Pam Groves of the University of Alaska Fairbanks looks at bones of ancient creatures she has gathered over the years from northern rivers. The remains here include musk oxen, steppe bison & mammoth. (Ned Rozell)

Pam Groves of the University of Alaska Fairbanks looks at bones of ancient creatures she has gathered over the years from northern rivers. The remains here include musk oxen, steppe bison & mammoth. (Ned Rozell)

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Dan Mann thinks it’s because our recent climate has been too stable, at least when compared to the wacky ups and downs of the last ice age.

Bạn đang xem: What killed the world's giants?

Mann, a University of Alaska Fairbanks professor, presented his idea at a recent lecture on campus. Mann is an expert on former worlds, especially that of Alaska, where along northern waterways he and UAF’s Pam Groves have found thousands of bones of extinct horses, along with the hardened remains of lions, giant short-faced bears, mammoths và mastodons. A couple of years ago, while paddling down a sluggish Arctic river, they found a 40,000-year-old steppe bison eroding from a hillside.

Groves is a co-author with Mann on a 2019 paper about what might have caused the extinction of so many large animals at the end of the last ice age, about 11,700 years ago. Many people blame humans, who had a need to lớn hunt và the tools to lớn get the job done.

Looking bachồng over what they can piece together about the past millennia, Mann, Groves and their co-authors say the Holocene — the era in which we live sầu — is too vanilla for large animals to dominate. Great swings in climate favor animals that weigh more than 100 pounds, he said.

Over the last 100,000 years, 64% of large animal species have gone extinct. The loss of large animals lượt thích mammoths, mastodons và giant ground sloths has been somewhat evenly spread over all the continents — except Africa, which has lost less than 20% of its large animals.

During the height of the last ice age, trăng tròn,000 years ago, much of Canadomain authority was beneath 9,000 feet of ice. That ice sheet extended well into mid-America, though much of Alaska was ice-không tính phí at the time.

Using records of ancient climate found in deep ice cores & from other sources, scientists have found that the last ice age was unstable compared lớn the most recent 12,000 years. During the ice age, every thousand years or so featured wild temperature swings.

“The ice age is a time of crazy, rapid change,” Mann said. “To keep up, you really have lớn be on the move sầu, whether you’re a plant or an animal.”

Mastodons and the like were good at dealing with huge environmental changes, Mann said. Large animals are better than small ones at hoofing long distances lớn find new food, & are more able khổng lồ get the most out of the food they are eating and endure periods of starvation. If their favorite forests went brown due lớn increased warmth, for example, giant ground sloths could shuffle on to find greenery.

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Smaller creatures, lượt thích foxes & voles, fit better in today’s world of predictable boreal northern forests, alpine meadows and low-lying deserts that have sầu not changed much. Since the ice age, large beasts have perhaps not been as able khổng lồ use their evolutionary advantages.

Mann bases his hypothesis in part on an idea of Pleistocene expert Dale Guthrie, who lives in Fairbanks. Guthrie said that northern summers during the ice age were longer, which allowed more plant species lớn live sầu in Alaska. With the shift toward our present climate, growing season became shorter & more nutritionless peat formed. This resulted in less food for large creatures, among muốn them mammoths, rhinos & horses.

As an argument against humans killing all the ice-age giants, Mann pointed out that wild horses, bison & mammoths disappeared from the Alaska landscape when there were few or no people around.

But Mann does not give people a pass. As soon as humans invented agriculture, we started altering the carbon balance of the atmosphere. We have sầu not stopped.

Warming the planet at a time it should perhaps be cooling, humans may have sầu snuffed out a few species that were “hiding out và waiting for it khổng lồ get cold,” Mann said.

“It is possible that without the anthropogenic delaying of the next ice age, perhaps the woolly mammoths surviving on Wrangel Island until (4,000 years ago) would have had time to lớn repopulate the boreal region,” Mann and his coauthors wrote in their 2019 Biological Reviews paper.

Ned Rozell

Ned Rozell is a science writer with the Geophysical Institute at the University of Alaska Fairbanks.

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