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.