Ants in obligate mutualisms with plants are a textbook example of coevolution. Extensive work has emphasized the ecology of the association, but the implications on ant behavior and colony organization are not well understood. This talk first focuses on behaviors that evolved in Central American acacia ants promoted by the association with acacia plants. Second, I will present some anatomical implications of those behaviors. Third, I will briefly discuss short-term responses of plants to ant defense. Finally, I will discuss how the acacia-ant mutualism became fertile ground for innovative behaviors in other organisms.