What Killed the Last Woolly Mammoths? Scientists Say It Wasn’t Inbreeding

Roughly 10,000 years ago, a small group of woolly mammoths found themselves stuck on an island off the coast of Siberia. While their mainland peers disappeared, this isolated herd multiplied. They became the only surviving members of their species and thrived for around 6,000 years—until they, too, died out.

But what killed the last known population of woolly mammoths? Some have previously speculated that inbreeding snuffed out the Wrangel Island colony. But new research suggests otherwise.

The Wrangel Island mammoths did have low genetic diversity—and individual creatures suffered from genetic diseases—but that’s not what killed them, according to a new paper published Thursday in the journal Cell.

“We can show that, in all likelihood, inbreeding and genetic diseases did not cause the population to gradually decline towards extinction,” says study co-author Love Dalén, an evolutionary geneticist at the Center for Paleogenetics, a collaboration between Stockholm University and the Swedish Museum of Natural History, to New Scientist’s James Woodford. “The population was doing OK despite the inbreeding.”

Scientists reached this conclusion by analyzing ancient DNA. The team studied the genomes of 14 individuals that lived on Wrangel Island between 4,333 and 9,219 years ago. Then, they compared those to the genomes of seven mammoths that lived on mainland Siberia until 12,158 years ago. Collectively, the 21 genomes gave them insights into approximately 50,000 years of genetic history.

While they found that the island-dwelling mammoths acquired some minor genetic mutations, any anomalies that would have caused major problems were gradually eliminated from the population. These mutations, as a result, could not have caused the species to go extinct, the researchers say. Most likely, that’s because the affected individuals didn’t—or couldn’t—reproduce.

Even with mutations from inbreeding off the table, researchers can’t pinpoint exactly what killed the Wrangel Island mammoths. Humans likely weren’t responsible, as they didn’t arrive on the island until 400 years later. But the team suggests the creatures died in an unlucky incident—perhaps because of a novel virus or natural disaster, like an Arctic volcano eruption or a tundra fire.

“It was probably just some random event that killed them off, and if that random event hadn’t happened, then we would still have mammoths today,” Dalén says in a statement.

Woolly mammoths once roamed throughout North America, Europe and Asia. But around 15,000 years ago, these behemoth creatures began to vanish—because of human hunting and habitat loss due to natural climate changes—until all that remained were a few small herds living on islands.

One group ended up on Wrangel Island, a landmass about the size of Delaware that got cut off from mainland Siberia by rising sea levels caused by melting glaciers.

Their isolation ended up being a blessing in disguise. Wrangel Island had no predators—including humans—and no other grazing animals, which gave the woolly mammoths carte blanche to munch and reproduce freely. Within just 20 generations, the colony ballooned from eight breeding individuals to between 200 and 300 mammoths.

“Wrangel Island was a golden place to live,” Dalén tells the New York Times’ Carl Zimmer.

But island life was not all good. Without access to other herds, the isolated woolly mammoths were inbreeding. Their low genetic diversity meant individual mammoths probably suffered from inherited diseases. Since no predators inhabited the island with them, they were able to survive in spite of their poor health—until whatever cataclysmic event took place that eventually killed them.

“Based on our results, the extinction must have happened rapidly,” the study’s co-authors write for the Conversation.

Not everyone is convinced by the findings. Even minor genetic mutations could have made the woolly mammoths less resilient in the face of other threats, like climate change or disease. So, it’s still possible that inbreeding contributed to their demise.

“It is really good evidence against the [genetic] meltdown model, but it doesn’t completely exclude that model,” says Vincent Lynch, an evolutionary biologist at the University at Buffalo who was not involved with the research, to Science News’ Claire Yuan.

More broadly, however, the findings could be useful for modern-day conservationists as they try to save endangered or vulnerable species facing extinction. When considering how to help populations rebound, they should make genetic diversity a top priority, according to the researchers.

“Mammoths are an excellent system for understanding the ongoing biodiversity crisis and what happens from a genetic point of view when a species goes through a population bottleneck,” says study co-author Marianne Dehasque, also an evolutionary geneticist at the Center for Paleogenetics, in the statement.

Sarah Kuta | READ MORE

Sarah Kuta is a writer and editor based in Longmont, Colorado. She covers history, science, travel, food and beverage, sustainability, economics and other topics.