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Why Some Woolly Rhinos Grew Ribs on Their Necks

These misplaced ribs don’t grow unless something unusual is going on in the gene pool

A woolly rhinoceros painted by a prehistoric artist on the wall of Chauvet cave in France (Public Domain)
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Some 350,000 years ago, the woolly rhino first browsed the shrubs and grasses of Europe and Asia with thick fleshy lips similar to their relatives living today. Unlike living rhinoceroses, however, these mammals sported shaggy coats and impressive humps. And just before they went extinct, it's possible the animals showed another morphological oddity: ribs growing from their neck.

A new study, published last month in the journal PeerJ, suggests that an unusually high proportion of woolly rhinos that lived between 35,0000 and 115,000 years ago sported these seemingly misplaced ribs in their neck, reports Susan Milius for Science News.

Researchers from the Naturalis Biodiversity Center in the Netherlands analyzed the neck vertebrae of 32 woolly rhinos in their collection as well as 56 skeletons from existing rhino species. In about 16 percent of the woolly rhinos, they found spots where ribs would have attached on the lowest neck vertebrae, or cervical vertebrae. This is not where ribs normally sprout. None of the modern rhino skeletons displayed such spots.

"Judging from the rib facets, the cervical ribs were quite large," the researchers write in the study.

In humans, such cervical ribs only affect about 0.5 to 1 percent of the population. A higher percentage can indicate the population is isolated, which have a higher chance of displaying abnormal genetic traits. Higher numbers also show up in children with cancer and in fetuses that did not survive.

The extra ribs may not have hurt the rhinos directly, but the researchers explain that such an unusually high percentage could indicate that the population was declining and inbreeding was increasing. That would have bumped up the number of genetic mutations, including some that could cause cervical rib development. Alternatively, stress during pregnancy could have created the extra ribs. "Diseases, famine, cold and other stressors can lead to disturbances of early development," the researchers write.

In earlier research,  Frietson Galis, one of the authors on the latest study, and colleagues found that woolly mammoths also had cervical ribs more frequently than expected. They suggest that cervical ribs could be a sign of a failing population.

Since the modern rhinoceros skeletons the researchers analyzed date from a time when populations were healthy, it's not surprising that they didn't find extra ribs. But sadly, with current rhino populations dwindling, it's possible they could soon have more data about these unexpected signs of the creatures' impending demise.

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