As a rule, all mammals have the same number of vertebrae in their necks, regardless of their necks’ length. Among other animals, like birds, reptiles and amphibians, there’s a little more variety: the long, slender necks of swans have 22-25 vertebrae, while bullfrogs’ necks have just one. Mammals, though – whether they’re a Kitti’s Hog-nosed Bat (the smallest mammal), a blue whale (the biggest) or anything in between – always have seven.
There appears to be good reason to follow the trend. Too many or too few neck vertebrae are associated with stillbirth, childhood cancer, neuronal problems and misplaced or crushed nerves, muscles and blood vessels in humans and some other mammals. Any change in the vertebrae number is probably selected against to avoid these problems, conserving basic mammal body plans in the process.
Rules are made to be broken, though, and both sloths and manatees have abnormal numbers of neck vertebrae. Two-toed sloths (Choloepus) have five to seven neck vertebrae, three-toed sloths (Bradypus) have eight or nine and manatees (Trichechus) have six.
Neither sloths nor manatees seem to suffer from the problems that other species have when they diverge from the seven-vertebrae template, though, and a team of scientists from Austria and the Netherlands think they know how they animals are getting away with it. [Read more]