Unusual rib bones that grow out of the neck are giving scientists new clues about what caused the woolly mammoth to become extinct roughly 10,000 years ago.
The so-called cervical ribs – extra rib bones that protrude from the vertebrae at the base of the neck – were about 10 times more common in mammoths living in the Late Pleistocene than they are in elephants alive today, according to a study by Dutch researchers published Tuesday in the journal PeerJ.
The cervical ribs themselves aren't necessarily dangerous, but they tend to appear in animals that failed to develop normally during the early stages of pregnancy, and other problems associated with abnormal development can be fatal. In humans, for instance, about 90% of babies born with a cervical rib die before they are old enough to reproduce, according to a 2006 study in the journal Evolution.
The authors of the new study became interested in cervical ribs in mammoths after mammoth fossils were dug up in the Netherlands during a public works project to extend Rotterdam Harbor into the North Sea. Three of the bones were from the lower portion of the neck, just above the part of the spine that connects with rib bones. But two of the three specimens showed signs of having been connected with ribs. (You can see the mark on the bones in the photo gallery above.)
That chance discovery made the researchers wonder just how common cervical ribs were."We knew these were just about the last mammoths living there, so we suspected something was happening," study leader Jelle Reumer, a paleontologist at Utrecht University and director of the Natural History Museum Rotterdam, said in a statement.
Reumer and two colleagues looked in the collections of the Natural History Museum and the Naturalis Biodiversity Center in Leiden, both in the Netherlands, and found 16 samples of mammoth vertebrae from the base of the neck. Seven of the samples were missing the part that would have clued the researchers in on whether a cervical rib had been attached. Of the remaining nine, six were normal and three once had a cervical rib. That worked out to an incidence of 33.3%.
For the sake of comparison, the researchers also analyzed bones from 21 Asian elephants and seven African elephants that were in museum collections in Europe. Among the 28 elephants, only one had a cervical rib. The incidence, therefore, was 3.6%.
The mammoths were nearly 10 times more likely than the elephants to have a cervical rib, leading researchers to conclude that the incidence of the errant ribs was "extremely high" among the mammoths. Not only were they unusually common, but the shape of the vertebrae suggested the bones were unusually large as well, according to the study. In mice with cervical ribs, those with bigger bones have more health problems than those with smaller bones, the researchers noted.
"There was indeed a problem in this population," Reumer said.
There are two explanations for the high incidence of cervical ribs in mammoths, and both are probably part of the story, the researchers wrote. The first is that the mammoths were interbreeding, which would be expected among a dwindling population. The second is that the mammoths were struggling with famine, disease and other environmental conditions that made it difficult for female mammoths to bear healthy offspring.
"A combination of inbreeding and harsh conditions may be the most likely explanation for the extremely high incidence of cervical ribs," the research team concluded. The animals' vulnerability "may well have contributed to the eventual extinction of the woolly mammoths."