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.