Cracking the Mystery of How Sloths Got Long Necks

NO RIBS? An anatomical quirk may tell why sloths have up to 10 vertebrae.

It is not quite how the elephant’s child got his long nose, but still it is research worthy of Rudyard Kipling: scientists said Monday that they have figured out how sloths got their long necks.

Throughout the animal kingdom, most mammalian creatures, from mice to giraffe, have a seven-vertebrae neck.

Sloths, however, are a puzzling exception. They can have as many as 10 vertebrae, posing one of the enduring enigmas for scientists, who have long wondered what explains the anatomical quirk.

Scientists at the University of Cambridge in England said they now think they have the answer.

After analyzing the development of the vertebral column in sloths they made a startling discovery: the part of the skeleton which they had long believed to be part of the sloth rib cage is, in fact, analogous to the bottom of the mammal’s “neck.”

In other words, the bottom neck vertebrae of sloths show a similar sequence of development as the top rib cage vertebrae of other mammals, both of which start at eight vertebrae down from the head.

The research, published Monday in the journal The Proceedings of the National Academy of Sciences, showed that the bottom “neck” vertebrae of sloths are developmentally the same as rib cage vertebrae of other mammals — just without ribs.

“Even though they’ve got eight to 10 ribless vertebrae above the shoulders, unlike the seven of giraffes, humans, and nearly every other species of mammal, those extra few are actually rib cage vertebrae masquerading as neck vertebrae,” said Robert Asher, of the zoology department at Cambridge.

By observing the position of bone formation within the vertebral column, they determined that all mammals, including sloths, are similar in when they develop the eighth vertebra down from the head — whether or not it is actually part of the neck.

The unusual anatomy has to do with how the sloth evolved millions of years ago, in contrast to other mammals.

The Cambridge researchers said the new results support the interpretation that the limb girdles and at least part of the rib cage derive from different embryonic tissues than the vertebrae, and that during the course of evolution, they have moved in concert with each other relative to the vertebral column.

In sloths, the position of the shoulders, pelvis, and rib cage are linked with one another, and compared to their common ancestor shared with other mammals, have shifted down the vertebral column to make the neck longer, the researchers said.


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