The Sickly Defender Hypothesis
The ability to detect and avoid socially transmitted infections has been
selected for in many species. Many studies demonstrate that individuals
can identify and avoid infected conspecifics based on visual cues,
chemical cues, or sickness behaviours (Kiesecker et al. 1999;
Behringer et al. 2006; Tobler & Schlupp 2008; Zylberberget al. 2013; Poirotte et al. 2017; Stephenson et
al. 2018). Were an individual to mimic being infected, it could
potentially deter others from approaching or from attempting to utilize
a shared environment. This could, for instance, result in a rival
foregoing aggression in favor of avoidance (Figure 1). The mechanism by
which this might occur would differ depending on how parasites are
transmitted and the sensory modalities by which infection is detected
and mimicked.
If infections are transmitted by direct contact with infected
individuals, or via transfer of blood, it might be beneficial for a
healthy individual to avoid combat or social interactions with an
apparently infected conspecific. This, in turn, could benefit an
infection-mimic by diminishing the frequency and intensity of aggressive
bouts. For males with low resource-holding potential (RHP: Parker 1974),
false infection might then allow access to more resources than would be
possible otherwise, at a reduced cost. Faking sick is likely to be a
particularly potent strategy for low-quality males to deter high-quality
rivals, as more attractive and higher-quality males are often more risk
averse (Hedrick 2000; Fowler-Finn & Hebets 2011; Ory et al.2015; Rypstra et al. 2015), likely because they stand more to
lose if their residual reproductive value is reduced due to infection or
predation (Stoehr & Kokko 2006; Engqvist et al. 2015).
Dishonest signals of infection may also decrease the odds that rivals
will invade an infection-mimic’s territory or attempt to consume his
resources. Many parasites adopt a sit-and-wait strategy, and
aggregations can form due to limited dispersal of certain life stages
(McCoy et al. 2003). This means that an infected resident could
indicate local infection risk to any would-be intruder, even if the
resident is ultimately displaced. In contests, the value of the
contested resource is an important determinant in an animal’s decision
to risk escalated combat for that resource (Parker 1974). If a territory
appears to carry with it a higher risk of infection than other
territories, its value should be discounted.