Extra rib may be sign of mammoth decline

by Timothy Oleson
Friday, August 8, 2014

High rates of a congenital defect in woolly mammoths may offer evidence that inbreeding and environmental stress contributed to the animals' demise during the Late Pleistocene, according to a new analysis of fossil mammoth neck bones.

Scientists led by Jelle Reumer of the Rotterdam Museum of Natural History in the Netherlands searched museum collections of neck vertebrae from Late Pleistocene specimens of Mammuthus primigenius that had inhabited the North Sea region. They found that one-third of the fossils showed evidence of stubby bones called cervical ribs projecting from the sides of the vertebrae. By comparison, the researchers reported in the journal PeerJ that only 3.6 percent (one in 28) of vertebrae analyzed from modern Asian and African elephants — the closest living relatives of the mammoths — featured a cervical rib, a 10-fold difference.

Most mammals have seven cervical vertebrae in their necks, which, unlike vertebrae farther down the spine, are not attachment sites for ribs. However, cervical ribs — protruding from the sixth or seventh neck vertebrae — do occur in small but variable proportions of different mammalian species; less than 1 percent of humans have them. Although the extra bones are generally considered benign themselves, their occurrence is often accompanied by other congenital defects, signifying the role of genetic or environmental stress during early embryonic development.

Reumer and his colleagues suggested that the apparently high occurrence of neck ribs among Late Pleistocene North Sea mammoths could be symptomatic of harsh environmental conditions at the time, as well as population stresses on the animals that led to inbreeding. Such a “vulnerable condition may well have contributed to the eventual extinction of the woolly mammoths,” they wrote.

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