The evolution of animal societies: An exploration of the effects of population structure, life-history and behavioural plasticity

Why do animals invest time and resources in social interactions and help each other, when the natural world is driven by fierce competition? Cooperation indeed abounds in the natural world, where organisms form societies whose fabric is made by the seemingly selfless actions of individuals. In this thesis, I used mathematical models to understand how evolution shapes animal societies, namely those of humans, insects, and fish. First, I found that human cooperation can evolve due to the competition between groups, when social behaviours are transmitted through culture, rather than genes. Furthermore, the forms of cultural transmission more favourable for cooperation depend on specific details of how groups compete. As for insect societies, I found that the most complex societies, those of bees, wasps, and ants, evolve in populations that share a set of genetic, ecological and behavioural characteristics. Those characteristics are haplodiploidy (the sex determination mechanism by which females and males are, respectively, derived from fertilized and unfertilized eggs), lifetime monogamy (when females mate with one male throughout their life), manipulation of the proportion of sons and daughters, and the production of two broods per year. Finally, inspired by investigations on fish societies, I developed a model showing that mutual cooperation can evolve when individuals negotiate with their peers. This negotiation is compromised when interactions occur among relatives, leading to less cooperation. Social life is full conflicts between individuals; in this thesis I explored a wide range of mechanisms that allow animals to surpass those conflicts, and organize their life around each other.