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.
One hypothesis on how sloths and manatees come up short or long in the vertebrae department is that one of the genes that controls the development and differentiation of the vertebrae regions (there are five) mutates or is expressed abnormally and causes incorrect patterning in the skeleton. These genes are often pleiotropic (that is, they influence multiple traits), so a mutation in a single gene can cause multiple abnormalities in different parts of the body.
Studying the skeletons of sloths and manatees and comparing them to related animals with the normal number of vertebrae and ones with odd vertebral patterns caused by mutation, Irma Varela-Lasheras and colleagues from the Netherlands Centre for Biodiversity Naturalis and the International Institute for Applied Systems Analysis found plenty of anatomical evidence that genes are the problem. They found that sloths and manatees have many of the same skeletal malformations that are common to other species with Hox gene (the genes that determine body structure) mutations, including lab mice engineered to have Hox problems. They also found other skeletal abnormalities – fused vertebrae, defective cartilage production and asymmetric ribs – that could only be explained as side effects of genetic malfunction.
These types of mutations are consistently selected against in all other mammals, but sloths and manatees lumber on with the associated skeletal problems and not a care in the world about them. They don’t suffer from incidences of cancer like other vertebral rule breakers do, either. The researchers think that being slow and steady is the trick. The Hox mutations aren’t selected against because the animals’ slowed-down lives protect them from the mutations’ negative effects. Low intensity lifestyles minimize the problems caused by skeletal malformations and slow metabolisms reduce their risk of cancer. Left to run wild, the Hox genes allow the animals to break free of the standard body mammal plan and stretch their strange necks in new directions.
Reference: Varela-Lasheras I, Bakker AJ, van der Mije SD, Metz JA, van Alphen J, & Galis F (2011). Breaking evolutionary and pleiotropic constraints in mammals. On sloths, manatees and homeotic mutations. EvoDevo, 2 (1) PMID: 21548920
Images: “Manatee at the Aquarium of Veracruz, Mexico” by AlejandroLinaresGarcia; “Sloth in the Amazon” by Praziquantel; “A Manatee Skeleton with Calf” by Sklmsta. Used under a Creative Commons License.