A wide variety of mammoth species once roamed North America, from the pony-sized pygmy mammoth to the huge, hairless Columbian mammoth. A new study suggests these different mammoths were open to diverse and exotic mates.

"Our results strongly suggest that various nominal mammoth species interbred, perhaps extensively," says a new study by Canadian and U.S. researchers published this week in the journal Frontiers in Ecology and Evolution.

The study, based on analysis of DNA from 67 specimens of species other than woolly mammoths, uncovers new insights about how mammoths evolved and calls into question whether they were really separate species.

A pygmy mammoth model is seen at the Ontario Science Centre. (Emily Chung/CBC News)

When most people think of mammoths, they generally think of the woolly mammoth, a hairy elephant-like animal that roamed the tundra. But in fact, North America was once home to many other kinds of mammoths.

The Columbian mammoth was a mostly hairless mammoth larger than an African elephant that is thought to have arrived in North America 750,000 to one million years before woolly mammoths, says Hendrik Poinar, a McMaster University geneticist. The Columbian mammoth was no tundra animal — it made its home in the temperate grasslands of the U.S. and Mexico.

At the other end of the scale were the pygmy mammoths that Poinar describes as "tiny little guys on the islands off California" that weren't much bigger than a pony.

Of course, there was the woolly mammoth, which entered North America much later, but whose range eventually overlapped with the Columbian mammoth.

And then there was Jefferson's mammoth, which lived in the woodlands of the Great Lakes region and was sort of like a woolly mammoth and sort of like a Columbian mammoth.

Most DNA studies up until now have been on woolly mammoths, as many of their remains and DNA have been well preserved in the permafrost.

Poinar and his team wanted to learn more about how the populations of other kinds of mammoths changed over time. While the DNA in those more southern species isn't as well preserved, new techniques developed in Poinar's lab allow DNA to be extracted and identified even when there is very little of it. The DNA was collected from specimens — mostly teeth — from Siberia and all over North America, including Yukon and B.C.

Woolly mammoths likely interbred extensively with other mammoth 'species,' a new study suggests. (Carl Buell)

While Columbian, woolly, and pygmy mammoths all looked different, lived in different environments and entered North America at different times, their DNA was surprisingly similar — especially the Columbian and woolly mammoths.

"That's not what you expect from a distinct species," Poinar said.

Many animals also showed some Columbian and woolly traits, while others showed something halfway in between — that was particularly the case for Jefferson's mammoth.

The one least closely related to the others was the pygmy mammoth, suggesting that it was most closely related to the first mammoths to enter North America, the steppe mammoths.

The results suggest that despite all the differences that evolved between the woolly mammoth and the Columbian mammoth during the million years or so that they were separated, "when they met back up, they could still interbreed," Poinar said — and weren't really separate species in a biological sense.

That said, there's no doubt the animals were very different and adapted to very different environments.

"They are different 'ecological' species," Poinar said. "You throw a Columbian up in the far north in the middle of winter at –80 and he's not surviving. You take a woolly and throw him down into the Everglades and he's sweating to death."

They also had a very different response to climate change, he added — woolly mammoths survived climatic change much better than Columbian mammoths, which went extinct earlier.