What can we learn from bonobos and bottlenose dolphins about the evolution of between-group cooperation?

Cooperation between unrelated individuals across social groups is a hallmark of human societies, underpinning our species' unique capacity for cultural transmission and large-scale cooperation. Although long considered rare among non-human animals, recent findings from bonobos and Indo-Pacific bottlenose dolphins demonstrate that members of both species will provide costly help to unrelated out-group conspecifics with no immediate return, revealing that such cooperation can also arise outside the human lineage. In this review, we synthesize current knowledge on the occurrence, mechanisms and drivers of between-group cooperation in these two species. Despite differences in evolutionary history and social structure, both species share convergent traits-such as protracted development, social flexibility, advanced socio-cognitive abilities and absence of range defence-probably fundamental for the emergence of costly cooperation across groups. We highlight future research priorities, including the ontogeny and stability of between-group relationships, behavioural and cognitive mechanisms supporting partner choice across groups and resilience of these relationships to demographic and ecological change. By integrating insights from these two model systems and placing them within a broader phylogenetic perspective, we outline emerging evolutionary and mechanistic frameworks for understanding the origins, maintenance and function of between-group cooperation, providing comparative perspectives on the evolutionary roots of human ultrasociality.