Facultative or Hidden Signals of False Infection
Until now we have treated infection mimicry as a permanent trait.
However, the evolution of facultatively expressed dishonest signals of
infection could be particularly beneficial. Many sickness behaviours, or
behavioural symptoms such as sneezing, could be exhibited flexibly, and
so could be shown to rivals, but concealed from potential mates or
social partners. This flexibility would mitigate the trade-off between
attraction and repulsion of different subsets of conspecifics.
Situational expression of sickness behaviours mediated by the social
setting has been demonstrated in some case studies (Owen-Ashley &
Wingfield 2006; Lopes et al. 2012). For instance, zebra finch
males who were made sick showed sickness behaviours in isolation, but
not in the presence of a female or in a group (Lopes et al. 2012;
Lopes et al. 2013), indicating flexibility in the expression of
these behaviours based on audience composition. Sickness behaviours and
behavioural symptoms are likely the most conducive to flexibility.
Permanent visual signals would likely not be able to be changed in
real-time, and longer-lasting signals such as scent marks on a territory
(Kavaliers et al. 2005) would not be amenable to flexible
deployment.
Just as flexibility in signal deployment could reduce the costs to
attractiveness of mimicry, so too could taking advantage of
polymorphisms in the sensory perception of receivers. Sensory systems
often differ markedly among species, and even within species (reviewed
in: Dangles et al. 2009). For example, in some New World primate
species, males and females differ in their ability to perceive colours
(Jacobs 1994). Situations such as this could allow certain signals to be
perceived disparately between the sexes. Taking advantage of such
differences in sensory capabilities might be especially useful in
interspecific infection signalling, such as deterring predators, as
mimicry in a signalling channel only available to predators would allow
species to be mimics to predators without compromising cues important
for intraspecific communication.
Flexible or “hidden” signals in intra-specific mimicry, however, raise
some theoretical difficulties: It is difficult to reconcile why the low
costs of these signals (i.e., near zero reduction in attractiveness to
mates) would not result in these signals spreading to high frequency
and, thus, select for receivers to ignore them. Maintenance could
perhaps be achieved if there are steep cognitive costs to receiver’s
ability to detect the trait, or if the signal itself were costly to
produce, meaning not all individuals can generate the signal.
Alternatively, what began as a signal exploiting a sensory bias against
infection cues could be coopted for other purposes, such as becoming a
general warning or courtship signal. The selective benefits leading to
the origin of a trait (including signals) can differ from the benefits
that cause a trait to be maintained or elaborated (Williams 1966;
Arnqvist 2006). Thus, even if the costs of mimic signal production are
low (Szamado 2003), if it is reliably deployed in specific circumstances
such as agonistic interactions, then what began as an
infection-mimicking signal, based on the arguments outlined above, could
subsequently evolve to be part of a normal species’ repertoire of
agonistic behaviours.