The impacts of out-group threats on within- and between-group behaviours in the cooperatively breeding white-browed sparrow-weaver

Abstract

Groups face a variety of potential threats from out-group conspecifics. Individuals from outside the group may challenge the breeding position and success of group members and groups often defend against neighbouring or unfamiliar groups. These groups might attempt to acquire the group's resources or encroach into their territory. Together, these threats from out-group conspecifics may give rise to costs affecting group members, resulting in a greater incentive to cooperatively defend against rivals. Few studies have investigated the long-term effects of threats from out-group conspecifics on group dynamics, cooperation and spatial competition. This thesis aims to address the need to investigate, not only the long-term impacts of inter-group conflict for competition over space, but also the consequences of the breakdown in monogamy resulting from out-group breeder replacement in the cooperatively breeding white-browed sparrow weaver (Plocepasser mahali). Firstly, I investigated the effects of out-group breeder replacement on helper retention and reproductive success, finding that neither is markedly negatively impacted by out-group breeder replacement, even though breeder replacement was expected to destablise cooperative groups due to reductions in relatedness. Secondly, I investigated the role of group size in between-group conflict over space and found that larger social groups dominate competition for space. To maintain a large group size relative to neighbouring groups, subordinate helpers may work to increase group size by improving the reproductive success of the dominant (known as group augmentation) in order to enjoy the benefits of living in a large group. As a result, there may be strong selection for cooperative territorial defence, especially if success in conflict relies on cohesion between group members, to overcome the threat from out-group conspecifics.