Mastodons moved as the climate changed

Mastodons migrated vast distances across North America in response to dramatic climate change during the ice ages of the Pleistocene, according to a new international study.

A team of evolutionary geneticists, bioinformaticians and palaeontologists reconstructed complete mitochondrial genomes from the fossilised remains of 33 animals, publishing their findings in the journal Nature Communications.

American mastodons (Mammut americanum) went extinct about 11,000 years ago, along with the likes of mammoths, sabre-toothed cats and giant ground sloths.

There is speculation about how and why, the researchers say, but many scientists believe it was a combination of climate change, increasing competition for food and overhunting by early humans.

At the time M. americanum was among the largest living land animals, roaming widely from Beringia, which historically joined Russia and America, east to Nova Scotia and south to Central Mexico.

It was primarily a browser, living in swampy settings, eating shrubs and low-hanging tree branches.

“The genetic data show a strong signal of migration, moving back and forth across the continent, driven, what appears to be entirely by climate,” says co-author Hendrik Poinar, from Canada’s McMaster University.

“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.”

The project was led by Poinar’s colleague Emil Karpinski and brought together researchers from Canada, the US, Mexico and Australia. The team teased out and reconstructed DNA from fossilised samples including teeth, tusks and bones.

Analysis shows, they report, that mastodons were moving vast distances in response to warming climate conditions and melting ice sheets, from warmer environments to the northernmost reaches of Alaska and the Yukon.

“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,” says Karpinski.

The researchers identified five distinct groups (or clades) of mastodons, of which two originated from eastern Beringia. They detected no overlap in the ages of these two, and suggest they likely resulted from separate expansions into the region.

This coincided with interglacial periods when warm climatic conditions supported the establishment of forests and wetlands.

The authors also found that the northern clades had lower levels of genetic diversity than that in groups south of the continental ice sheets, making them more vulnerable to extinction.

They argue that similar northward population expansions today, owing to climate change, likely consist of a subset of a species. This may leave them vulnerable if more genetically diverse southern populations are eventually lost.