Decoding the disappearance of bears and lions using ancient DNA

Tens of thousands of years ago in the Pleistocene, North America was roamed by fearsome beasts, including ancient cave lions and grizzly bears. The lions went extinct, never to return, but the bears are still there in force. The history of these carnivores’ existence on the continent, however, is peppered with disappearances and reappearances in the archaeological record.

Now, a team of researchers from the University of Adelaide have found a changing climate may be behind the mysterious disappearance of ancient lions and bears from parts of North America for a thousand years or more prior to the last Ice Age.

The study, published in Molecular Ecology, sequenced DNA extracted from ancient bones of cave lions and bears found in North American and Eurasia to understand their past abundance and movements between continents.

“There’s a common perception that outside of mass extinctions or direct human interference, ecosystems tend to remain stable over thousands or even millions of years,” says study co-author Kieren Mitchell, from the University of Adelaide’s Australian Centre for DNA. “As illustrated by our study of the fossil record, that’s not necessarily the case.”

Mitchell says the bears went extinct multiple times on the continent before their reappearance.

“Previous research has shown that brown bears (or grizzly bears) disappeared from some parts of North America for thousands of years prior to the last Ice Age. They later reappeared, walking from Russia to Alaska across the Bering Land Bridge – possibly at the same time as people moved across the Bridge into North America too.

“But no-one knows exactly why they disappeared in the first place, which is why studying this event is important.”

The Bering Land Bridge is an intermittent causeway between Russia and North America that is ordinarily undersea, but has been exposed in past ice ages, following the rhythms of sea level rise and fall. It’s this crucial land bridge that brought megafauna like bears, elk and lions – as well as humans – onto the frigid continent.

A key finding of the new research is that cave lions from the same area also became extinct more than once – before their final extinction they had already disappeared and reappeared thousands of years later, following the same pattern as the bears. There is no evidence that people caused these temporary disappearances, nor were cold Ice Age conditions to blame.

“Instead, it looks like a smoking gun pointing to some kind of change in their ecosystem,” Mitchell says.

As it turns out, the timing of these bear and lion extinctions in parts of North America coincides with evidence of a widespread shift in vegetation in the region. The authors say warming temperatures prior to the last Ice Age may have changed the composition of plants, interrupting the flow of the food chain and denting the population of herbivores on which the bears and lions preyed.

As the Ice Age encroached, cooling temperatures may have reversed this change, making the area more hospitable for herbivores and their predators.

“Overall, these findings demonstrate just how changeable past ecosystems have been, and also how the abundance of different species can be very sensitive to changes in climate,” Mitchell says.

Lead author Alexander Salis agrees: “While many might think that species arrive in a region and stay put, we show that the past was much more dynamic, involving multiple waves of dispersal and local extinctions in this case.”