for National Geographic News
The plot thickens.
New findings suggest that an ongoing, epic whodunit may actually be a whatdunit. That is, climate change, not humans, may be what killed off Ice Age mammoths, horses, and other large animals in North America.
(For the previous plot twist, see "Ice Age Horses May Have Been Killed Off by Humans, Study Finds" [May 1].)
Dale Guthrie, a researcher at the University of Alaska in Fairbanks, has spent some 20 years examining more than 600 bones of large mammals from Alaska and the Yukon Territory.
His analysis points toward climate as the culprit.
Guthrie's data, published in tomorrow's issue of the journal Nature, shows that increases in moisture and warmer temperatures 13,500 to 11,500 years ago allowed for edible plants to migrate north.
This plant exodus provided more food sources for horses, mammoths, bison, and elks living in the far north, he suggests. (See a classic photo of a mammoth find, circa 1900.)
But then the milder climate backfired on the big mammals. It paved the way for trees, which eventually outshaded and outcompeted the low-lying plants the animals depended on, Guthrie says.
The upstart forests transferred the landscape's nutrients to the treetops, out of the reach of large mammals. Elks and bison, it seems, adapted better to the new landscape than mammoths and horses.
By the time the forests were established, human hunters had also ridden the vegetational tide, migrating into Siberia and then Alaska and the rest of the Americas. Moose followed the same pattern around the same time.
Soon after humans arrived in North America, the prehistoric horse and then the mammoth dropped out of the picture. Finally, even bison and elk numbers declined. But those animals, along with moose, survived in places where the habitat remained suitable.
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