A Case for the Role of Infection Cues in Signal
Evolution
Animal signals often evolve to exploit pre-existing sensory biases in
receivers (reviewed in Ryan 1990; Endler & Basolo 1998). For instance,
novel signals may resemble stimuli to which a receiver is already
attracted, such as food-mimicking orange spots on male guppies and
appendages on male tetras (Rodd et al. 2002; Arnqvist & Rowe
2005). Sensory bias can also select for signals that exploit receivers’
aversion to certain stimuli, such as harmless Batesian mimics resembling
a dangerous model (Bates 1862). Sickness behaviours (Hart 1988) and
physical symptoms of infection are often used to identify infected
conspecifics (Zylberberg et al. 2013). Symptoms and sickness
behaviours have also recently been framed in terms of signal evolution,
with the possibility of dishonesty in these signals being conjectured
(Shakhar & Shakhar 2015; Steinkopf 2015; Tiokhin 2016). However, to our
knowledge, risk-factors for parasitic infection as a template for
sensory exploitation in the context of competition and sexual selection
have not been demonstrated in any species. We propose that signs of
infection could be promising and flexible sources of dishonest signal
evolution.
First, infection-mimicking signals have the benefit of their being
weighed and interpreted differently by different receivers. For
instance, often some members of a population are more susceptible to
infection than others or may incur greater costs upon infection (Zuk
2009; Hawley et al. 2011). We would predict these highly
susceptible hosts would avoid infected conspecifics more so than less
susceptible individuals. This allows dishonest signals of infection to
potentially target their effects towards specific subsets of a
population, or at least differ in the strength of their effects on
different receivers. Facultatively expressed dishonest signals of
infection are more powerful still because, even in the absence of
differential susceptibilities of receivers, they could be expressed or
even directed towards (e.g., sneezes) certain individuals whilst being
concealed from others. Additionally, in extreme cases, infection mimicry
has the potential to attract some receivers but repel others. These
possibilities are discussed in detail later.
Second, dishonest signals of infection have the benefit that they are
unlikely to interfere with species recognition systems (e.g.,
identification by potential mates). Contrast this to a hypothetical
scenario where individuals evolve to resemble their predators. Such
predator-mimics run the risk of being misidentified as heterospecifics,
which could hamper their ability to engage in beneficial social
interactions, e.g. mating.
In the following sections, we lay out more specific scenarios in which
dishonest signals of infection might evolve and be maintained, or in
which aversion to infection cues could be an initially exploited sensory
bias resulting in subsequent signal elaboration.