit that extravagant sexual signals evolved as honest indicators of the signaller’s lack of parasites and underlying ability to resist infection (Hamilton & Zuk 1982; Andersson 1994). This suggests that females should select against individuals who appear infected, particularly if parasites can be transmitted directly (Able, 1996). There are, however, factors that may reduce such costs or eliminate them entirely, as discussed below.
There is often sexual dimorphism in susceptibility to, and costs of, infection. For many kinds of parasites, males are at greater risk due to immune suppression via testosterone (Folstad & Karter 1992), higher stress (reviewed in Sapolsky 2005), or energy allocation trade-offs (reviewed in Nunn et al.2009). In these cases, males may have a higher susceptibility to becoming infected when they encounter infectious material (Zuk 2009; Hawley et al. 2011). These sex-differences could result in male and female receivers being deterred to differing degrees, allowing false infection to be a powerful deterrent to male rivals while only imposing modest costs in terms of attractiveness to females, at least for certain kinds of parasites.
Parasites may also evolve to be more virulent and costly for one sex than other, causing there to be disparate costs to becoming infected. Many infections are differentially spread by males because of their wider ranges, greater contact with conspecifics, and greater immune susceptibility (Hawley et al. 2011). Therefore, we may expect parasites to optimize their pathology on male bodies as opposed to females (Duneau & Ebert 2012; Duneau et al. 2012), leading to differences in the costs of infection in hosts of different sexes (e.g. Blanco et al. 2001; Tseng 2004). Sexual dimorphism in infection outcomes is especially pronounced in polygynous species, in which males must compete intensely for mates, and this is associated with greater parasite-induced male morality (Moore & Wilson, 2002). When fitness costs from infection are sufficiently steep for males relative to females, males should be more averse to risk of infection (Stoehr & Kokko 2006). Thus, in certain scenarios, we should predict the behavioural elements to immunity (Schaller & Park 2011) to be particularly active in males relative to females, and this should be especially true for high-quality males who have a greater residual reproductive value to protect (Engqvist et al. 2015). This could further enable the evolution of dishonest signals of infection with comparatively modest costs to a male mimic’s attractiveness relative to their intra-sexual deterrent ability.
An ideal case of asymmetrical risks of infection is when a parasite targets sex-specific tissue. For instance, certain species of myxosporeans, a microscopic parasite, specifically parasitize male gonads in fish and amphibians (reviewed in Sitjà-Bobadilla 2009). When infection is pronounced, this can produce externally visible infection cues (Sitjà-Bobadilla 2009). These parasites can occasionally be transmitted via direct contact, but infection is more commonly acquired via free-floating spores. As such, faking sick could make a territory unappealing to could-be opponents, while the cues could be mostly irrelevant to prospective females, which lack the tissues necessary to harbor the infection.
For the arguments presented above, the infection being mimicked need not be entirely benign to females, as long as the costs of female deterrence are outweighed by the benefits of male deterrence. As was discussed in the section describing the Sickly Defender Hypothesis , costs in terms of loss of attractiveness may not be equivalent for males of high and low quality. Thus, even a slight decrease in the immediate attractiveness of high-quality males may be enough to make infection-mimicry suboptimal, while the factors described in this section may allow mimicry to be a more plausible condition-dependent strategy for low-quality males.