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Study Solves Mystery Of Long-necked Sloths

October 19, 2010

New research by scientists at the University of Cambridge in England gives insight into how sloths, one of the few mammals with more than seven neck vertebrae, evolved their uniquely long necks.

The mystery of how the three-toed sloth came to have as many as 10 neck vertebrae has long puzzled scientists, given that most of the 5,000 mammal species have exactly seven vertebrae in their necks.

Other animals, such as birds and lizards, vary greatly in the number of vertebrae in their neck.   A swan, for instance, may have twice as many neck vertebrae as a songbird. 

But mammals show far less variation, with giraffes, humans, mice, elephants and armadillos ““ all mammals ““ having exactly seven neck vertebrae.  Sloths are a rare exception, having as many as 10 neck vertebrae.

To determine if patterns of bone formation in sloths might provide clues to their divergent vertebral anatomy, the scientists investigated the skeletal development among mammals, focusing on the vertebral column in three-toed sloths.

They found that 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. However, in all mammals except for sloths, bone formation always took place earlier in the body of the first few vertebrae of the ribcage, rather than in the neck.  

Sloths, on the other hand, show early bone-formation in the bodies of their distal, ribless neck vertebrae, before those of the ribcage.

The scientists found that the part of the skeleton long believed to be part of the sloth ribcage 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 ribcage vertebrae of other mammals, both of which start at eight vertebrae down from the head.   Therefore, the bottom “neck” vertebrae of sloths are developmentally the same as ribcage 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 ribcage vertebrae masquerading as neck vertebrae,” said Dr. Robert Asher of the Department of Zoology at the University of Cambridge.

The sloths’ atypical anatomy is tied to differences in how they evolved millions of years ago compared with other mammals.  In sloths, the position of the shoulders, pelvis, and ribcage 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 Cambridge scientists said.

The study supports the interpretation that the limb girdles and at least part of the ribcage 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, they said.

The research was published Monday in the journal Proceedings of the National Academy of Sciences.

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