Division of labour within social groups and the interspecific relationships within mutualisms have traditionally been treated as separate research areas. In this opinion, we align terminologies and concepts between the two fields, by comparing within-group division of labour to the outsourcing of functions in mutualisms. Division of labour and interspecific outsourcing share fundamental similarities. Both are built from specialization of some individuals within the relationship on tasks or functions required for survival, growth and reproduction. Both also generate variable fitness outcomes. A key difference is that mutualisms generally generate direct fitness gain, while benefits from cooperative sociality often accrue from a mix of direct and indirect fitness. Additionally, the levels of physical and physiological specialization within many mutualisms expand far beyond the levels of differentiation seen in cooperative social groups, with the exception of reproductive division of labour. The consideration of between-species outsourcing in the context of division of labour allows expansion of our understanding of both fields and beyond, to consider general principles as drivers of division of labour, and role differences more broadly across levels of complexity.This article is part of the theme issue 'Division of labour as key driver of social evolution'.