Prey preferences may be driving a process still in its early stages
Killer whales (Orcinus orca) can be surprisingly finicky eaters. In the North Pacific and Antarctic, some feed only on fish; others, only on mammals—dietary preferences that seem to have led to new species of orcas. Some researchers think that a similar process is occurring in the killer whale populations of the Northeast Atlantic. But speciation there may be a long time in coming. A new paper examining these orcas’ diets over the last 10,000 years reveals that most are not as picky as their relatives; those eating herring today may be feasting on baby seals tomorrow. The study shows that the Northeast Atlantic whales may only be at the beginning of the speciation process.
Evolutionary biologists have long argued about whether it’s possible for a new species to arise in a population that isn’t separated by geographic barriers, such as an ocean or a mountain range—a process called sympatric speciation. “Killer whales have been thought of by some as something like the poster child” for the process, “because there are multiple genetically distinct populations [which have not yet been formally described as separate species] with different prey preferences in the North Pacific and Antarctic,” says Phillip Morin, a cetacean biologist at the Southwest Fisheries Science Center in San Diego, California, who was not involved in the new study. Scientists have suggested that the orcas separated into distinct species because of what they chose to eat. In this scenario, fish-eaters would mate only with other fish-eaters, and mammal-eaters only with other mammal-eaters. Given enough time, the two populations would become genetically distinct and unable to reproduce.
Some killer whale observers have proposed that the orcas in the Northeast Atlantic also likely comprise two species, because some pods appear to be fish specialists, while others prefer marine mammals. They point out that the orcas’ hunting tactics for the two types of prey differ dramatically and are learned behaviors—cultural differences that may also help drive populations apart.
“When hunting herring, the whales travel in large groups and vocalize a lot,” says Andrew Foote, an evolutionary biologist at the University of Copenhagen, and lead author of the new study. “But they travel in small groups, three to five animals, and hunt in complete silence when going after seals,” because of the seals’ acute hearing.
To find out if differences in diet and culture have also led to two species of killer whales in the Northeast Atlantic, Foote and his colleagues studied the dietary choices and genetic relationships of orcas from Greenland to Norway. They compared 20 tissue samples taken from modern (1865 to 1995) whale bones and teeth with 23 more ancient samples, dating from 2800 to 6800 years ago, that were collected from archaeological sites and dredging operations. The scientists analyzed the samples’ isotopic ratios—a telltale chemical signature of what the animals ate during their lifetimes. They also extracted DNA from the specimens, and from skin samples of living orcas to reconstruct the whales’ genetic lineages. And they examined the wear-patterns on the animals’ teeth; killer whales that feed heavily on herring have badly worn teeth.
The scientists’ analysis revealed a mixed picture. Although two of the orca populations eat only fish, there was no genetic evidence that they have diverged from those that dine on seals, the team reports online this week in the Proceedings of the Royal Society B.
“It might be that the whales are at the beginning of what is a very slow process, since all the ecological ingredients are there for a speciation event,” Foote says. He points out that only 10,000 years ago these waters were under ice, and no orcas were to be found, so it may not be a surprise that most of the orcas haven’t differentiated as they have in the North Pacific and Antarctic, where they’ve lived far longer.
Some of the study populations that previously were considered fish-only diners also feast on mammals, the team’s research shows. For instance, the scientists collected samples from a family pod of six orcas that aboriginal hunters in East Greenland had killed. “Each whale had a seal pup in its stomach,” Foote says, “yet their teeth were worn like those of the herring-hunting whales” seen off Iceland. In fact, the researchers’ subsequent analysis showed that these orcas belonged to the population of animals previously considered strictly fish-eaters. Tilikum, SeaWorld’s notorious orca that has killed three people, including one of his trainers, was captured as a youngster in these same waters, Foote notes; while he is fed only fish at SeaWorld, in the wild, he may have eaten marine mammals, too.
“Their data indicate that prey preferences are less fixed in North Atlantic killer whales than in those in the North Pacific and Antarctic,” Morin says. So, in these waters “prey preferences alone don’t appear to be sufficient to cause genetic divergence,” leading to distinct species. The populations may need to be physically separated, too. Still, that may be happening, Foote says. “If the strict fish-eaters stick to their diet, and the groups hunting fish and seals become specialized on mammals, in time, they’ll have fewer encounters with each other, and that could lead to speciation.” And if that’s the case, then Foote and his team have documented one of the rarest of biological events: a population at the very beginning of becoming two species.