Dire wolves menaced Pleistocene landscapes for tens of thousands of years, eventually going extinct at the end of the last ice age. Despite the long success of this species, very little is known about them, including their origins and reasons for their demise. New research is helping to fill these gaps.
Portrayals of dire wolves in fantasy films and TV are exaggerated versions of the real thing, but that’s not to say these extinct creatures weren’t formidable predators. These animals were slightly bigger and stockier than modern grey wolves, and they had powerful bites, with teeth well-suited for shearing meat. These large carnivores were thus able to prey upon the many large herbivores that shared their ice age habitats.
A paper published today in Nature provides the first genome-wide data on dire wolves, in a research project that involved nearly 50 contributors.
“Our new results show that the reign of the dire wolf extended much further back in time than we previously thought,” Kieren Mitchell, a co-author of the study and an evolutionary biologist from the University of Adelaide in Australia, explained in an email. “That means they had a very long time to become specialised and reach an equilibrium with their environment, competitors, and prey. When dire wolves became extinct, they left no direct heir. Their line and legacy disappeared forever.”
To which he added: “Grey wolves may occupy that throne today, but they descend from a different branch of the family tree.”
What’s more, the new paper, co-authored by biologists Laurent Frantz from Queen Mary University of London and Angela Perri from Durham University, suggests dire wolves long lived in genetic isolation, which may have played a critical role in their extinction.
The remains of dire wolves have been found across many parts of North and South America, perhaps most famously in the La Brea Tar Pits of California. Despite the fossil evidence, however, scientists haven’t been able to pinpoint their temporal or geographical point of origin, nor the reasons for their extinction. Prior to the new study, researchers primarily focused on dire wolf skeletons, which, while helpful, didn’t tell the whole story.
“This study actually started with a few different research groups all independently trying to get dire wolf DNA,” said Mitchell. “We were all interested in trying to better understand the origin and extinction of the dire wolf, and we thought we might find some clues in their genomes. Eventually we realised we all had different pieces of the puzzle and joined forces to get the best picture of dire wolf evolution that we possibly could.”
By pooling their resources, the team managed to assemble DNA — both nuclear and mitochondrial genomes — from five dire wolf specimens dating from between 13,000 and 50,000 years ago. This allowed the team to partially reconstruct the dire wolves’ evolutionary history. (As an interesting aside, none of the DNA was taken from specimens pulled from the La Brea tar pits, as the heat destroyed all genetic evidence.)
The team then compared these samples to the genomes of living wolf-like species, by referencing research published by other scientists. That said, the scientists did sequence new genomes for some species that hadn’t previously been studied, like the black-backed jackals and side-striped jackals of Africa, explained Mitchell. In total, the team compared dire wolf DNA to 22 genomes, belonging to modern North American grey wolves, coyotes, prehistoric dogs, and African jackals.
The scientists were unable to find evidence of gene flow between dire wolves and American grey wolves, among other similar species. This strongly suggests that dire wolves lived and evolved in isolation from related species, which prevented hybridisation — a critically important and often undervalued contributor to natural selection (modern humans, by example, may be the product of much hybridisation, the result of multiple human species interbreeding in Africa). By mating with similar species, animals can acquire a plethora of desirable traits, while at the same time increasing their genetic diversity. Even today, modern grey wolves and coyotes interbreed, for example.
Mitchell said it was odd not to find any evidence of hybridisation.
“Hybridisation seems to happen very commonly between closely related species wherever they encounter each other. The fact that we didn’t see this with the dire wolf and other wolf-like species led us to conclude that the dire wolf must have been geographically isolated for a long time, giving them no opportunity for hybridisation,” he said. “By the time dire wolves did encounter other wolf-like species, they’d likely evolved to become too different for hybridisation to be possible.”
Indeed, dire wolves did share habitats with grey wolves and coyotes, but as the new research suggests, dire wolves weren’t able to mate with them owing to genetic dissimilarities. The shared ancestor of grey wolves and coyotes evolved in Eurasia, migrating to North America some 1.37 million years ago. Evidence presented in the new paper suggests dire wolves originated in the Americas long before that, in a finding that corroborates previous suspicions on the matter.
As the new research also shows, the last common ancestor for all of these groups dates to 5 million years ago, which is further back in time than scientists presumed. This divergence happened early, pointing to the uniqueness of the dire wolf. The shared ancestor spawned three primary lineages: the group that led to dire wolves, the group that led to grey wolves and related wolf-like species, and the group that led to African jackals.
That said, the authors weren’t able to determine “which two of these lineages or lines were most closely related to each other,” and specifically “whether it is the dire wolf or jackals that are the closest relatives to that third [wolf-like] lineage,” said Mitchell.
The inability to interbreed, as the new paper suggests, may have contributed to the extinction of dire wolves.
“Most scientists agree that the dire wolf likely became extinct because the large herbivorous mammals they hunted — like bison, horses, and camels — either became extinct or drastically declined in the areas where the dire wolf was distributed around 13,000 years ago,” Mitchell said. “Our study helps to explain this by showing that dire wolves likely had millions of years to evolve their own specialised behaviour and biology that was very different to grey wolves and coyotes. Presumably dire wolves weren’t able to adapt successfully to preying on smaller animals — like deer, rabbits, or even mice — and couldn’t migrate to other areas with more abundant large prey.”
As for grey wolves and coyotes around at the time, they didn’t have to depend on large prey to survive, so they weren’t affected by the end of the last ice age in the same way, according to Mitchell.
A limitation of the new paper is that the genetic data analysed wasn’t detailed enough to identify potentially problematic mutations in dire wolves. Consequently, the researchers don’t know if dire wolves lacked genetic diversity, which could have resulted the accumulation of many deleterious or disease-causing mutations. Scientists studying other extinct species, such as the woolly mammoth, have been able to do exactly this, finding evidence of extensive inbreeding. It’s also possible, however, that dire wolves were “genetically very healthy,” said Mitchell.
“That’s a hypothesis that future studies with more data might be able to test.” he added.