Distance defines us: determining dolphin group membership through analysis of inter-individual distances

Abstract Group spacing and cohesion are fundamental aspects of social structure in group-living species, shaping interactions and ecological dynamics. Defining what constitutes a group can be problematic in highly mobile group-living species such as delphinids. Spatial proximity among individuals is often used to delimit group membership, yet threshold distances lack justification. In this study, we combined machine-learning methods with measurements extracted from drone-based observations to investigate how the sighting size, age composition, and behaviour influence nearest-neighbour distance (NND) and inter-individual distance (IID) among Indo-Pacific bottlenose dolphins ( Tursiops aduncus ) in Coffin Bay, South Australia. Our aim was to identify threshold distances for NND and IID that objectively define group membership based on spatial proximity. We identified threshold distances of 18 m for NND and 24 m for IID, based on the 95th percentile of their distributions across all sightings. Mixed-effects models showed statistically significant effects of sighting size, age composition and behaviour on both NND and IID, with effect sizes ranging from < 1 m to 12.6 m. Behaviour had the strongest impact, with dolphins exhibiting closer IID during socialising and travelling compared to foraging. Larger sighting sizes were associated with significant increases in both NND and IID, while higher proportions of juveniles and calves influenced spacing differently, and depended on sighting size. These findings reveal that dolphin spatial structure is dynamically shaped by sighting size, age composition and behaviour, advancing our understanding of the mechanisms governing group cohesion and coordination. The approach presented here provides an objective framework for defining delphinid groups, which is broadly applicable to other group-living mammals.