When faced with a killer whale should you fight or flee?

THAT the best form of defence is attack is an old maxim. In reality, it is frequently untrue; running away is a far better option. But it seems to be the approach taken by pilot whales when faced with a pod of killer whales which are looking for dinner.

That, at least, is the conclusion of Matthew Bowers of Duke University in North Carolina. He came to it as the result of a study, just published in the Journal of Experimental Biology, which he and his team conducted on pilot whales (pictured) and Risso’s dolphins—two closely related species of small cetacean. He knew that killer whales, which are partial to snacking on both of these species, chat with one another during the normal course of events, even if they tend to stay quiet when making an attack. He therefore speculated that potential prey would react to distant killer-whale communications, it being risky to do nothing. What he did not know was what the reaction would be.

To find out, he and his colleagues gathered a library of cetacean calls. These included those of killer whales, pilot whales, Risso’s dolphins and humpback whales. They then played these calls to pods of their target animals, in order to find out what happened.

In a series of experiments—ten on pilot whales and four on Risso’s dolphins—the researchers tagged one member of the pod under observation, in order to gather detailed information about what was happening to it. They also employed a team of experienced marine biologists to make observations of the entire pod from boats.

The tags were fitted with several instruments: pressure sensors to measure depth; magnetometers to record orientation with respect to Earth’s magnetic field; accelerometers to measure movement; and microphones to record chatter. They were attached to the animals’ bodies by specially designed suckers and detached themselves four hours after attachment, to float to the surface for recovery.

The team employed two small vessels to tag the target animals and to observe as far as possible from the surface the positions and behaviours of pod members. One or other of these boats played the recorded calls, in random order, modified to sound as if the creatures making them were about a kilometre away. The observers were not told which calls were being broadcast, in order not to bias their observations. Once the detached tags were recovered, the data therein were correlated with the biologists’ records of events.

As might be expected, neither pilot whales nor dolphins reacted much to the calls of conspecifics, of the other prey species or of humpbacks. Both, though, reacted rapidly to killer-whale calls.

The dolphins did so by forming into a tight cluster and then bolting at top speed away from the observation vessel that was playing the calls. The pilot whales also formed a tight cluster on hearing the killer-whale calls. But in contrast to the dolphins they increased their chatter, turned in the direction of the boat broadcasting the calls and moved at a steady, almost threatening, pace directly towards it.

Since no actual killer whales were involved in this experiment, Dr Bowers cannot say what would have happened in any subsequent confrontation. But he suspects that it would have led to a form of attack called mobbing. This is a tactic employed by terrestrial and aerial prey species but not widely recorded under water. As the name suggests, it involves members of a group of potential prey attacking a predator as a mob, confusing it and threatening it with injury. This generally causes it to retreat, permitting the potential prey to get on with their lives in peace.

Why pilot whales and Risso’s dolphins behave differently in response to the threat from killer whales is unclear. Both live in groups of several dozen animals and so have the numbers needed for collective defence. It may be because pilot whales are bigger than Risso’s dolphins. But the dolphins are not so small as to be a negligible threat to a killer whale. As to seeing a pod of pilot whales take on killer whales, that would surely be the money shot of any wildlife documentary that managed to get the footage.