Mastodons Driven to Extinction by Tuberculosis, Fossils Suggest

Kimberly Johnson
for National Geographic News
October 3, 2006
Tuberculosis was rampant in North American mastodons during the late Ice Age and may have led to their extinction, researchers say.

Mastodons lived in North America starting about 2 million years ago and thrived until 11,000 years ago—around the time humans arrived on the continent—when the last of the 7-ton (6.35-metric-ton) elephantlike creatures died off.

Scientists Bruce Rothschild and Richard Laub pieced together clues to the animals' widespread die-off by studying unearthed mastodon foot bones.

Rothschild first noticed a telltale tuberculosis lesion on a bone at an excavation site in New York in 2001.

"His eye was caught by a particular bone—a metacarpal, the equivalent to one of the long bones in the palm of the hand," said Laub, who is also curator of geology at the Buffalo Museum of Science.

"He noticed a feature on the bone indicative of tuberculosis."

Rothschild, a practicing physician and an expert on ancient diseases, went on to study 113 mastodon skeletons across the continent. He found that 59 of them, or 52 percent, had the tuberculosis lesions.

Based on the finding, it's likely that virtually every late Ice Age mastodon in North America had tuberculosis, Laub says.

Tuberculosis causes grooved erosion in bones. While the disease didn't kill the ancient animals directly, it certainly weakened them, Rothschild says, likening their deaths to events that lead up to an airplane crash.

"You don't have a crash from one thing going wrong," he said. "It's multiple factors."

The research findings were published recently in the German science journal Naturwissenschaften.

Large, Adaptive Animals

Remains of mastodons have been found throughout North America from Alaska to Mexico.

"That's interesting in itself," Laub said. "All the evidence shows that mastodons were exceedingly adaptive."

The appearance of tuberculosis in an animal as large and adaptive as the mastodon could give scientists insight on how it spread, Laub says.

"There's interest in trying to understand why so many species of large North American mammals became extinct at the end of the Ice Age."

A popular theory explaining their demise is that they were hit with a hyperdisease—a disease that spreads rapidly through an entire species.

Tuberculosis alone, however, was not powerful enough to bring down the mastodons.

"It would have been one of a number of factors that conspired against them," Laub explained.

These findings also show that it's possible for a disease to work its way rapidly through a species, he adds.

Scientists believe there were other contributing factors leading up to their extinction, such as the environmental change brought on by the end of the Ice Age.

Humans also may have hunted them, the stress of which could have had a negative effect on their reproductive cycle. Mastodons are thought to have had long gestation periods and produced few offspring in a lifetime.

"If they were infected with tuberculosis, that could have had an weakening effect on their physiology," Laub said.

Mass Extinction Mystery

Findings such as this latest report can help scientists shed light on the age-old question of why species die out.

The mass extinction that included the end of the mastodons was among the most recent to occur.

(See a National Geographic magazine feature on the massive Permian extinction 250 million years ago.)

"Extinction is part of evolution, and if we can understand some of the processes that cause these mass extinctions, then we've learned quite a bit about an important mechanism that drives evolution," Laub said.

Scientists are now able "to really go after the genome of ancient tuberculosis and have the option of comparing it" to modern strains of the disease, Rothschild said.

With further study, scientists could possibly identify what's causing resistance in modern strains of tuberculosis.

"It's given people a better understanding of what the disease's effects are," he said.

According to Alex Greenwood, a researcher of DNA found in fossils based at Old Dominion University in Norfolk, Virginia, hyperdisease is the minority theory.

He admits he was initially skeptical of the report's findings.

"It's hard to call [tuberculosis] a pandemic disease in a hyperdisease context based on these results," he said.

"This is a pandemic that existed over a long period of time rather than … something that became more common at the end of the Pleistocene [Ice Age], putting new pressure on mastodon populations.

"There is also a possibility [that] more recent soil and environmental contaminants could have affected the bones after the animals died," he said.

"Still, it's an interesting find."

It also lends some rare and welcome physical evidence to the ongoing debate over mastodon extinction.

Some competing theories of what killed the animals, such as climate change or overhunting by humans, offer only sparse physical evidence, Greenwood says.

"If you find something like this, especially if you could get clear molecular data, it is actual hard data. It's a nice start," he said.

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