By Nicola Jones

Killer whales are notoriously picky eaters. Now one type of killer whale, or orca, has been found to dine on an unusual dish: shark. But these 'offshore' killer whales of the northeastern Pacific pay a high price for their tough-skinned preference -- their teeth become worn right down to the gums.

The documenting of their unusual diet adds weight to the notion that the region's three orca lineages are separate species, which has implications for both future studies and conservation strategies.

Killer whales (Orcinus orca) have been seen hunting sharks on a one-off basis before (see video). "They can take pretty much whatever they want," says John Ford of the Pacific Biological Station in Nanaimo, British Columbia, whose team reports the new evidence in Aquatic Biology. But it is unusual for these top predators to specialize in this fare. One other orca population in New Zealand is thought to prefer shark meat, although researchers don't have solid evidence.

The three types of orca in the northeastern Pacific Ocean off North America's west coast are the most-studied orca populations in the world. Two have been known since the 1970s: the so-called 'resident' whales, which eat fish, and 'transients', which prefer to dine on marine mammals such as sea lions. Offshore killer whales were first identified in the late 1980s, but what they prefer to eat has been a mystery up to now.

The different killer-whale lineages are also known to have different genetics, do not mate with each other, and have learned different hunting strategies. Transients, for example, travel in small packs and are generally quiet, to help them sneak up on their prey, whereas residents are far more vocal.

Separate tables

Ford's work adds to a growing rallying cry for the different killer-whale lineages to be considered separate species, which could become a key factor in future conservation decisions. "The fact that these populations have such different diets means they can't just swap places," says Robin Baird, an orca researcher at the Cascadia Research Collective in Olympia, Washington, who was not involved in Ford's study.

A southern population of fish-eating resident orcas is considered endangered, says Ford, and other populations are locally classified as threatened, including the offshore orcas living between California and Alaska, estimated to be fewer than 500 individuals. "These are small populations that are very vulnerable to changes in their food," he says.

Spotting the offshore orcas' fearsome feast took some doing. Ford's colleagues and contacts were able to observe the whales on 98 occasions from 1988 to 2009. But as they feed at depths of hundreds of metres, their actual dining habits were difficult to discover. The researchers only caught them hunting on two occasions: the fact that they were feeding was given away by their quick turns, long, deep dives, and the blobs of pink meat that floated to the surface. Genetic testing of these blobs confirmed the remains of at least 16 Pacific sleeper sharks (Somniosus pacificus).

Although choosing shark as a main course might seem dangerous, Baird points out that transient killer whales haven't necessarily chosen an easier prey. "Steller sea lion skulls are similar to those of grizzly bears," he says. "Attacking one of them is probably more dangerous than attacking a 2-metre-long sleeper shark.""This is the first good documentation of what they're feeding on," says Baird. The other giveaway of their abrasive choice of food is the orcas' dental wear and tear. From examination of dead stranded orcas and museum specimens, Ford found that the average offshore orca has worn its teeth right down, exposing their pulpy centres.

The effect on the whales' teeth could be more problematic. "It may be that the young whales have to do most of the work," says Ford. "The older ones are probably just gumming away at the liver." Or it could be that older animals switch foods, says Baird -- just as some humans switch to softer fare in later life.