Symbiotic relationships where both parties can benefit from interactions are called mutualism. The mutualism typically involves the exchange of different commodities such as nutrition, defense, and housing. Because commodities impose some costs for providers, the non-rewarding “free-riders” can overwhelm the cooperators from same parties, which leads the collapse of mutualistic relationships. The apparent spread of mutualism in nature, however, indicate the existence of regulatory mechanisms to avoid the invasion of free-riders. It has been revealed, both in theoretically and empirically, that punishment and sanction toward free-riders allows for selective interaction with cooperative partners, and thus enforced cooperative behavior is one of the mechanisms to maintain the mutualistic interactions on the evolutionary time scale. In addition, recent studies suggested that mutualistic relationship can maintain by restricting or manipulating the partner behavior, depriving the free-riding options.

Typical example of mutualistic interactions can be seen in the association between ants and lycaenid butterflies, in which the butterfly larvae provide the nutritious rewards in exchange of defensive behavior from the ants. In this talk, I will introduce the regulatory mechanisms of this mutualistic association from the view point of both ant and butterfly sides, and discuss how partner manipulation explain the evolutionary stability of this interactions.