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Mastodons Marched North When Ice Retreated

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Fossil mount of the Warren Mastodon on display in the fourth floor dinosaur halls at the American Museum of Natural History. The Warren mastodon, which was the first complete American mastodon skeleton found in the United States, on display in the Paul and Irma Milstein Hall of Advanced Mammals at the American Museum of Natural History.
D. Finnin/© AMNH

New research suggests that American mastodons traveled extreme distances in response to dramatic climate change and melting ice sheets, from warmer environments to the northernmost parts of the continent.

But the populations that headed north were less genetically diverse, making them vulnerable to extinction—and an important case study for scientists investigating how today’s warming climate impacts animal ranges. 

“Today, you might think that it’s great to see animals like brown bears in northern Canada and the Arctic islands well beyond their historical range. They are obviously benefitting, just like these mastodons did for a time, as a result of natural climate change,” said Ross MacPhee, a senior curator in the Museum’s Department of Mammalogy and one of the authors on the study, published today in the journal Nature Communications. “But that benefit can be very limited. It’s important to realize that what we might think is beneficial change at one level for some species is not necessarily all that good for others.”

Mastodons, which belong to a group closely related to mammoths and modern-day elephants, went extinct about 11,000 years ago. Mastodon fossils discovered previously in northern climates indicate that the species likely had a large range, from present-day Alaska and the Yukon east to Nova Scotia and south to Central Mexico. But it was not known when or how frequently these migrations happened.

To find out more, an international team of scientists led by McMaster University in Ontario reconstructed genomes from the fossilized teeth, tusks, and bones of 33 mastodon specimens donated from museums and research institutions across North America, including the Museum.

The results show that the animals migrated vast distances in response to dramatic climate change during the ice ages of the Pleistocene, which began about 2.6 million years agoc. The Pleistocene included cold glacial periods interspersed with warmer periods during which ice sheets would retract. During these warm “interglacials,” previously frozen regions grew new forests and wetlands that provided new food sources for animals like the mastodon, enticing them northward.

“These weren’t stationary populations. The data show there was constant movement back and forth,” said evolutionary geneticist Hendrik Poinar, director of the McMaster University Ancient DNA Centre and an author on the study.

The researchers suggest that examining how mammoths and other Pleistocene megafauna responded to climate transitions can provide valuable information on how climate change is affecting modern-day species.

“It’s really interesting because a lot of species presently, like moose and beaver, are rapidly expanding their range northwards by as much as tens to hundreds of kilometers every century,” said Emil Karpinksi, lead author on the study and a graduate student at the Ancient DNA Centre and the Department of Biology at McMaster University.

The scientists also analyzed the genetics of the “pioneer” populations that made it to the north, finding that their genetic diversity was very low.

“That is always a danger signal for vertebrate species,” said Grant Zazula, an author on the study and paleontologist with the Government of Yukon. “If you lose genetic diversity, you are losing ability to respond to new conditions. In this case, they were not up there long enough to adapt to northern conditions when they cycled back to cold.”