Supplementary material from "Deep-diving beaked whales dive together but forage apart"

Echolocating animals that forage in social groups can potentially benefit from eavesdropping on other group members, cooperative foraging or social defence, but may also face problems of acoustic interference and intra-group competition for prey. Here, we investigated these potential trade-offs of sociality for extreme deep-diving Blainville′s and Cuvier's beaked whales. These species perform highly synchronous group dives as a presumed predator-avoidance behaviour, but the benefits and costs of this on foraging have not been investigated. We showed that group members could hear their companions for a median of at least 91% of the vocal foraging phase of their dives. This enabled whales to coordinate their mean travel direction despite differing individual headings as they pursued prey on a minute-by-minute basis. While beaked whales coordinated their echolocation-based foraging periods tightly, individual click and buzz rates were both independent of the number of whales in the group. Thus, their foraging performance was not affected by intra-group competition or interference from group members, and they did not seem to capitalize directly on eavesdropping on the echoes produced by the echolocation clicks of their companions. We concluded that the close diving and vocal synchronization of beaked whale groups that quantitatively reduces predation risk has little impact on foraging performance.