How did the wolf cross the ocean?
It was a mystery of nature even before Charles Darwin reached the Falklands Islands in the South Atlantic: How did the Falklands Islands wolf, the only resident mammal, reach the islands, about 460 kilometers off the Argentine shore?
The great naturalist and writer wisely wondered what was weird with the wolf:
“As far as I am aware, 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,” Charles Darwin, 1834.
After Darwin left, whalers, sealers and farmers hunted and poisoned the Falklands wolf to extinction.
Nobody thought the wolf, AKA Dusicyon australis, had crossed the strait. Did it cross on a raft made of trees, walk on the winter ice, or catch a ferry ride from obliging people now lost to history?
The question of colonization of remote islands recurs often in natural history: The journeys of plants, parasites, people and other mammals beg for explanation, especially on remote islands like the Falklands.
Now, a team from the University of Adelaide in Australia has combined forensic genetics with paleogeography to come up with an answer, based on shifting sea levels and the wolf’s relationships to relatives on the mainland.
Welcome the newbie!
The study began by analyzing genetic sequences to identify when the Falklands wolf split off from its last common ancestor, a now-extinct mainland fox called Dusicyon avus. Logically, the wolf made its crossing after it diverged from its ancestor.
Instead of dating the fork in the road with standard rates of mutation, the researchers carbon-dated the ancestor’s remains. After counting the genetic differences between the wolf and the fox, they arrived at a rate of molecular changes per year for those animals, at that time, says senior author Alan Cooper of the Australian Centre for Ancient DNA. “We think this is a much better approach than standard mutation rates. This is quite an important difference from other studies.”
Previously, scientists concluded that the wolf had reached the Falklands about 330,000 years ago, but the new analysis — focusing on a different last common ancestor — pointed toward a much more recent arrival, about 16,000 years ago.
That was during the height of the Wisconsin ice age, when ice sheets, especially on Antarctica, locked up ocean water, causing sea level to fall. That exposed “land bridges” linking islands and continents.
As the ocean surface declined about 130 meters, the strait separating the Falklands (known in Argentina as the Malvinas Islands) narrowed to a couple of dozen kilometers. Presumably this water froze during some winters, and the wolf either roamed across or rafted on icebergs.
It’s possible that other mammals reached the Falklands but left no traces — or that the wolf was the only mammal to cross the strait. “The wolves were after marine resources, like seals, penguins and seabirds, on the edge of the ice,” says Cooper. “Other mammals such as rodents, which are common in South America, were not interested in crossing 20 to 30 kilometers of ice, as there is no food or habitat.”
I go, you stay
The study brings into focus the role of climate and sea level in colonizing remote islands. “Strangely enough,” says Cooper, the role of sea level change “was the key observation that led Alfred Russel Wallace to discover evolution independently from Darwin.”
In 1858, Wallace, a naturalist, mailed his theory of natural selection to Darwin, who recognized it as strikingly similar to his own, unpublished theory. Darwin helped arrange a simultaneous reading of the theories at a scientific meeting in 1859.
Wallace, who was still collecting specimens in the southwest Pacific, noted that similar animals lived on islands that were separated by shallow seas. But when he looked at organisms on opposite sides of what’s now called the “Wallace line,” he saw that animals like those in Australia lived to the east. The animals living to the west were related to those living in Asia.
And dividing the two regions is the Lombok Strait, which was deep enough to remain open during glacial times, and too warm to freeze over, even during the Ice Ages. Although Darwin is rightly credited as the originator of evolution through natural selection, “he didn’t really seem to grasp the role of sea level change to the same degree,” says Cooper.
But let’s give Darwin the last word: The Falklands wolf DNA used in the study came from a specimen Darwin collected in 1834.
— David J. Tenenbaum
- The origins of the enigmatic Falklands Islands wolf, Jeremy Austin et al, Nature Communications, 5 May 2013. ↩
- Looking for lost letters between Darwin and Wallace ↩
- The Darwin Correspondance Project ↩
- Learn about the HMS Beagle, Darwin’s adventure ship! ↩
- Wallace & Darwin: Friends or Foes? ↩
- A naturalist’s notes: Wallace’s original journal ↩