Image caption The woolly mammoth finally died out approximately 4,000 years ago

Woolly mammoths died out because of dwindling grasslands - rather than being hunted to extinction by humans, according to a Durham University study.

After the coldest phase of the last ice age 21,000 years ago, the research revealed, there was a dramatic decline in pasture on which the mammoths fed.

The woolly mammoth was once commonplace across many parts of Europe.

It retreated to northern Siberia about 14,000 years ago, where it finally died out approximately 4,000 years ago.

The reasons for its extinction are unclear and have been a matter of heated scientific debate.

Some scientists have argued that it was principally the result of climate change while others say that it was driven by pressures of a growing human population, or even a cataclysmic meteor strike.

Now, according to Professor Brian Huntley of Durham University, that debate has been settled.

"What our results have suggested is that the changing climate, through the effect it had on vegetation, was the key thing that caused the reduction in the population and ultimate extinction of mammoths and many other large herbivores," he said.

Professor Huntley and his colleagues created a computer simulation of vegetation in Europe, Asia and North America over the last 42,000 years.

They did this by combining estimates of what the climate was like during this period with models of how various plants grow under different conditions.

They found that the cold and dry conditions during the ice age, with reduced concentrations of carbon dioxide, didn't favour the growth of trees.

So instead of forests there were vast areas of pasture, which was ideal for large herbivores, such as woolly mammoths. But as a result of a warmer, wetter climate and rising concentrations of carbon dioxide at the end of the ice age, trees emerged at the expense of the grasslands.

"During the height of the ice age, mammoths and other large herbivores would have had more food to eat," said Professor Huntley.

"But as we shifted into the post-glacial stage, trees gradually displaced those herbaceous ecosystems and that much reduced their grazing area."