Cooperation and non-linear dynamics: An ecological perspective on the evolution of sociality

Using the theory and methods of non-linear dynamics, I explore the consequences of cooperation on the size and dynamics of social groups. I present a model that incorporates into a discrete growth equation a positive density-dependent factor to represent the synergistic effects of cooperation. Analysis of this model shows that, by increasing the net reproductive output of group-living organisms, cooperation could either stabilize or destabilize the dynamics of a social group. At one end of the spectrum, group-living and cooperation could make persistence possible under harsh demographic or ecological conditions. At the other end of the spectrum, in populations already organized in social groups, cooperation could lead to more highly integrated social groups that are subject to a boom-and-bust pattern of growth. When groups last for multiple generations, such a pattern could take the form of periodic or chaotic dynamics. It is suggested that dynamical instability could result in rates of group turnover large enough for selection among the highly integrated social groups to take over as the primary evolutionary force. Consideration of the dynamical effects of cooperation, therefore, may shed light both on the ecological and demographic conditions leading to the origin and maintenance of group-living as well as on the forces responsible for shaping the diversity of animal societies.