One of the oddest sights on Charles Darwin's famous voyage of the Beagle must have been the Falkland Islands wolf, a remarkably tame animal the size of a Labrador retriever that roamed these specks of land 460 kilometers off the coast of Argentina. Differences in the size and color of wolves on different islands—and with wolves on the mainland—sparked Darwin's thinking about the mutability of species. He was also perplexed by how these curious creatures got to the islands in the first place when no other mammals could be found there. "As far as I am aware," he wrote in 1839, "there is no other instance in any part of the world, of so small a mass of broken land, distant from a continent, possessing so large a quadruped peculiar to itself."
Now, researchers may finally have solved the mystery. By comparing ancient mitochondrial DNA (maternally inherited DNA from the powerhouses of the cell) from several species of extinct and living canids—the family that includes wolves, foxes, and dogs—an international team has found that the animals are most closely related to an extinct wolf from South America, and that the two split apart about 16,000 years ago, just after the end of the Last Glacial Maximum. This was a time when sea levels were dramatically lower and the wolves could have crossed the icy straits on foot, unknowingly taking the first steps toward becoming a new species, according to a report today in Nature Communications. "There was almost certainly a shallow and frozen strait between the islands and the mainland, allowing the Falklands wolf to cross when the sea was frozen over, probably while pursuing prey, like penguins or seals," says molecular evolutionist Alan Cooper of the University of Adelaide in Australia, leader of the study.
Although humans drove the Falklands wolves to extinction soon after Darwin's visit (an outcome he predicted, after seeing how unused to humans the animals were), the crew of the Beagle brought several specimens to England; the skeleton of one is still stored at the Natural History Museum in London, with Darwin's handwriting on the label. A protective museum curator would not let Cooper drill a hole in the specimen's tooth to remove DNA, but Cooper spotted a tiny sinus hole, about 1 millimeter wide in the skull. Using tiny tweezers, he extracted a "little chunk of dried nerve and blood vessels," he says. An analysis of that DNA, as well as the mtDNA from five of the seven other known specimens of Falklands wolves, confirmed that it was a new species of canid, dubbed Dusicyon australis—and not a fox or a dog brought to the islands by humans, as some had thought.
But when did the wolves reach the Falklands? To find out, the researchers needed DNA from a close living relative for comparison. But only one species of the large South American canid still survives -- the maned wolf, Chrysocyon brachyurus, a strange animal that looks like a large red fox on stilts that eats rodents, birds, and fish. But it turned out to be a distant relative whose ancestors diverged from the Falklands wolf 7 million years ago. To get a precise estimate, Cooper's team collaborated with paleontologists in Argentina who collected teeth from six individuals of a similar looking ancient wolf, D. avus, which lived in Argentina and Chile until it went extinct about 3000 years ago (probably due to humans). By comparing the mtDNA of these and a half dozen South American fox species, the researchers confirmed that the Falklands wolves were closely related to D. avus: The two species had diverged from a single founder population just 16,000 years ago.
That date serves as a marker for the origins of the Falklands wolves, Cooper says. At that time, low sea levels had exposed large areas of the gentle slope of the east coast of South America. Indeed, submarine terraces suggest that the Falkland Islands may have been separated by a strait of water just 20 to 30 kilometers wide, which may have frozen over periodically. Some of the wolves could have ventured out on the ice, searching for food like modern Arctic foxes that cross the sea ice today. But other mammals, such as rodents or other land mammals, would have been deterred by the open expanse, making the land bridge an icy filter for large animals that might make it to the Falklands. When the glaciers melted and the sea level rose, some of the wolves were trapped in the islands, where they survived by hunting penguins, geese, and scavenging the seashore for grubs, nests, and marine life. As time passed, they evolved separately. By the time British explorers arrived in the 17th century, they had never seen humans and were so tame that they were particularly vulnerable to clubbing and rapid extinction, as Darwin predicted sadly.
Such colonization over ice "seems a very plausible idea," says Graham Slater of the Smithsonian Institution's National Museum of Natural History in Washington, D.C., who was not involved in the new work. The new report is important, he says, because it helps researchers pinpoint when the wolf reached the islands as well as revealing how a new species arose. That's significant because it was the distinct differences between the wolves on the different Falkland Islands that suggested first to Darwin that species are not fixed entities but mutate and evolve into new species as they separate and adapt to different environments. He noted that the wolves on West Falkland were smaller, redder, and darker, with finer fur than those on East Falkland.
After he had seen the various adaptations of finches on different islands in the Galapagos, the famed naturalist wrote that the differences in the wolves on different islands, similar to the various shapes in bird beaks on different islands in the Galapagos, "undermine the stability of Species." But he saw the Falklands wolf first on the voyage of the Beagle and it clearly "fostered his thinking," Cooper says. "It's just great to be able to tie up the loose ends that perplexed Darwin—this wolf has been one of natural history's conundrums."