No Grass, No Horse

Research published in Nature shows that climate changes and the highly specialized digestive system of the wild horse Equus ferus might have contributed to its extinction in North America.

At the end of the Pleistocene epoch, E. ferus was abundant and wide-ranging in North America. However, E. ferus, which stood at about 14 hands, shrank about 12% before disappearing entirely around 12,500 years ago, according to R. Dale Guthrie, PhD, a professor emeritus at the University of Alaska, Fairbanks.

The disappearance of this species and other herbivores has long puzzled researchers, who have posed two conflicting theories for the extinctions--overhunting by humans or climate change.

New evidence found in the fossil record of Alaska seems to let people off the hook. E. ferus became extinct nearly 600 years before undisputed archaeological evidence of human hunters in the region. Guthrie based his findings on measurements and radiocarbon dating of the cannon bones of mature animals from the Alaskan permafrost.

Other evidence also supports the climate change theory of extinction. At the end of the Pleistocene period, the arid steppes, consisting of a mixture of grasses, sedges, and sage, became warmer and moister and gave way to shrub tundra, which was covered with unpalatable plants. Since large mammals like the horse require a great volume of food and have a slow breeding cycle, they suffered the most from such changes. First, E. ferus decreased in size, and then the species died out completely.

"All across the continents, the horse dropped out except in the very arid regions of central Asia's grasslands, a warm steppe," Guthrie said.

"Horses are almost obligatory grazers," he wrote, "and these northern horses appear to have been the most specialized grassland-dependent component of the large mammal fauna."