Behavioral reduction of infection risk

Evolutionary biologists have long postulated that there should he fitness a dvantages to animals that are able to recognize and avoid conspecifics infe cted with contact-transmitted disease. This avoidance hypothesis is in dire ct conflict with much of epidemiological theory, which is founded on the as sumptions that the likelihood of infection is equal among members of a popu lation and constant over space, The inconsistency between epidemiological t heory and the avoidance hypothesis has received relatively little attention because, to date, there has been no evidence that animals can recognize an d reduce infection risk from conspecifics. We investigated the effects of C andida humicola, a pathogen that reduces growth rates and can cause death o f tadpoles, on associations between infected and uninfected individuals. He re we demonstrate that bullfrog (Rana catesbeiana) tadpoles avoid infected conspecifics because proximity influences infection. This avoidance behavio r is stimulated by chemical cues from infected individuals and thus does no t require direct contact between individuals, Such facultative modulations of disease infection risk map have critical consequences for the population dynamics of disease organisms and their impact on host populations.