Some types of orca calls—having similar characteristics to the screams of humans—spur dolphins and whales to run for cover, while other types of orca calls are less threatening and do not cause cetaceans to flee.
Regarded as one of the ocean's most formidable predators, killer whales are pack hunters, with some orcas hunting other marine mammals while others prefer to eat only fish. In a new study published in the Journal of Experimental Biology, Matthew Bowers from Duke University and colleagues speculated whether aquatic mammals that are potential killer whale prey could distinguish calls of the predatory killers from those of other marine mammals.
For the study, the team investigated how pilot whales and Risso's dolphins reacted to the calls of killer whales. They discovered a subset of orca calls, with similar characteristics to human screams, which triggered the flight of whales and dolphins, while less-threatening calls prompted no evasive action.
Monitoring pods of pilot whales off North Carolina and Risso's dolphins off California’s Catalina Island, Bowers and his colleagues played recordings of killer whales and social calls from pilot whales, Risso's dolphins and humpback whales to the animals while observing their reactions. One individual from each group was tagged with a data logger that recorded the sounds heard by the animals, in addition to their depth and movements.
When most of the sounds, including many of the killer whale calls, were played, the pilot whales and dolphins remained calm. When four specific killer whale calls were broadcast, their reaction was entirely different. "It was crazy to see a group of animals respond so strongly to something you're doing," said Bowers, describing the response of the Risso's dolphins as a stampede. “The strong and differential responses to this subset of killer whale calls was eye-opening,” he added.
Back in the lab, Bowers and Nicola Quick reconstructed the dolphin and pilot whales' movements and noticed the two species' reactions were completely different. While the pilot whales came together in a tight group that dived down toward the alarming sound, the dolphins flocked together and dashed off in the opposite direction at high speed for more than 10 km.
Correlating the animals' movements with the sounds they heard, the team discovered unique features in the distressing killer whale recordings that did not occur in calls by members of their own species, the humpback whale calls or the killer whale calls that had not incited panic. The distressing calls featured many sound structures that are prevalent in mammalian distress cries, including the wails of humans.
“The signal starts to jump around in an unpredictable fashion,” said Bowers. He explained that the features are disconcerting because our brains cannot weed out the erratic sounds and ignore them. “We suggest that these calls convey information about the predators' behaviour or intent," he said. The calls could warn potential victims of "the killer in their midst”.