Woolly mammoths went extinct at the end of the last ice age, about 10,000 years ago. But were the animals wiped out by increasing human populations, or were other forces at work? New research shows woolly mammoths may have been driven to extinction, at least in part, by inbreeding. 

The species appears to have suffered from a dramatic increase in the frequency of birth defects in the centuries leading up to its demise, new study says. 

Fossils of mammoths show a large number of the animals were born with extra vertebrae called cervical ribs in their necks. These features are usually harmless, but indicate unusual changes occurred during development of the fetus. 

Cervical ribs can also point to other skeletal problems, including failure of vertebrate bones in spinal columns to fuse properly. The extra ribs could also indicate neck muscles in the ancient animals were fused together and may suggest an increased risk of cancer. 

Investigators compared 12,000-year-old fossils of woolly mammoths found near the North Sea to modern elephants. They found cervical ribs were present in one-third of the nine fossils studied, making the deformity 10 times more common in the ancient animals than they are in their modern-day descendent. Just one of the 21 modern elephants studied showed a similar feature.

"It had aroused our curiosity to find two cervical vertebrae, with large articulation facets for ribs, in the mammoth samples recently dredged from the North Sea. We knew these were just about the last mammoths living there, so we suspected something was happening. Our work now shows that there was indeed a problem in this population," Jelle Reumer, paleontologist at the Natural History Museum Rotterdam in the Netherlands, said

Cervical ribs are dangerous birth defect in humans, killing up to 90 percent of victims before they reach adulthood. 

"The incidence of abnormal cervical vertebral numbers in mammoths appears to be much higher than in other mammalian species, apart from exceptional sloths, manatees and dugongs," Reumer and his co-authors wrote in the article announcing their results. 

Rising temperatures in the Late Pleistocene era, combined with an increase in birth defects, may have doomed the species to extinction. Researchers speculate that as climate change forced these animals into smaller regions, inbreeding became more common. Within generations, the genetic diversity decreased, making populations more susceptible to parasites and viruses. 

These giant animals are among the most-studied of all species from the period. As technology for genetic sequencing improves, it may be possible to test this theory using fragments of DNA from the animals. 

The study was published in the open access journal PeerJ.

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