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.

A team of researchers at the Institute of Evolutionary Sciences at the University of Montpellier, France who surveyed the existing literature on the evolutionary perspectives of left-handedness, including its mechanisms and the costs and benefits acting as selective forces on the left-handed, say they may have found the secret to southpaw survival. We lefties simply had a tactical advantage in one-on-one competition.

The team’s study suggests that because lefties are in the minority, right-handed opponents may not have been used to the way they fight, and the element of surprise gave lefties an advantage. Their very uncommonness, and a good left hook jab, gave them an edge.

Because the advantage allowed them to survive physical confrontation and win resources and mates, left-handedness became more frequent over the generations through natural selection.

We can see sort of the same thing happening in the success of left-handed boxers like “Gentleman Jim” Corbett and Oscar de la Hoya and left-handed tennis players like John McEnroe and Jimmy Connors. Death isn’t on the line at Wimbledon, but everyone loves a winner, so they attract more sex partners and are more likely to reproduce.

The researchers also noted that lefties in many European countries have higher average incomes and are well represented among gifted children with high IQs. Although an advantage in fist fighting doesn’t explain that, a place at the top of the socio-economic ladder certainly promotes reproductive success, so smarts and cash should result in higher birth rates for lefties and the passing along of left-handedness.

Reference: Llaurens, V., Faurie, C. and Raymond, M. 2009 : Why are some people left-handed? An evolutioanry perspective. Philosophical Transaction of the Royal Society B. doi:10.1098/rstb.2008.0235