• cetaceans
• gregarious behaviour
• killer whales
• laterality
• leadership
• Orcinus orca
• swimming

One of the most frequently observed gregarious behaviours in free-ranging cetaceans is swimming in formations. However, there is still a lack of data on its determinants. Formation swimming relies on the ability of each group member to perceive stimuli from conspecifics and from the surrounding environment to maintain a precise layout. Killer whales (Orcinus orca) of the Southern Residents population (SRKW) have been monitored since the 1960s with detailed data are available on whales life history and population trends, and represent an ideal model to study these phenomena.
Taking advantage of the availability of videos of swimming formations acquired with drones in 2018 and 2019 around the San Juan Island archipelago (US), I analysed 47 videos with 29 different whales, identified the group leaders, and described the geometry, and laterality of formations. Using 3D trigonometry, I described whale positional arrangement. I could identify who was leading in each group (i.e. a female in most of the sightings). I then tested whether the leadership role and positioning of the orcas were random using Chi-squared type statistics with a permutation approach. Leaders showed a significant trend in occupying an off-centred position, supporting the hypothesis of a lateralization influence at group level. Group members were also positioned in a non-random order from left to right, with a preference for swimming beside specific nearest neighbours.