Social hosts evade predation but have deadlier parasites

Abstract Parasites exploit hosts to replicate and transmit, but overexploitation kills host and parasite ( 1 ): predators may shift this cost-benefit balance by consuming hosts ( 2–4 ) or changing host behavior, but the strength of these effects remains unclear. Modeling both, we find a primary, strong effect: hosts group to defend against predators ( 5 ), increasing parasite transmission, thus multiple infections, and therefore favoring more exploitative, virulent, parasites ( 6 ). Indeed, among 18 Trinidadian Gyrodactyus spp. parasite lines, those collected from high predation guppy populations were more virulent in common garden than those from low predation populations. Our model accurately predicted this result when parametrized with our experimentally demonstrated virulence-transmission trade-off, implicating the behavioral effects of predation. Broadly, our results indicate that reduced social contact selects against parasite virulence. One-Sentence Summary Our theory and data show predators cause increased host social grouping; the resulting transmission favors parasite virulence.