The Evolution Of Non-Kin Cooperation In Joint-Nesting Taiwan Yuhinas, Yuhina Brunneiceps
Cooperation among individuals occurs throughout nature. However, how diverse forms of social organizations are shaped by natural selection in different ecological and social conditions remains a puzzle. In this thesis, I use evolutionary game theory and inclusive fitness theory to model social conflict and its resolutions. I use Taiwan yuhinas, a passerine bird species in which multiple, non-related females lay eggs jointly in the same nest and then all group members cooperate in rearing the young, as a model system to study the formation and stability of non-kin groups. Based on the theory of reproductive skew, I derive the necessary prerequisite condition for the evolution of any stable social group: The reproductive productivity of the group must exceed that of the sum of all of its members if they attempted to reproduce solitarily. I argue that different types of group benefits have different influences on the properties and composition of cooperative breeding groups. I develop the "boarded tug-of-war model", which synthesizes previous transactional and tug-of-war models of reproductive skew, to obtain the general ecological and social conditions for a stable social group to evolve. This synthesis model predicts that different conflict resolution mechanisms will be selected in different ecological and social conditions. I also model the general effect of parental investment on reproductive skew by relaxing the restrictive assumptions of existing models. My expanded model clarifies how adjustable parental effort can influence the resulting reproductive skew and social conflict. Finally, I use Taiwan yuhinas as an empirical case study of the evolution of non-kin groups. I find that kin are more likely than non-kin to join groups, but only when group sizes are larger than the most productive sizes. I interpret this as suggesting that yuhinas still prefer to cooperate with kin. The reason yuhinas typically form non-kin groups is that maturing young have excellent opportunities for successful dispersal and reproduction elsewhere. This results in a high offspring dispersal rate, which limits the chance of adults recruiting their young (kin) into cooperative breeding groups. Studying the evolution of non-kin cooperation can help us understand the general principle of stable cooperation.
dissertation or thesis