Seals discern foes' from neighbor-whales' calls. (Ear for Killers).

Killer whales that eat fish chatter in dialects with up to 17 kinds of calls. Researchers now say that harbor seals eavesdrop on the whales and can tell the harmless neighborhood fish eaters from roving gangs with a taste for fresh seal.

The recent experiments also suggest how the seals' predator alarm develops, says Volker B. Deecke of the Vancouver Aquarium Marine Science Centre in British Columbia. The seals start with an aversion to all killer whales but learn to ignore the local fish eaters, Deecke and his colleagues contend in the Nov. 14 Nature.

Killer whales can live a variety of life styles. Those that cruise the western coast of North America typically either stay home eating fish or roam the coast preying on seals and other mammals. Most fish can't hear the high whale-call frequencies, and fish eaters make a lot of noise. Seals and other mammals can tune in, and mammal-killing whales typically hunt silently and call after they've made a kill.

To investigate the eavesdropping seals, Deecke and his colleagues made recordings of calls from whales off the coasts of British Columbia and Alaska. The mammal eaters' wails feature "long downward slides" that strike Deecke as "melancholy" and "very haunting" He says the fish eaters sound "more upbeat."

During the experiment, Deecke spent days playing the recordings underwater to seal congregations in British Columbia. Local fish-eater chatter made fewer than 5 percent of seals swimming at the surface dive for safety. In contrast, the haunting sounds of the visiting mammal eaters drove away some 40 percent of the seals.

Deecke asked whether the seals had learned to fear the roving killers or were innately afraid of killer whales but had learned to ignore the harmless locals. So, he played recordings offish eaters from Alaska, which chatted in dialects that Deecke's seals had never heard before. Those sounds caused as much alarm as the wails of the roving seal killers.

Deecke says that he doesn't know of another animal that distinguishes between dialects of a different species. However, for defense, it's probably advantageous to start with a generalized fear and then recognize the dialect of a benign neighbor. Learning too slowly who's an enemy can be fatal, whereas learning too slowly who's harmless just wastes effort, Deecke points out.

Ecologist James Estes of the U.S. Geological Survey in Santa Cruz, Calif., welcomes the work as part of a growing body of research on how intricate predator-prey relationships can be (SN: 10/17/98, p. 245). He says that, like Deecke, he's seen seals ignore some killer whales. Estes calls the new paper's explanation for the phenomenon "elegant in its simplicity."