Lucy Roberts, PhD student in the Museum of Zoology writes:
I often spend swathes of the working week wandering around the museum galleries and stores, marveling at the extent of the collections and regularly being surprised by new (to me), amazing specimens.
My research is focused on axial skeleton (spine) diversity, so on my museum wanders I habitually count vertebral elements.
When I first walked around the museum’s lower gallery after the sloths had been mounted I made a beeline, squinted toward their necks and started counting. This was because, despite the fact that I study reptiles, the tree sloths have my respect as known vertebral mavericks. A characteristic trait of mammals is that we have seven vertebrae in our necks. Long-necked or short, guinea pig to giraffe, seven neck (cervical) vertebrae is the rule. This rule is followed in all mammals except for manatees (which consistently have 6 cervical vertebrae) and tree sloths.
Tree sloths blow the rule to pieces. Three-toed sloths (Bradypus) have 8 to 10 cervical vertebrae, which enables them to rest their head on their chest and turn their heads through 270˚. Two-toed sloths (Choloepus) have 5 to 6 cervical vertebrae, which contributes to their stiffened neck posture. Moreover, the number of neck vertebrae varies not only between species of sloth but also between individuals of the same species. The conclusion of my peering at the museum’s mounted specimens is that the three-toed sloth (left of picture) has 10 cervical vertebrae, and the two-toed specimen (right) has 6.
Why is it then that these particular mammals appear to be able to break the mould? Changes to the number of cervical vertebrae can occur in other mammals, but when they do, they are associated with further developmental abnormality and medical problems. This includes, for example, increased rates of juvenile cancer. The famous slow-pace of life that sloths enjoy, in regard to physical exertion and metabolic rate, reduces the negative impacts of their vertebral modifications.
The mechanisms underlying these shifts in spinal structure are endlessly fascinating. During the embryonic development of any animal, certain genes act to orchestrate each part of the process. One particular group, the Hox genes, determine the pattern of vertebrae that form from neck to tail. This Hox code is pretty consistent in mammals, but the wild variation we see in tree sloths shows that their developmental toolkit differs from the mammalian norm.