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Three-Toed Sloths Saved From Excess Necking

Posted by Richard Conniff on October 19, 2010

One of the great mysteries in mammalogy appears to be solved, and it gives me an excuse to republish this adorable photo.  Here’s part of the press release from the University of Cambridge:

By examining the development of bones in the vertebral column, limbs, and ribcage, scientists at the University of Cambridge have discovered how sloths evolved their unique neck skeleton.

From mice to giraffes, mammals are remarkable in that all but a handful of their 5000 species have exactly seven vertebrae in the neck. Among the few that deviate from this number are three-toed sloths, which may have up to ten ribless vertebrae in the neck.

Traditionally, vertebrae above the shoulders that lack ribs are known as cervical or neck vertebrae. Animals such as birds and lizards show great variety in the number of vertebrae in their neck. For example, a swan may have twice as many as a songbird.Mammals, on the other hand, are much more conservative. A giraffe has the same number of neck vertebrae as a human, mouse, elephant, or armadillo; all have exactly seven.

To find out why the three-toed sloths was an exception, zoologist Robert Asher and his co-authors studied its embryological development.  They found that in other mammals, the top vertebrae of the ribcage develop first, followed by the bottom vertebrae of the neck.  But the sloth develops the bottom vertebrae of the neck first.  And that is because they are actually rib vertebrae, minus the ribs.  That is, the sloth is not so odd after all, at least in this regard, and conforms to the larger pattern of mammalian developed.  The press release continues.:

Sloth skeleton with "neck" in redTo find out why the three-toed sloths was an exception, zoologist Robert Asher and his co-authors studied its embryological development. They found that in other mammals, the top vertebrae of the ribcage develop first, followed by the bottom vertebrae of the neck. But the sloth develops the bottom vertebrae of the neck first. And that, stupid, is because they are actually rib vertebrae, minus the ribs. That is, the sloth is not so odd after all, at least in this regard, and conforms to the larger pattern of mammalian developed. The press release continues.

All mammals, including sloths, show early development of the body of the eighth vertebra down from the head, whether or not it is part of the neck.In other words, the bottom neck vertebrae of sloths show a similar sequence of development as the top ribcage vertebrae of other mammals, both of which start at eight vertebrae down from the head. This shows that the bottom “neck” vertebrae of sloths are developmentally the same as ribcage vertebrae of other mammals, but lack ribs.

Dr Robert Asher, of the Department of Zoology at the University of Cambridge, said: “The remarkable conservatism of the mammalian neck is apparent even in those few species that superficially seem to be exceptions, like sloths. Even though they’ve got eight to ten ribless vertebrae above the shoulders, unlike the seven of giraffes, humans, and nearly every other species of mammal, those extra few are actually ribcage vertebrae masquerading as neck vertebrae.”

Shoulders, pelvis, and ribcage are linked to one another in three-toed sloths, and have simply “shifted down the vertebral column to make the neck longer,” according to the press release.

The press release does not suggest why the sloth might have developed this way, and unfortunately I am unable to access the original publication at the moment.  But the traditional interpretation of the three-toed sloth’s excess neckiness is that it evolved to enable the sloth to reach around and eat the available leaves as it hangs from the branch of a tree.   The new study does not appear to change that interpretation.

You can read more about sloths in my book Every Creeping Thing:  True Tales of Faintly Repulsive Wildlife.

The paper ‘Skeletal development in sloths and the evolution of mammalian vertebral patterning’ by Lionel Hautier, Vera Weisbecker, Marcelo R. Sánchez-Villagra, Anjali Goswami, and Robert J. Asher appears the 18 October 2010 edition of the Proceedings of the National Academy of Sciences.

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