American mastodons migrated many times across thousands of miles up and down the North American continent over 800,000 years due to climate change, before going extinct 11,000 years ago, according to a new study.
Mastodons were once some of the largest living land animals on Earth.
This 800,000-year period was a bit of a climate roller coaster of ice ages. Cold glacial periods were followed by warmer interglacial periods when ice sheets would retract, creating forests and wetlands where once there was only ice.
Researchers never knew how mastodons reacted to these changes until now.
“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 Hendrik Poinar in a statement. Poinar is an evolutionary geneticist and director of the McMaster University Ancient DNA Centre in Ontario, Canada.
The study published Tuesday in the journal Nature Communications.
These changes attracted mastodons that ranged northward during these warmer times to seek out shrubs and low-hanging tree branches. Unlike woolly mammoths that thrived in colder settings and icy grasslands, mastodons – a relative of mammoths and elephants – preferred wet and warm wooded and swampy environments.
But as the mastodons roamed from Central Mexico up to present-day Alaska, the Yukon and Nova Scotia, the climate would shift again. The warm temperatures of the interglacial period, similar to temperatures we experience today, would drop and the ice would return.
The temperature fluctuations between these periods, researchers estimated, could shift more than 18 degrees Fahrenheit. And glacial periods could cause an expansion of the ice sheet, covering 50% of North America’s habitable land.
These groups of mastodons that migrated north would die out. Then, another group of them would migrate north again when the temperatures changed. The researchers know this because there is no continuity to suggest that they survived the cyclical deep freezes over the 800,000-year period, the study said.
And the groups that migrated northward to the Arctic during these warm periods were less genetically diverse, meaning they were very similar to one another. This also made them more vulnerable to extinction, the scientists said.
Mastodons went extinct about 11,000 years ago about the same time that other large land animals, like woolly mammoths, giant ground sloths and saber-toothed cats also disappeared.
But understanding why these large land animals went extinct is more difficult. Scientists believe a combination of competition for food, climate change and overhunting by early humans is to blame.
These animals preferred different environments, “so we would expect them to have very different responses to the repeating glacial/interglacial cycles,” said Emil Karpinksi, lead study author and a graduate student at the Ancient DNA Centre and the department of biology at McMaster University.
While the warm periods provided a new food source for mastodons, it also removed environments like grasslands that were suited for horses, mammoths and bison as more trees encroached.
The researchers studied the fossils from 33 different mastodons and were able to reconstruct DNA from their teeth, tusks and bones. The DNA revealed five distinct groups of mastodons. Two came from eastern Beringia, or a region that once joined Russia to the US, and those groups resulted from migrations to this area during warm interglacial periods.
And these ancient mastodons’ lack of genetic diversity made them vulnerable to further changes in the environment.
Animals facing climate change
Previous research has largely focused on mammoths, as well as bison and musk ox, which thrived in the cold grasslands called tundra steppes.
The new study not only “greatly increases the amount of genetic information for the American mastodon and provides a solid framework for future studies on the animal,” Karpinski said.
“I think it further illustrates that the extinction of mastodons and other big animals was probably much more nuanced than many people think,” he said. “It’s difficult for me to think climate didn’t play a role when these animals clearly had such drastic responses.”
But the study also provided takeaways for animals currently experiencing and reacting to climate change, like moose and beavers.
“A big part of why we study these extinct species is to see if we can’t find clues to how extant species may respond to events like the anthropogenic warming we’re experiencing now,” Karpinski said.
As the climate warms, these current-day populations have expanded their ranges northward by as as much as 10 to hundreds of kilometers every century, the researchers said. While many have seen this as a positive change for these species, ultimately, these animals could suffer if their southern populations are lost.
If these expanding northern populations are genetically similar to one another, like the northward-ranging mastodons, that could have long-term consequences for their conservation.
“That is always a danger signal for vertebrate species,” said Grant Zazula, study coauthor and paleontologist with the Government of Yukon, in a statement. “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.”
This change applies to other animals as well that react to climate change by expanding into new territories.
“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 benefiting, just like these mastodons did for a time, as a result of natural climate change,” said Ross MacPhee, study coauthor and a senior curator in the American Museum of National History’s department of mammalogy, in a statement.
“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.”