(CN) – A subset of killer whales that dine on marine mammals telegraph their presence by emitting alarming and identifiable calls, a new study finds.
In the report, published Tuesday in the Journal of Experimental Biology, a team of researchers examined how aquatic mammals that live alongside killer whales distinguish between the calls of these predators and those of other marine mammals.
While killer whales are known for hunting stealthily in packs, the study suggests some of these ferocious aquatic mammals may be less sneaky than previously believed.
To investigate whether marine mammals react differently to the calls of killer whales, the team sailed in inflatable boats 40 miles off the coast of North Carolina to observe pods of pilot whales and to Catalina Island – off the coast of California – to monitor small groups of Risso’s dolphins by tagging one member from each group with a data logger, which recorded the sounds heard by the animals. The devices also recorded their movements and depth.
The researchers then played recordings of killer whales’ cries and social calls from humpback whales, pilot whales and Risso’s dolphins.
They then captured the mammals’ reactions by tagging a member from each monitored group with a data logger that recorded their movements and depths and the sounds they heard. Co-author Danielle Waples, a research analyst at Duke University, observed the mammals’ movements from a second inflatable boat.
“Each playback experiment was an all-day endeavor,” said author Matthew Bowers, a research scientist at Colorado State University.
Bowers said the pilot whales and dolphins seemed to remain calm when most of the sounds – including many of the killer whale calls – were played.
However, he was surprised by the mammals’ reactions to four specific killer whale calls, which led some of the dolphins to “stampede” away.
“It was crazy to see a group of animals respond so strongly to something you’re doing,” Bowers said. “The strong and differential responses to this subset of killer whale calls was eye-opening.”
Bowers and co-author Nicola Quick, a research scientist at Duke University, then used their recording and observations to estimate how much energy the mammals were using in order to establish a sense of their urgency. After reconstructing the mammals’ movements, they realized the species reacted completely differently.
While the pilot whales assembled closely and dove down toward the distressing sounds, the dolphins grouped together and rapidly fled more than six miles in the opposite direction.
The team also matched the mammals’ movements with the sounds that they had heard and identified unique features in the alarming killer whale recordings that were not found in the cries of other members of their species. The features also did not occur in humpback whale calls or killer whale cries that did not elicit panic.
The troubling calls feature many sound structures that are found in other mammalian distress cries, such as human wails.
“The signal starts to jump around in an unpredictable fashion,” said Bowers, who added that the features are upsetting because our brains can’t filter out and ignore the erratic sounds. “We suggest that these calls convey information about the predators’ behavior or intent,” which could forewarn potential victims.
The research was funded by the Department of Defense’s Strategic Environmental Research and Development Program, the Naval Facilities Engineering Command Atlantic, and the National Oceanic and Atmospheric Administration Fisheries Southeast Region.