Alaskan horses shrank, then disappeared.

Toward the end of the last ice age, a group of horse species that lived in Alaska shrank in body size over several millennia before going extinct. That finding implicates changing environmental conditions as the stimulus for equine die-offs in the region, one researcher suggests. Some scientists previously proposed that the arrival of human hunters was to blame for the demise of many species of large North American mammals (SN: 12/4/99, p. 360).

Caballoid horses, a group of stocky species related to some Eurasian horses, were once common in what is now Alaska. But they, along with 70 percent of North America's large mammal species, disappeared by the end of the last ice age, says R. Dale Guthrie of the University of Alaska in Fairbanks.

At the height of that ice age, about 24,000 years ago, the shinbones of caballoid horses averaged 21 centimeters in length. By the time the horses went extinct about 14,500 years ago, the average caballoid shinbone measured only 19 cm.

Ecological factors can best explain the decline in horse size and the species' ultimate extinction, Guthrie suggests in the Nov. 13 Nature. A sudden shift in pollen species about 15,500 years ago chronicles the evolution of the landscape from an arid, windswept grassland--an ecosystem to which the horses were particularly well adapted--to a less nourishing mosaic of lakes, bogs, forests, and tundra.

Archaeological evidence so far dates the arrival of people in the area at about 5 centuries after the disappearance of the horses. Even if some people roved through the area before horses went extinct, it's unlikely that just a few hunters could wipe out the horses, says Guthrie. Furthermore, he notes, any theory that blames people for the demise of these horses must also explain why mammoths, another supposed target of such hunters, outlasted horses in the area by about 1,300 years.

Despite the current lack of evidence, human predation may have been at play in the horses' Alaskan extinction, says Stephen M. Rowland of the University of Nevada at Las Vegas. Ancient bands of nomads whose traces archaeologists have yet to find could have driven horses from the area, but that hypothesis would be hard to test, Rowland notes.

Elsewhere in North America, people may indeed have contributed to the extinction of horses at the end of the last ice age. For instance, researchers have found the residue of horse blood on spearheads at an 11,000-year-old archaeological site in western Canada.

At Nevada's Gypsum Cave, northeast of Las Vegas, scientists unearthed bones from horses, camels, and other large mammals. Preliminary examinations of some of those bones have revealed scratches that "look suspiciously like cut marks" says Rowland. Also, many of the bones had been burned at temperatures that suggest they were cooked over a fire, he notes.

Rowland and Elizabeth M. Glowiak presented their findings last week at the Geological Society of America meeting in Seattle.