Teeth worn down to the gums!

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A scientist from Canada has discovered a why a type of killer whale has such bad teeth – a love of shark meat!

The discovery of the whales with teeth worn down to their gums, lends weight to the notion that the region's three orca Orcinus orca lineages are separate species, which has implications for both future studies and conservation strategies.

The three types of orca in the north eastern Pacific off Canada and Alaska can be divided into three distinct groups; the "resident" fish-eaters; "transient" marine mammal-eaters; and "offshore" whales.

Although the latter group were first discovered in the late 1980s, until now very little has been known about what they eat as they feed hundreds of feet below the surface.

John Ford, a biologist at the University of British Columbia, is quoted in the Canadian paper The Star:

"A few years ago, a colleague found a group of offshore whales on the beach in Alaska. The teeth were worn flat to the gums. We thought it was odd. Then a couple of years later, a colleague found a whale floating off the coast of Tofino and towed it in. We did a necropsy – an animal autopsy – on it. This one also had teeth worn flat, right to the gums.

"We started to speculate perhaps it’s sharks that they’re eating, and sharks have very rough skin – when it dries out, it’s like coarse sandpaper."

Later, while Ford and colleagues were observing the whales, they noticed them engaging in feeding behaviour. Immediately following this they discovered chunks of pink meat on the water’s surface which upon genetic analysis turned out to be the meat of at least 16 different Pacific sleeper sharks (Somniosus pacificus).

This discovery, together with the fact that the different lineages have different genetics, do not mix, socialise, or mate together and have learned different hunting strategies (transients travel in small packs and are generally quiet, to help them sneak up on their prey, whereas residents are far more vocal) as well as the fact that they are thought to have different 'languages', means that some scientists are calling for a reclassification into three separate species.

Robin Baird, an orca researcher at the Cascadia Research Collective in Olympia, Washington reiterates: "The fact that these populations have such different diets means they can't just swap places".

Ford points out that a southern population of fish-eating resident orcas is considered endangered, 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.

For further information see: Ford, J. et al. Aquatic Biol. 11, 213-224 (2011)