MODELING THE POPULATION-DYNAMICS OF A CUCKOO-HOST ASSOCIATION AND THEEVOLUTION OF HOST DEFENSES

Cuckoo parasitism in Nagano Prefecture in Japan has shown dramatic cha nges in the parasitism rate, host usage by the cuckoo, and defensive b ehavior of hosts during the past 60 yr. To gain insights into these ph enomena, we model the population dynamics of a cuckoo-host association together with the population genetics of a rejecter gene in the host population. Analysis shows that both the dynamical change in the host- parasite association and the establishment of the host's counteradapta tion crucially depend on the product of two factors, the carrying capa city of the host and cuckoo's searching efficiency. When the product i s less than a critical value, the host population cannot evolve a coun teradaptation even if parasitized by the cuckoo. Hence, the lack of co unteradaptation does not necessarily imply that the host population on ly recently has become parasitized. As the product becomes larger, the rejection behavior will be eventually established at higher levels in the host population. In this case, the spreading of rejection behavio r is very fast, which suggests that the cuckoo-host association reache s an equilibrium state within a relatively short period. These results make possible new interpretations of several circumstances reported a bout cuckoo-host associations.