The visual ecology of selective predation: Are unhealthy hosts less stealthy hosts?

Predators can strongly influence disease transmission and evolution, particularly when they prey selectively on infected hosts. Although selective predation has been observed in numerous systems, why predators select infected prey remains poorly understood. Here, we use a mathematical model of predator vision to test a long-standing hypothesis about the mechanistic basis of selective predation in a -microparasite system, which serves as a model for the ecology and evolution of infectious diseases. Bluegill sunfish feed selectively on infected by a variety of parasites, particularly in water uncolored by dissolved organic carbon. The leading hypothesis for selective predation in this system is that infection-induced changes in the transparency of render them more visible to bluegill. Rigorously evaluating this hypothesis requires that we quantify the effect of infection on the visibility of prey from the predator's perspective, rather than our own. Using a model of the bluegill visual system, we show that three common parasites, , , and , decrease the transparency of , rendering infected darker against a background of bright downwelling light. As a result of this increased brightness contrast, bluegill can see infected at greater distances than uninfected -between 19% and 33% further, depending on the parasite. and also increase the chromatic contrast of . These findings lend support to the hypothesis that selective predation by fish on infected could result from the effects of infection on 's visibility. However, contrary to expectations, the visibility of was not strongly impacted by water color in our model. Our work demonstrates that models of animal visual systems can be useful in understanding ecological interactions that impact disease transmission.