Miser-slave symbiotic relations

In a number of natural associations, such as nitrogen-fixing bacteria in legumes, the host organism receives a significant benefit, but the symbiont partner gets little in return. Two researchers from New Zealand have recently suggested a simple model explaining such unequal relationships. Based on the Prisoner’s Dilemma game, it allows both host and symbiont to adopt a cooperation strategy that is conditioned on the partner’s actions. The asymmetry arises because the symbiont has a much faster reproduction rate than the host and therefore frequently engages in evolutionary struggles. If the host’s cooperation strategy gives the symbiont greater net benefit with greater cooperation, then the symbiont will be driven to fully cooperate, even if that increased benefit is minimal. Over the longer time scale of their own battles for survival, hosts evolve to give the minimum level of cooperation that still induces full cooperation from their partners. (M. R. Frean, E. R. Abraham, Phys. Rev. E 69, in press.)