A tuberculosis pandemic affecting an ancient elephant-like creature probably contributed to the great beast's demise, a new study suggests.
Scientists examining American mastodon skeletons found a type of bone damage in several foot bones that is a unique symptom of tuberculosis.
The disease would have weakened and crippled the animals, making them more vulnerable to humans and climate change, two factors that scientists have long speculated were behind the mastodon's extinction in North America.
Mastodons were ancient pachyderms that resembled woolly mammoths, but were shorter, less hairy and more distantly related to modern elephants.
Both mastodons and several species of mammoths lived in North America and disappeared mysteriously, along with other large mammals, around the end of the last major Ice Age 10,000 years ago.
A crippling disease
Researchers Bruce Rothschild of the Northeastern Ohio Universities College of Medicine and Richard Laub of the Buffalo Museum of Science in Buffalo, N.Y., looked at 113 American mastodon skeletons and found signs of tuberculosis in 59 of them — 52 percent of the total.
Tuberculosis is a bacterial infection that commonly infects the lungs. It can also affect other parts of the body, including organs and bones.
In humans, only about 1 to 7 percent of infected individuals develop bone damage.
The fact that more than half of the mastodon skeletons examined had the bone lesions suggests that tuberculosis was a "hyperdisease" for American mastodons, afflicting a large percentage of the population.
When tuberculosis infects bone, it creates a tell-tale type of damage in which bone beneath cartilage is carved out, or "excavated."
The infected mastodons were different ages and sizes, came from all over North America and 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 it went extinct.
That the disease was widespread and yet persisted for so long in the species suggests it was not immediately lethal, Rothschild said. Instead, it was probably a chronic disease, one that gradually weakened rather than killed the animals.
In humans, tuberculosis can lay dormant for several years after initial infection, repressed by the body's own immune system. But it can flare up into full-blown disease during times of stress.
A similar flare-up probably happened with the mastodons during times of stress, Rothschild said.
Mastodons living at the end of the last Ice Age had reasons to be stressed. They faced not only a drastically changing world brought about by rapid climate change, but also the arrival of a new threat: weapon-wielding humans who hunted them for food.
Together, these three factors — disease, climate change and humans — might have been too much for the creatures.
Weakened by tuberculosis, the beasts would have been less able to ward off other diseases, and the crippling bone damage would have affected their ability to walk.
"Extinction is usually not a one-phenomenon event," Rothschild told LiveScience.
A route of infection
But how did North American mastodons first get tuberculosis, a disease for which the first known documentation is in a 500,000-year-old buffalo from China?
Rothschild thinks he knows the answer. In a separate study, he and Larry Martin from the Natural History Museum in Kansas found similar tuberculosis-caused bone damage in North American bovids, a group of animals that included 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.
Bison and other bovids are believed to have originated in Asia and crossed into North America using the Bering Land Bridge, which connected the two continents. Humans made the same journey much later.
The researchers speculate that some of the bovids were probably already infected with tuberculosis when they migrated into the New World.
Once in North America, the bovids could have spread to mastodons and other species, possibly even humans, Rothschild said.
Both the mastodon and bovid studies will be detailed in upcoming issues of the German science journal Naturwissenschaften.
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