False Infection & Species Recognition
Due to parasite-host specificity, false signals of infection could evolve as means of reinforcing species recognition. Hybrid zones and hybrid reproduction by allopatric species are common phenomena (Harrison 1993). Often hybrids are at a fitness disadvantage compared to non-hybrid individuals of either species (Sage et al. 1986; Bleeker & Matthies 2005). This favors reliable mechanisms of species recognition (Andersson 1994). Parasites are often highly species-specific, even when closely related species occur sympatrically (Van As & Basson 1987; Bittencourt & Rocha 2003; Dick 2007). As parasites are obliged to identify their correct hosts, and do so by directly sampling the host’s physiology, the presence of visible infection, or symptoms of a host-specific parasitic infection, could be used to increase the reliability of conspecific recognition.
Females tend to be especially attentive to reliable indicators of species identity, because the costs of mistakenly mating with heterospecifics are typically greater for females (Parker, 1979; Parker 1983; Parker & Partridge 1998). Thus, attending to cues of species-specific infection could provide indirect benefits to females via increased offspring vigour, and direct benefits due to reduced mate searching costs. Signals to emphasize or exaggerate, or even “imitate” being infected could then be advantageous for increasing male reproductive success. Over time, a runaway process could cause these signals emphasizing or imitating species-specific cues of parasitism to become species-identifying sexual signals and possibly spread to fixation if costs are low (Fisher 1930). In this hypothetical, infection mimicry is likely not to be dishonest as such, because males displaying false infection would do best to be most attractive to their own species to avoid reduced hybrid fitness in their offspring.