Interactions between an Irrawaddy dolphin Orcaella brevirostris mother-calf pair and Indo-Pacific humpback dolphins Sousa chinensis in southern Cambodia

Reports of interspecific encounters among cetaceans are common and range from affiliative behaviours such as allo-parental care to aggression, including lethal encounters. These interactions are complex and may lead to the formation of mixed-species groups, particularly among sympatric delphinids. In Cambodia’s coastal waters, Irrawaddy dolphins Orcaella brevirostris and Indo-Pacific humpback dolphins Sousa chinensis share overlapping ranges and face similar threats, including habitat degradation and bycatch. During a boat-based survey, we observed a rare interaction between an Irrawaddy dolphin mother-calf pair and a group of seven Indo-Pacific humpback dolphins (five adults, one subadult and one juvenile). This event was documented through photography and drone footage, which allowed individual identification of all the humpback dolphins. Behaviours resembling allo-parental care by the humpback dolphins toward the Irrawaddy calf were initially observed. As the interaction continued however, instances of aggression emerged, including calf separation attempts and physical contact. While these were agonistic, there was no evidence of infanticidal intent or serious harm. Interspecific interactions like these are difficult to interpret and remain poorly understood, particularly in understudied regions like Cambodia. To our knowledge, this is the first documentation of interactions between the two species in Cambodia and one of the few detailed accounts globally. Continued monitoring is essential to better understand these behaviours and their implications for conservation.