Maintenance of Mimicry in the Sickly Defender Hypothesis
The plausible maintenance of dishonest signals of infection are contingent on several factors. The transmissibility of parasites and their associated fitness costs to its host must be weighed against the benefits gained by displacing or attacking infection-mimicking opponents. In cases where an opponent’s genuine infection status is ambiguous, evolution is anticipated to select against making the costlier error (Wiley 1994). Thus, if the costs of acquiring a certain infection are higher than the costs of losing a subset of winnable contests, then animals should err on the side of “believing” any infection cues they see.
The frequency of mimics relative to truly infected individuals will alter the relative costs to receivers of ignoring or believing mimics, meaning that the success of mimicry is likely contingent on mimics remaining beneath a certain frequency threshold (negative frequency-dependent selection). When mimics become too common, this will select for receivers to ignore mimics. As mimics decrease in frequency, however, the benefits of ignoring mimics is predicted to be outweighed by the costs of mistakenly confronting genuinely infected opponents. The precise stable frequency of mimics will depend on the fitness impacts of the parasite, its transmissibility, and the relative benefits of acquiring territories or other resources (see Box 1A).
The frequency of mimics might also be shaped by the costs of the strategy (see Box 1A). For instance, there may be costs associated with increased conspicuousness to predators or, very likely, decreased attractiveness to mates. These costs may differ depending on male condition. Seeing as low-RHP males in species with large differences in RHP are perhaps the most likely mimics (Mokkonen & Lindstedt 2016), it may be that their attractiveness to females is already sufficiently low that the costs of exhibiting false infection might be comparatively modest. Conversely, the larger reproductive potential of high-quality males will cause any decrease in their attractiveness to have a greater absolute cost to reproductive success (Engqvist et al. 2015), likely causing infection-mimicry to be a suboptimal strategy. In terms of benefits, high-quality males will presumably be capable of competing for mates and resources by conventional means, reducing any benefits of mimicry. Additionally, due to greater risk aversion in high quality males (Engqvist et al. 2015), the efficacy of mimicry for deterring high-quality opponents is predicted to be greater than for low-quality opponents (Figure 1). In aggregate, infection mimicry is anticipated to impose steep costs to high-quality males with low returns, whereas low-quality males are anticipated to experience lower costs and greater returns. These differential costs and benefits of mimicry may be sufficient to ensure that the mimic strategy is not optimal for all males, thus preventing the frequency of mimics from crossing the threshold at which receivers begin to ignore apparent infection cues. The attractiveness costs of mimicry, and ways in which they may be altered or mitigated, are discussed in more detail later.
If infection mimicry is most beneficial and least costly to low-quality males, then it is most likely to evolve as a condition-dependent strategy. Condition-dependent models of alternative reproductive tactics posit that an individual’s condition will affect an animal’s developmental decision to go down alternative strategic routes (e.g., if big, be a fighter, if small, a sneaker) (Repka & Gross 1995; Gross & Repka 1998). This allows for the maintenance of different tactics with different fitness outcomes. If costs to low-quality males of conspecific aggression exceed the benefits of maximizing their attractiveness, then infection-mimicry could be a fitness optimizing strategy, even if their overall fitness is lower than high quality males. Mimicry, then, becomes a classic “making the best of a bad situation” strategy.