Ancient bears evolved to become giants so they could scavenge prey killed by others, a new study suggests.
The findings, published in the Royal Society journal Biology Letters, found that short-faced bears weighing over a tonne evolved independently in North and South America.
"We think their large size was a particular advantage that let them exploit carcasses from other predatory species," said evolutionary biologist Dr Kieren Mitchell from the University of Adelaide, who was lead author on the new paper.
Dr Mitchell said giant bears from the Tremarctinae group were among the largest land-based carnivorous mammals that ever lived. The bears roamed the grasslands and open woodlands of the New World from about 2.5 million years ago to about 12,000 years ago.
The animals strode around on their long legs, eating whatever they could find, but the shape of their teeth suggests they were better adapted for eating meat than plants, he said.
At the time the giant bears lived there would have been lots of large herbivores, such as bison and mammoth in North America, and horses and giant ground sloths in South America.
There would also have been specialized predators like wolves, lions and sabre-tooth cats, but there would have been no specialized scavengers, Dr Mitchell said.
"This opened up a bit of a gap that looks like the bears took advantage of separately in North and South America."
By turning into giants, the bears could spot vultures circling and quickly get to any newly-killed prey and get a meat meal without the risk and energy of having to do their own hunting.
"They could just get anything out of the way that was there originally, and just tuck in," Dr Mitchell said.
The giant bears in North America belonged to the genus Arctodus while in the South America it was the genus Arctotherium.
Both were members of a group, which today has only one living member - the much smaller and largely vegetarian Andean spectacled bear (Tremarctos ornatus).
To shed light on the evolution of the giant bears, Dr Mitchell and colleagues compared the mitochondrial DNA of Arctotherium and Arctodus bears with the DNA of the living spectacled bear.
This included DNA extracted from a femur of a giant bear found in a cave in Chile and DNA from the largest species of North American giant bear Arctodus simus.
The genetic analysis showed the South American giant bear was more closely related to the modern bear, even though it looked more like the North American giant bear, indicating the two extinct species evolved independently as an adaptation to exploit a similar ecological niche.
Article first appeared on ABC Science Online.