Naoyuki Iseki
(Dept. Biology, Kyushu Univ.)

08/09/30, 15:00 at Room 3631 (6th floor of building 3 of the Faculty of Sciences)

Many living things defend themselves by having poison. Such trait seems to be an appropriate weapon. However, to add to cost for poison, the fact that most poisonous preys seriously damage by predation indicates evolutionary difficulty for this weapon. In some cases, these preys are a kind of altruist because they kill predator at the expense of their life. Fisher (1953) called such preys “heroic individual” and suggested that gregariousness and kin selection may be important for evolution of these species. Theoretical study showed that if predators have learning skill, poisonous trait can evolve with gregarious behavior (Leimer et al 1986). However in initial stage of invasion of poisonous prey, its density may be insufficient to evoke learning. Some studies suggest that spatial structure promotes altruism (Nowak and May 1992, Nakamaru and Iwasa 1997). To investigate evolution of poisonous prey, we build two prey-one predator models. Those are i) spatial lattice model and ii) complete mixing finite population model. Numerical simulation found that less-competitive mutant can make successful invasion by kin aggregation (in model i) and by asymmetrical predation bias (in model ii). In both models, when wild type has an intermediate birth rate, successful invasion of mutant tends to occur. This is because higher birth rate of wild type makes amplitude of population dynamics large. This makes initial survival of few invaders hard. Stable coexistence of three species is observed for long time in wide parameter range.

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