When hunting down their prey in ruthless fashion, orca let out a “human-like” scream, and it turns out that the dolphins they hunt know that this means they could be next. By playing wild dolphins the death cry of the killer whales, researchers were able to observe as the cetaceans swam for their lives.
There are a handful of different subpopulations of orca cruising the oceans. One of the main defining factors is the type of prey that they feed on, as some only eat fish, some only marine mammals, and a few that aren’t picky either way. So delineated are these differences that scientists argue that they should be separate species in their own right.
Some groups of killer whales include up to 32 different species of cetacean in their diet. Considering orcas themselves are simply massive dolphins, it raises the question of whether or not the smaller dolphins and porpoises that fall victim to the predators can understand them, and whether or not this could give the prey an advanced warning of potential attacks.
In order to test this, a team of marine biologists sailed 40 miles off the coast of North Carolina to see how both pilot whales and Risso’s dolphins react to the calls of killer whales, publishing their results in the Journal of Experimental Biology.
First of all, they tagged members of the pods with a data-logger. This recorded a whole host of information, including the sounds heard by the animal, as well as other useful data such as their depth and movement. Then they played a range of sounds that the cetaceans were likely to experience in their daily lives, including the calls from other members of their species, humpback whales, and a range of killer whale vocalizations.
For the vast majority of the calls played to the animals, they remained calm and carried on doing what they were doing, even when some of those calls were from orcas. But when the team then played the killer whale hunting call, there was an immediate and sudden shift in behavior.
“It was crazy to see a group of animals respond so strongly to something you're doing,” said lead author Matthew Bowers. As soon as the Risso's dolphins heard the noise, they effectively stampeded in the opposite direction. “The strong and differential responses to this subset of killer whale calls was eye-opening.”
By using the data collected from the trackers, they could recreate how the two different species of cetaceans responded to the calls, demonstrating that they had strikingly different tactics. While the Risso’s dolphins turned tail and fled as fast as they could for over 10 kilometers, the pilot whales doubled down. They formed a tight group and dived down towards to sound, trying to confront the whale they thought was making it.