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