Josefine Bohr Brask:
Social systems in nature are characterised by heterogeneous social structures. The pattern of social interactions or associations between individuals within populations (i.e. their social network) is typically non-random. Such structuring may have important implications for the expression and evolution of behaviour, and for individual fitness. In this thesis I investigated implications of social structure for fitness and behaviour, with focus on three main areas: social structure & fitness, social structure & communication, and social structure & cooperation. These areas were investigated in four separate studies. In the first study, we propose a simple framework that may be used as a base when studying the link between social structure and fitness. We furthermore review current evidence for fitness effects of social structure, and find good support for such effects. In the second study, we investigate empirically the role of the social environment of individuals for their communication patterns. Our study species is a song bird, the black-capped chickadee (Poecile atricapillus). The results suggest that individual communication in this species is influenced by features of the local social environment of the individual. In the last two studies, we investigate the role of social structure for cooperation in a classic natural system for behavioural research, the Trinidadian guppy (Poecilia reticulata), by means of computer simulations. Cooperation contradicts evolutionary theory and requires special explanations. Theory predicts that heterogeneous social structure can support and promote cooperation in populations. Our simulations suggest that social structure may not be able to explain the presence of cooperation in the guppy system. Overall the thesis provides new insights into implications of social structure for fitness, communication and cooperation.