When Earth Warmed Between Ice Ages, Mastodons Migrated North

Dramatic environmental changes led American mastodons to migrate: Their reaction to the interglacial periods could help predict what we may expect in this time of climate change

Ruth Schuster
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An artist's impression of the American mastodon (Mammut americanum).
An artist's impression of the American mastodon (Mammut americanum).Credit: Charles R. Knight
Ruth Schuster

American mastodons evolved in the bitter cold, and when the ice ages were in abeyance and the temperatures turned warm, they would simply move north. Masses of mastodons would migrate vast distances to the northernmost plains of Alaska and the Yukon – but not all the mastodons, it seems. Some would stay behind, and the result was diminished genetic diversity in the northern mastodon line, rendering the species as a whole more vulnerable to extinction.

That conclusion was reached by a genetic study of the remains of 35 mastodons (Mammut americanum), published Tuesday in Nature Communications.

When environmental conditions become intolerable, species that can migrate do so. This isn’t theoretical, it’s empirical and obeys an imperative that has applied since time immemorial. If you stay put when the conditions worsen, you risk premature death. At this very time we are observing species on the move as climate change descends upon us. How many species?

“Climate change is impelling a universal redistribution of life on Earth,” a team wrote in Science in 2017, adding that policy has utterly failed to keep abreast of the changes, let alone adapt to them. National Geographic put it at half of all species, warning that as climate change “displaces everything from moose to microbes,” it is changing life as we know it.

By the way, the northward spread by disease-bearing mosquitoes is exactly this – climate change-induced population shifting.

If there’s one species that seems not to be doing much about climate change, it’s us – and one difference is that we’re reacting (or not reacting) to climate change caused by ourselves, that seems to be ramping up quickly. The mastodons were reacting to fluctuations in the Earth’s climate.

Mastodon fossil on display at the American Museum of Natural History
Mastodon fossil on display at the American Museum of Natural HistoryCredit: Denis Finnin / ©American Museum of Natural History

While it cannot be said that the cause of ice ages has been scientifically nailed down, if we leave volcanic winters and other sundry calamities out of it, ice ages were apparently triggered by the cycle of the Earth’s tilt vis-à-vis the Sun and its orbit, as well as the level of carbon dioxide.

For over 2.5 million years, the global climate has been swinging violently between long ice ages, which could last up to 100,000 years, and shorter warming periods called the interglacials, which generally lasted about 10,000 years. At the peak of the ice ages, the northern hemisphere was heavily glaciated. Recent research estimated that during the Last Glacial Maximum, around 20,000 years ago, the average global temperature was 6 degrees Celsius (about 11 degrees Fahrenheit) lower than the average global temperature now. 

LiveScience evocatively points out that during the last glaciation episode, the area now known as Chicago was under an ice sheet nearly 3 kilometers (almost 2 miles) thick.

As always when it comes to climate, one snorts at the mere 6-degree change – but in fact it was a huge deal, and local changes could be much more extreme. The northern hemisphere could be called a hot spot for climate change: it is extremely affected by changes in climate. The Arctic averaged about 14 degrees Celsius colder than now, the separate research estimates. Hair-covered elephants apparently liked it that way.

The beasts of Beringia

The mastodon of North America thrived in woody, swampy environments, and when it was sufficiently cold they ranged as far south as Central America.

The new analysis of 33 mastodons by Emil Karpinski of Hamilton’s McMaster University and colleagues, plus separate analysis of two other mastodons (thus totaling 35), split the extinct elephantids into five groups (or clades).

"Mastodon" by Thomas Quine, at the American Museum of Natural History.
"Mastodon" by Thomas Quine, at the American Museum of Natural History.Credit: Thomas Quine

To be clear: Karpinski and the team analyzed the mastodons' mitochondrial genomes, not the nuclear genomes, using material extracted from fossil teeth and bones. Almost all eukaryotic (non-bacteria, non-virus) cells contain mitochondria, which are characterized by having their own DNA separate from the cellular DNA. The mitochondrial DNA generally passes along the matriarchal line. We inherit genes from mother and father, but our mitochondria, by and large, derive only from mama.

So: the mitochondrial analysis showed that two of the five mastodon clades originated from eastern Beringia (the land bridge that joined Russia and America when ice was high and sea levels were low). But the two Beringian mastodon groups didn’t overlap in the ages of the specimens: they seem to have resulted from separate migration to that far-northern, low-lying land bridge.

And when did they do that? During the interglacials, when the heating caused water levels to rise and their beloved freezing swamps and woods to turn into lush forests and wetlands.

Also, even though the sample group is very small for far-reaching conclusions, the team suggests that the northern clades exhibited lesser genetic diversity than the groups south of the continental ice sheets. That indicates, they suggest, that not all moved north.

Likely the ones left to swelter in that nasty green forest died off, which all begs a frightening hypothesis: that as animal and other species creep north today – they are not all going, but only a subset of the species. And that could leave them as vulnerable to extinction as the mastodon was.

By the way, in case you’re wondering what the difference is between mastodons and mammoths, mastodons apparently had shorter legs but were stockier in build, and had shorter, straighter tusks, paleontologists explain. Mammoths had humps of fat including on their heads and mastodons didn’t. Mastodon molars were adapted to eating woody bushes, while mammoths ate grass. Though obviously they had a common ancestor about 25 million years ago (very roughly), their evolution followed different paths, and thus mastodons arose in North and Central America, while mammoths evolved in the Old World.

Both went extinct at about the same time.

It begs adding that while humans definitely played a role in some mega-faunal extinctions – especially on islands where the hapless huntees had nowhere to go – that doesn’t seem to have been the case for the American mastodon in the far north at least. Its extirpation in the Arctic and sub-Arctic regions predated our arrival, even factoring in intense disagreements about the date of our arrival in the “New World.”

Mastodons last lived in high latitudes during the interglacial about 125,000 to 75,000 years ago. But further south, the giant elephantids soldiered on, ultimately going extinct once and for all about 10,000 years ago – by which time we were most definitely around.

In fact, most of the world’s mega-fauna (aka “big animals”) went extinct by that time. In some cases, the evidence indicates that the mega-fauna that, like the mastodon, evolved in cold weather, couldn’t take the heat. In other cases, we ate them.