A new study of ancient woolly mammoth fossils suggests that they have suffered from high-rates of birth defects involving an extra cervical rib. This finding implies that there was a problem with their population that might have led to their extinction.

Researchers from the Natural History Museum of Rotterdam and Utrecht University discovered that remains of the several mammoths found near the North Sea from the late Pleistocene period has more than the average, which is constant at seven. This extra cervical rib is situated along their neck vertebrae.

In the study, they looked at the collections of the Natural History Museum of Rotterdam and the Naturalis Biodiversity Center in Leiden, Netherlands. They scoured the mammoth fossils and found 16 neck vertebrae -- six pieces of the sixth cervical vertebrae and 10 of the seventh cervical vertebrae. They analyzed nine of those and found that three had once been attached to the cervical ribs.

They were only able to examine a small sample of bones, but the results surprised them, especially when they compared it to the rate of the same birth defects in modern elephants. They found that the rate in ancient mammoths is 10 times higher.

The high rate of birth defects in the mammoths is either caused by inbreeding that brings a higher rate of genetic defects and extra cervical ribs than the average or prenatal stress that were probably caused by scarcity or illnesses in female mammoths in the years preceding their extinction.

"The high incidence and large size of the cervical ribs indicates a strong vulnerability, given the association of cervical ribs with diseases and congenital abnormalities in mammals," said study researcher Jelle Reumer, a paleontologist, and his colleagues in a statement to LiveScience. "The vulnerable condition may well have contributed to the eventual extinction of the woolly mammoths."

Further details of this study can be read in the March 25 issue of PeerJ.