Groups of animals interact in a variety of different ways, from lethal conflict to cooperation and fusion. While intergroup conflict is relatively widely studied, the mechanisms and selective forces underpinning cooperation and group fusion are less well understood. The lower termites are a group of eusocial insects that displays a spectrum of intergroup relationships from violent battles to avoidance to colony fusion, making them a valuable model system with which to study intergroup interactions. In Chapter 2, I present a trial of a novel method of marking termites (Visible Implant Elastomer), finding that it causes slight changes to survival and behaviour associated with reproductive disinhibition. In the following chapters, I investigate two aspects of intergroup interactions in a single-piece nesting species of lower termite, Zootermopsis angusticollis: nestmate recognition and soldier caste ratio. In Chapter 3, I investigate the implications of colonies sharing similar nesting material on the ability of Z. angusticollis pseudergates to discriminate between members of their own and a different colony. I find that Z. angusticollis pseudergates discriminate between nestmates and non-nestmates but that this does not appear to be dependent on whether they encounter a non-nestmate raised on the same or a different wood type. Contrary to prediction, I also find that non-nestmates experience the same levels of cooperative allogrooming as nestmates. In Chapter 4, I use a theoretical model to examine the consequences of colony fusion on the sterile caste, the soldiers, at the 1 colony level. This model predicts that the reported increase in soldier number from some studies is supported under a narrow range of cost and benefit parameters, and that termites can benefit from fusion at the colony level under two scenarios: when fusion results in higher net benefit from soldiers, or when a colony can take advantage of the other’s soldiers. This suggests two pathways to the evolution of colony fusion, which might be applicable to lower termites of different ecological habits. Evidence from both Chapter 3 and Chapter 4 suggest that colony fusion in the lower termites could be driven by selection on pseudergates, which can gain direct fitness benefits from fusion. I suggest that this might be a general phenomenon and that the presence of conflict within a group can facilitate cooperation between groups across taxa.