Grasslands suddenly spreading across the Arctic about 10,000 years ago helped killed off the woolly mammoth and other prehistoric mammals, suggests a study of ancient Arctic vegetation.

The new DNA analysis of Arctic vegetation over the past 50,000 years, published in Nature by a team led by Eske Willerslev of the University of Copenhagen, offers a new wrinkle on the climate-warming theory: The great beasts vanished because they weren't getting enough of the right food.

Some 10,000 years ago, the researchers found, the flowering, broad-leafed plants known as forbs—including sagebrush, yarrow, mums, and tansies—disappeared from Arctic steppes, which became more dominated by grasses. That vegetation change was "a likely key reason for the decline and extinction of many megafuana species," Willerslev says, by email.

DNA Detectives

The new theory is surprising because mammoths were thought to thrive on grass. The Nature study challenges a long-held picture of a grass-covered "mammoth steppe" covering the polar regions of Europe, Asia, and North America during the last Ice Age.

In the study, the team sampled permafrost cores dating back to 50,000 years ago from 17 locations in northern Russia, Canada, and Alaska. They found that DNA signatures in the cores indicate that the flowering species, and the tiny roundworms associated with them in the soil, once predominated over grasses on the ancient steppes.

What's more, the mammoths and other beasts seem to have favored those forbs. Willerslev and colleagues analyzed 18 preserved samples of stomach contents and scat from mammoths, woolly rhinos, horses, reindeer, and elk. They found that flowering species were a large part of the animals' diet.

The forbs could have been a key source of protein, Willerslev says, and they may have been easier to digest than grass. Previous research had missed that component of the mammoths' diet, he and his colleagues argue, because those studies relied on pollen counts to estimate past Arctic vegetation. Grasses produce abundant pollen, so such studies gave a misleading picture of the makeup of the steppes.

"I think this is (a) preliminary conclusion," says Sergey Zimov, director of Russia's Northeast Science Station, who was not part of the study. He notes some overlap of the DNA results showing forbs flourishing after the Ice Age, and the limited number of prehistoric animal remains involved in the work.

Farewell to Flowers

When the last Ice Age reached its peak around 20,000 years ago, the diversity of all plants in the Arctic declined, but flowering plants continued to dominate over grasses. The warming that ended the Ice Age, however, also brought a wetter climate that was more friendly to grasses. "This is the likely reason for the vegetation change in the Arctic into a system dominated by shrubs and grasses we see today," Willerslev says. Indeed, much of the ancient steppe has given way to mossy tundra.

The herds of mammoths and other megabeasts had probably also helped maintain the steppe they lived in, whether it was dominated by grasses or, as the new research suggests, by forbs. Grazing and trampling of vegetation allowed new seedlings to take root, and manure fertilized the plants. Human hunters, by reducing the population of mammoths, may thus have helped complete the vegetation transition that climate change began.

"There are dangers in making broad generalizations about herbivore diet," says paleobotany expert Robert Crawford of Scotland's University of St. Andrews. Forage conditions in winter and local conditions for fodder across the Ice Age Arctic have to be examined more closely in more studies, he suggests, before forbs would look like an answer to the mystery of the mammoth.

Zimov, a founder of the "Pleistocene Park" effort to recreate Siberia's Ice Age steppes, agrees that the numbers of mammoths, bison and horses once seen in Siberia would have completely transformed the landscape. But he suggests that the ancient range of these creatures extended from Spain to China to North America, making it unlikely that climate change alone could have changed their habitat enough to wipe out the species.

Woolly mammoths had roamed the northern steppes for at least 300,000 years before they died off. In parts of Alaska, at least six times more animals—such as mammoths, horses, and bison—lived in the region during that era than live there today. A last, isolated population of mammoths is thought to have finally gone extinct on Siberia's Wrangel Island about 3,700 years ago.