Mastodons travelled north when climate changed, study finds

Researchers say dramatic environmental changes accompanying the shift or melting of continental glaciers played a key role as American mastodons moved north from their southern ranges.

The animals were among the largest living land animals on Earth at the time, roaming from Beringia (present-day Alaska and the Yukon) east to Nova Scotia and south to central Mexico.

They were primarily browsers, living in swampy settings, and eating shrubs and low-hanging tree branches.

In the first large-scale genetic study of American mastodons, published in the Nature Communications journals, researchers studied fossilised samples, including the teeth, tusks and bones, of 33 individual animals.

Around 11,000 years ago the species died out during the megafaunal extinctions which took out many of the large mammals such as mammoths, sabre-toothed cats and giant ground sloths.

Evolutionary geneticist Hendrik Poinar, director of the McMaster Ancient DNA Centre and author on the study, said: “The genetic data show a strong signal of migration, moving back and forth across the continent, driven what appears to be entirely by climate.

“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.”

According to the researchers, dramatic and repeated temperature fluctuations took place routinely on the planet for millions of years.

Cycles of glacial and interglacial periods over the last 800,000 years resulted in periodic expansion of the ice sheet across approximately 50% of the habitable land in North America.

Researchers identified five distinct groups – or clades – of mastodons.

Two originated from eastern Beringia (a region that historically adjoined Russia and America).

The authors found no overlap in the ages of the specimens from the eastern Beringia groups and suggest that the two clades likely resulted from separate expansions into this region.

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

The research suggests that mastodons were travelling vast distances in response to warming climate conditions and melting ice sheets, from warmer environments to the northernmost reaches of Alaska and the Yukon.

However, despite these massive increases in territory, northern populations were much less genetically diverse, rendering them more vulnerable to extinction.

Emil Karpinski, lead author on the study and a graduate student at the Ancient DNA Centre and the Department of Biology at McMaster University, said: “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.

“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 kilometres every century.”