Mastodons migrated over extreme distances in response to climate change

A new study has revealed that American mastodons migrated back and forth over long distances, and this movement was driven by the extreme climate changes of the Pleistocene. The researchers found that mastodons who expanded their range all the way into the Arctic became much less genetically diverse. As a result, these populations were less able to adapt to environmental changes, which contributed to their extinction.

Study co-author Ross MacPhee is a senior curator in the Department of Mammalogy at the American Museum of Natural History.

“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 MacPhee. “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.”

In their day, mastodons were among the largest animals on the planet. Mastodons were related to modern-day elephants and extinct mammoths, which they closely resembled.

The species went extinct about 11,000 years ago, along with many other animals such as mammoths, giant sloths, and sabre-toothed cats. While the exact cause of these extinctions remains unclear, the most prevalent theory is that the Earth warmed up at the end of the Pleistocene too quickly for many large mammals to fully adapt.

In previous studies, scientists found that mastodons had a large range across northern regions based on fossil evidence. However, it remained unclear whether the animals migrated in and out of their northern destinations, or simply arrived and never left. 

For the current investigation, an international team of experts used the fossilized teeth, tusks, and bones of 33 mastodons to reconstruct complete mitochondrial genomes. The analysis showed that the animals traveled extreme distances in response to warming climate conditions and melting ice sheets.

“The genetic data show a strong signal of migration, moving back and forth across the continent, driven, what appears to be entirely by climate,” said study co-author and evolutionary geneticist Hendrik Poinar.

Beginning about 2.6 million years ago, the Pleistocene was characterized by cold glacial periods interspersed with warmer periods with retreating ice sheets. During the warmer interglacial periods, regions that were previously frozen developed into forests and wetlands with new foraging opportunities, which enticed animals like mastodons to move northward.

“These mastodons were living in Alaska at a time when it was warm, as well as Mexico and parts of Central America. These weren’t stationary populations, the data show there was constant movement back and forth,” explained Poinar.

According to the researchers, understanding how different types of Pleistocene-era mammals responded to extreme climate transitions – both genetically and ecologically – will provide valuable information on how climate change is affecting modern-day species in the north.

“By looking genetically at these animals which lived for the last 800,000 years, we can actually see the make-up of these populations that made it up to the north,” said study lead author Emil Karpinksi. “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.”

When the scientists conducted a genetic analysis on the mastodons that moved in and out of the Arctic, they discovered very low genetic diversity among these populations.

“That is always a danger signal for vertebrate species,” said study co-author Dr. Grant Zazula. “If you lose genetic diversity, you are losing the 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.”

“Analysis of DNA preserved in these fossil mastodon bones gives us so much more information on how these now-extinct beasts lived and died in comparison to what we know based on traditional paleontological approaches. These data hold the key to our understanding of how ancient animal communities like mastodons adapted to changes in the past, and provide clues to how arctic ecosystems will respond to future warming scenarios.”

The study is published in the journal Nature Communications.

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By Chrissy Sexton, Earth.com Staff Writer