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Odd-Looking Orcas May Be a Distinct Species
- 6:30 AM
A strange-looking and mysterious killer whale living in the heaving seas ringing Antarctica might be a distinct species. Known as Type D orcas, the whales are so seldom seen that scientists relied on a 60-year-old museum specimen to unravel their ancestral story.The tale places the black-and-white toothed whales among the most genetically distinct orcas on the planet. Roughly 400,000 years ago, it concludes, Type D orcas diverged from the rest of the lineage. Their closest relatives are the transient, mammal-eating orcas of the north Pacific. Together, the two groups form a long branch in the evolutionary tree of killer whales and suggest that with more sequencing, more species will come.“This is a great study,” said biologist Robin Baird, of the Cascadia Research Collective, about the work, recently published in Polar Biology. “None of this would have been possible without that museum collection.”Type D orcas live in a frigid band of choppy water that rings Antarctica, known as the sub-Antarctic waters; here, the southern latitudes have earned ominous nicknames like the Roaring Forties and the Furious Fifties.“They just live in such bad water most of the time that people haven’t been out looking for them,” said study coauthor Robert Pitman, a marine ecologist with NOAA’s Southwest Fisheries Science Center. “We really don’t know very much about this animal at all.”The first record of Type D orcas dates back to 1955, when 17 of the odd-looking whales stranded on Paraparaumu Beach on New Zealand. Blunt, bulbous heads, tiny white eye spots, and delicate, curved dorsal fins made the orcas look unlike any others that had been observed. Scientists collected a skeleton and brought it to the Museum of New Zealand Te Papa Tongarewa in Wellington, where it’s been for almost 60 years.For half a century, scientists didn’t know whether the stranded orcas were an example of strange mutations within a single family group, or a distinct type of killer whale. Then, in 2004, the odd-looking whales were seen again at sea. Six sets of photographs from different southern hemisphere locations depicted strange-looking whales that looked like the ones that had stranded in New Zealand.“We realized that this animal is alive, and that’s it’s distinct, and that it has a fairly wide distribution,” Pitman said.After studying the evidence, Pitman and his colleagues began referring to the whales as Type D, or sub-Antarctic, orcas. Since 2010, more sightings at sea – mostly by tour boats heading toward Antarctica – have notched the total sightings to somewhere near a dozen. Scientists don’t know how big the population is or what it eats, but they suspect that Type Ds are fish-eating orcas, owing to an observation of them picking Chilean sea bass (“or Patagonian toothfish, whatever you want to call it,” Pitman says) off fishermen’s longlines.But finding out how the sub-Antarctic whales were related to other orcas presented a different challenge. There are no tissue samples from living animals – the whales have been too hard to find. Even Pitman, who’s studied Antarctic orcas for years, has never seen one.So, scientists went back to the New Zealand museum. There, they extracted DNA from bone and soft tissue clinging to the skeleton from the 1955 stranding. They ground up the samples, releasing decades-old genetic material. From that pool emerged the whale’s mitochondrial DNA, small rings of no more than 17,000 base-pairs that live within the energy-producing organelles in cells. Unlike nuclear DNA, which is inherited from both parents, mitochondrial DNA is passed down through the maternal lineage; there is little to no recombination, and the sequence only changes when mutations occur.Scientists can use this kind of genetic sequence to construct evolutionary relationships between organisms. In this case, geneticist Phillip Morin at NOAA’s Southwest Fisheries Science Center compared the mitochondrial sequence of the museum whale to a reference sequence from Southern Ocean killer whales.The Type D sequence wasn’t like the others. “It was extremely different,” Morin said. “I was actually surprised.”When Morin looked to see which killer whales the Type Ds were most closely related to, he found that they shared a most recent ancestor with the mammal-hunting North Pacific transients. Based on the number of mutations that had built up in their mitochondrial DNA, Morin estimates that the two whales diverged from their last common ancestor around 400,000 years ago.“That’s recent in evolutionary terms,” Morin said. “But it’s certainly long enough for divergence to occur, just through random mutation in the genome, and for selection to actually result in adapted differences.”Scientists aren’t sure whether the orcas might be a distinct species, or a subspecies. Now, the team is hoping to collect samples from living animals and construct a more complete genomic picture using sequences from the DNA inside the nucleus of their cells. “Clearly they’re on a divergent path,” Pitman said. “It’s just a matter of how far down that path they’ve gone.”Rus Hoelzel, a molecular ecologist at Durham University, in the United Kingdom, suggests that the 400,000-year branch point could be much more recent if the mitochondrial mutation rate the authors used to rewind the molecular clock is off. He’d also like to see more genetic sequences – as would the authors. “As the authors rightly point out, any discussion of new species would need to take into account variation among type D genotypes, and data from nuclear DNA,” Hoelzel said.What is clear is that Type Ds probably aren’t the only orca species swimming around – scientists estimate there could be as many as six or seven different kinds of killer whales in the world’s oceans with distinct morphology and behavior.Like humans, killer whales engage in complex social interactions. They share food preferences, learn hunting strategies from their parents, and pass on cultural variations in their vocalizations. Possessing some of the most creative hunting strategies of any whale, orcas quickly specialize in capturing different types of prey: Some prefer seals and other mammals, while others go for fish.Baird speculates that, rather than geographic distributions or barriers, these behavioral distinctions are helping drive speciation in orcas. “It’s because of these foraging specializations, the social organization and social structure of the animals — the importance of learning, the importance of teaching, the importance of cooperative hunting,” Baird said.As for Pitman, he’d like to organize an expedition and sail around the world at sub-Antarctic latitudes, looking for Type Ds. That sounds insane. But if he gets it together, we’d love to tag along.