Seals hear predators despite undersea noise

Researchers say they have found the first evidence that harbor seals can learn to tell the difference between the calls of roving, seal-hunting killer whales and their safer fish-eating killer whale cousins.

Distinguishing between the dialects of the complex vocalizations is difficult, but allows the seals to save time and energy by feeding shoulder-to-shoulder on the same salmon runs with the local whales for weeks at a time, they said.

To understand how the seals learned, researchers at the Vancouver Aquarium Marine Science Center used underwater equipment to play the calls of various killer whales near reefs off southern British Columbia where the seals emerge to rest.

In one experiment, the researchers played the calls of seal-eating transient killer whales or background noise. When seal-eating whale calls were played, the researchers found a decrease in the number of seals at the surface, where they are more likely to be caught. More seals were identified fleeing the source of the sound.

In a second experiment, the researchers played tapes of the calls of three different killer whale groups: those of seal-eating transients, fish-eating locals and a separate pod of fish-eaters that live off Alaska and whose calls were unfamiliar to the seals.

The responses to the seal-eating and Alaskan fish-eating killer whales were virtually the same. Researchers said the results suggest the seals use a cautious strategy in which they learn over time to ignore certain calls from fish-eating whales, but initially treat all unfamiliar whale vocalizations as life-threatening.

The idea that animals learn to distinguish between closely related friends and foes through auditory cues has been demonstrated with birds. However, the seal study is the first time the behavior has been shown in mammals, said researcher Volker Deecke.

"Seals are capable of quite complex learning tasks," Deecke said, noting fish-eating killer whales use a variety of calls that branch into different dialects. "Even in spite of all these variations, seals seem to make sense of it."