Parasite-induced warning coloration: a novel form of host manipulation

There is currently considerable interest in the evolution of parasite manipulations of hosts that facilitate transmission. The classic examples of such adaptive manipulations typically concern trophically transmitted parasites that increase host conspicuousness or in other ways maximize the likelihood of their intermediate host being consumed by the parasite's predatory definitive host, thereby completing the parasite's life cycle. However, examples of manipulations not involving trophically transmitted parasites are rare. Here we present evidence of a novel parasite-induced change in host coloration that we suggest acts as a warning signal to deter predators from consuming infected hosts, which would result in termination of the parasite's life cycle. When insect larvae are infected by the entomopathogenic nematode Heterorhabditis bacteriophora they become bioluminescent and change to a vivid pink-red colour. We used field trials involving wild European robins, Erithacus rubecula, and showed that infected larvae were rarely handled by these avian predators and, on the rare occasions that infected larvae were handled, they tended to be rejected. Our results suggest that the parasite-induced colour change in the host may act as a visual deterrent or warning signal to avian predators, which may be reinforced by the production of a distasteful chemical that, together, reduce the likelihood of infected prey being consumed. To our knowledge this is the first reported case of a parasite-induced host manipulation involving warning coloration, and so broadens the range of examples of this fascinating adaptive behaviour. (C) 2010 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.