Costly signalling — along with other adaptive mechanisms, including reciprocity and kin selection — supports altruism in human societies. Because literary works can reflect the lives, motivations and ideals of real cultures, the same adaptive forces governing the actions of actual persons may drive the interactions depicted in these stories. Based on this reasoning, we analysed the interactions in the Old-English poem Beowulf, asking whether the beneficent behaviour exhibited by the characters functions as costly signalling or as exchange-based interactions. We found that both mechanisms play a role but costly signalling provides benefits beyond those from relationships based on exchange. Specifically, gift exchange promoted comrade allegiance but costly signalling additionally provided status increase to the signaller. Furthermore, boasting about oneself forged alliances whereas telling tales about the exploits of others increased speaker status. We show that hypotheses derived from evolutionary theory can be explored through quantitative text analyses of period-specific literature.