Researchers found that climate change, rather than hunting, may have triggered the extinction of Alaska's native horses about 12,500 years ago.
     The cause of the disappearance of about 70 percent of North American large mammals, including all horse species, has been hotly debated by scientists. Some think hunting contributed to their disappearance but R. Dale Guthrie, of the University of Alaska in Fairbanks, said climate change and a shift in vegetation from grasslands to tundra was probably to blame. "Horses underwent a rapid decline in body size before extinction and I propose that the size decline and subsequent regional extinction are best attributed to a coincident climate/vegetation shift," he said in a report in the science journal Nature.
     Horses, which evolved in Asia, crossed into North America via the Bering Land Bridge that once linked Alaska and Siberia. After their extinction, they were reintroduced into the Americas by the Spanish in the 1500s.
     Guthrie, who studied carbon-dated fossil bones from two species of extinct Alaskan horses, discovered that bones from a horse dating back 12,500 years were about 12 percent shorter than those from another horse that lived nearly 15,000 years earlier. Even if hunters were in Alaska when the horses disappeared, Guthrie said there probably would not have been enough of them to kill all the horses. The shift in vegetation, which would have diminished their food supply, could account for the decrease in the horses' size and their eventual extinction.

Sources: Reuters, Yahoo! News (12 November 2003)