BENEFITS OF INDUCED HOST RESPONSES AGAINST AN ECTOPARASITE

As a consequence of the deleterious effects of parasites on host fitne ss, hosts have evolved responses to minimize the negative impact of pa rasite infection. Facultative parasite-induced responses are favoured when the risk of infection is unpredictable and host responses are cos tly. In vertebrates, induced responses are generally viewed as being a daptive, although evidence for fitness benefits arising from these res ponses in natural host populations is lacking. Here we provide experim ental evidence for direct reproductive benefits in flea-infested great tit nests arising from exposure during egg production to fleas. In th e experiment we exposed a group of birds to fleas during egg laying (t he exposed group), thereby allowing for induced responses, and kept an other group free of parasites (the unexposed group) over the same time period. At the start of incubation, we killed the parasites in both g roups and all nests were reinfested with fleas. If induced responses o ccur and are adaptive, we expect that birds of the exposed group mount earlier responses and achieve higher current reproductive success tha n birds in the unexposed group. In agreement with this prediction, our results show that birds with nests infested during egg-laying have (i ) fewer breeding failures and raise a higher proportion of hatchlings to hedging age; () offspring that reach greater body mass, grow longer feathers, and hedge earlier, and (iii) a higher number of recruits an d first-year grandchildren than unexposed birds. Flea reproduction and survival did not differ significantly between the two treatments. The se results provide the first evidence for the occurrence and the adapt iveness of induced responses against a common ectoparasite in a wild p opulation of vertebrates.