Choreography on the Sea: Decision Making, Synchrony, and Formations in Groups of Long-finned Pilot Whales

Abstract

The goal of my thesis is to improve our understanding of the group lives of social cetaceans through the quantitative exploration of collective decision making, synchrony and formations in long-finned pilot whales (hereafter also referred to as ‘pilot whales’; Globicephala melas) off Cape Breton Island, Nova Scotia, Canada. Pilot whales live in stable, long-term (likely matrilineal) groups termed “social units” that ephemerally associate with each other, providing a good model for investigating group life in cetaceans. Furthermore, pilot whales frequently mass strand for reasons that are unclear, and knowledge of their natural social behaviour may help us to understand and perhaps mitigate these events. I began with a systematic review of decision making across both terrestrial and aquatic mammals, where I contributed a modified framework to the study of decision making by non-humans and highlighted the scarcity of studies on cetaceans. I then looked at collective decision making in long-finned pilot whales during group dives using frame-by-frame analysis of aerial footage and discovered that dive initiators were significantly more likely to be females in flanking positions. The leadership of these individuals may function akin to flanking horse riders in cattle drives, maintaining cohesion within the group. Using this same aerial footage, I then went on to explore within-group synchrony in breathing and how it varies across social contexts, discovering an increase in synchrony shortly before collective dives and in larger groups. This apparent pre-dive anticipation represents the first evidence of pre-departure behaviour in cetaceans and likely serves multiple purposes: enhancing group cohesion and preparing members for the energetically demanding dive. Finally, I explored group formations in pilot whales by extracting the positions of individuals whales from overhead aerial images, calculating measures of group spatial formation. The formations of pilot whales varied across behavioural states, as well as in the lead up to collective dives, and across group sizes. Overall, my findings elucidate the complex interplay between collective decision making, synchrony, and formations in the day-to-day group life of pilot whales and should help us understand and mitigate mass stranding events.