Scientists examining mastodon skeletons found a type of bone damage in several of the animal's foot bones that is found only in sufferers of tuberculosis. Bones attacked by tuberculosis suffer a type of damage in which bone beneath cartilage is scooped out, or "excavated."

Tuberculosis is a bacterial infection that commonly infects the lungs, but can also affect other parts of the body, such as organs and bones.

Only about 1 to 7 percent of infected humans develop bone damage. The fact that more than half of the mastodon skeletons examined had the bone lesions suggests tuberculosis was a "hyperdisease" that afflicted a large percentage of the North American mastodon population.

The disease would have weakened both animals, making them easier for humans to hunt and kill. They also would have been more vulnerable to changes in the climate.

Scientists have often theorized that consumption by humans and the onslaught of the Ice Age caused their extinction in North America.

Researchers Bruce Rothschild of the Northeastern Ohio Universities College of Medicine and Richard Laub of the Buffalo Museum of Science in New York looked at 113 mastodon skeletons and found that 52 percent showed signs of tuberculosis.

So, how did tuberculosis -- first documented in a 500,000-year-old buffalo in China -- migrate to North America and infect mastodons and mammoths?

In a separate study, Rothschild and Larry Martin from the Natural History Museum in Kansas found similar tuberculosis-caused bone damage in North American bovids. Bovids are a group of animals that include bison, musk oxen and bighorn sheep.

Tuberculosis appears to have been just as prevalent in the bovids as in the mastodons, but the record of infection for this group of animals stretches back much further -- at least 75,000 years.

It is believed bison and other bovids originated in Asia and crossed into North America by way of the Bering Land Bridge, which connected the two continents.

Rothschild and Martin think some of the bovids were probably infected with tuberculosis when they crossed the land bridge. The bovids could have spread the disease to mastodons and other species, possibly even humans, Rothschild said.

The infected mastodons were different ages and sizes and came from all over North America. They also lived at different times. The disease appears to have struck the creatures as early as 34,000-years-ago and persisted in the species until as recently as 10,000 years ago.

Both the mastodon and bovid studies will be detailed in upcoming issues of the science journal Naturwissenchaften.

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