The social context of individual foraging behaviour in long-finned pilot whales (Globicephala melas)

Long-finned pilot whales (Globicephala melas) are highly social cetaceans that live in matrilineal groups and acquire their prey during deep foraging dives. We tagged individual pilot whales to record their diving behaviour. To describe the social context of this individual behaviour, the tag data were matched with surface observations at the group level using a novel protocol. The protocol comprised two key components: a dynamic definition of the group centred around the tagged individual, and a set of behavioural parameters quantifying visually observable characteristics of the group. Our results revealed that the diving behaviour of tagged individuals was associated with distinct group-level behaviour at the water's surface. During foraging, groups broke up into smaller and more widely spaced units with a higher degree of milling behaviour. These data formed the basis for a classification model, using random forest decision trees, which accurately distinguished between bouts of shallow diving and bouts of deep foraging dives based on group behaviour observed at the surface. The results also indicated that members of a group to a large degree synchronised the timing of their foraging periods. This was confirmed by pairs of tagged individuals that nearly always synchronized their diving bouts. Hence, our study illustrates that integration of individual-level and group-level observations can shed new light on the social context of the individual foraging behaviour of animals living in groups.