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.