There are over 5,400 species of mammals living on Earth today. Despite huge differences in shape and size, the vast majority have one anatomical feature in common: the number of bones in their necks. In fact, nearly all mammals, from cats and dogs to elephants and giraffes (and humans!) have seven cervical (neck) vertebrae. It is simply the scale, rather than the number, of those bones that makes a difference to their anatomical proportions.
The only known exceptions are manatees and two-toed sloths, which have five or six cervical vertebrae, and three-toed sloths, which have up to ten. Interestingly, it is common among birds and reptiles to have drastically different numbers of cervical vertebrae; a swan has 24 neck bones, while a duck has just 16.
Scientists have conducted extensive research in an attempt to discover why sloths are among the few mysterious mammalian outliers. One prevailing theory about three-toed sloths is that the extra vertebrae in their necks are actually ribcage vertebrae (but without ribs). Due to the position of their shoulders, pelvis, and ribcage, which are all linked with one another, their vertebral column has shifted downward, thus extending the neck.
According to evolutionary biologist Frietson Galis of the Naturalis Biodiversity Center in the Netherlands, "We think that it is the slow lifestyle and low metabolic rate which has allowed evolution to alter the neck length of sloths without any of the side effects seen for other mammals. Their low metabolic rates protect them from cancer and their low activity rates protect them from thoracic outlet syndrome."
Magnificent mammals:
- Despite their differences, all mammals derive from one common ancestor that lived around 200 million years ago.
- Giraffes use their necks in courtship battles. In a behavior known as “necking”, male giraffes will swing their heads into each other’s ribs and legs while battling over a mate.
- Unlike most mammals, sloths have lost the ability to thermoregulate. Their body temperatures can drop to as low as 84°F (28°C).
