Which of these mammals do NOT have 7 neck bones?

Nearly all mammals have the same number of cervical vertebrae no matter how long or short their necks are – humans, giraffes, mice, whales, and platypuses all have exactly seven cervical vertebrae.

There are only three exceptions to this rule out of over a thousand genera of mammals – manatees (which usually have six) and both types of living sloths (two-toed sloths usually have six, three toed sloths nine).

One theory behind these abnormalities in the number of cervical vertebrae for these animals is that their slow lifestyles and low metabolic rates allowed them to survive mutations that resulted in changes in the lengths of their necks. Whereas for most other animals, such a mutation likely would have been fatal, these slow-moving animals were able to adapt to these mutations more easily.

One consequence of having these unusual number of neck bones is that the two-toed sloths have more limited abilities to rotate their heads, about 90 degrees, while the three-toed sloths can rotate their heads up to 270 degrees. Manatees cannot rotate their heads, so they must turn their entire body around to look around.