Climate shifts, not over-hunting, killed off the woolly mammoth and wild horse, a carbon-dating study suggests.

What caused the animals to become extinct at the end of the last Ice Age more than 10,000 years ago has been one of prehistory's greatest whodunits. Biologists have often pointed the finger at over-hunting by expanding populations of humans.

But new radiocarbon dates give a more precise account of what happened at the time of the mass extinctions, and shift the focus to global warming.

Paleobiologist Dale Guthrie analyzed bone samples from bison, moose and humans, which lived through the extinction period, and from wild horse and mammoth, which did not survive. The more than 600 samples were recovered in Alaska and the Yukon. He also studied preserved samples of pollen from the period.

He found that by the time Homo sapiens started pushing into the region around 12,300 years ago, the wild horse had already died out and woolly mammoth were in decline.

Meanwhile, populations of bison, moose and white-rumped elk called wapiti were increasing, said Guthrie, professor emeritus with the Institute of Arctic Biology at University of Alaska, Fairbanks.

By analyzing pollen samples, he concluded that a naturally occurring shift in climate caused the animals to change their diet.

Like their modern cousins, the wild horses and the woolly mammoth of the past had a large intestinal pouch, or caecum, suited to feeding on low-quality forage on the steppe.

But as the frozen landscape thawed, higher-quality grasses started to grow. Those grasses were favoured by the bison and wapiti but were indigestible to the mammoth, Guthrie suggested.

His results were published in Thursday's edition of the journal Nature.