This quirky ability is due to the fact that this species of sloth possesses a much thinner and less muscular neck than most other species. All sloths also have an extra vertebra in their spines. As vertebrates develop, their bones ossify as solid material is laid upon them. Strangely enough, a sloth’s bottom neck vertebra solidifies much earlier than those found in other mammals.
This bone, normally located at the top of a mammal’s ribcage, seems to have been modified from the sloth’s ribs themselves as a fetus! It is not known if this additional bone provides any sort of advantage for sloths, but the brown-throated varieties boast a much wider range of neck rotation than the rest of their brethren.