When the first crocodile ancestors took to the water, they moved more like upright creatures than the colossal crouched reptiles that we know today. Since then the crocodile's posture has become more sprawled as they have spent more time immersed in water waiting to ambush prey. Intrigued by the factors that have led animals to migrate from the land to an aquatic life, Julia Molnar from the Royal Veterinary College, UK, says, ‘We chose crocodilians as an example of something that started out very terrestrial and has become more aquatic’. Explaining that there has been a reasonable amount of work on the limbs of crocodilians, Molnar and her PhD supervisors, John Hutchinson and Stephanie Pierce, decided to focus on the reptile's backbone. ‘There has not been a whole lot of work on the backbone, which is obviously important for supporting the body and transmitting forces between the body and the limbs’, says Molnar. So, as many creatures take advantage of the flexibility of their spines to enhance movement, the trio decided to investigate the stiffness of the thoracic and lumbar joints in juvenile crocodiles (p.758).
Working on the bodies of young Nile crocodiles that had died of natural causes at a conservation centre in France, Molnar carefully removed the animals' spinal columns and painstakingly divided the columns into two-vertebrae portions, each with an intact vertebral joint. Then Molnar carefully attached one vertebra in a horizontal position to a wooden frame before hanging a weight from the unattached vertebra and measuring the amount that the vertebra bearing the weight had moved to calculate the stiffness of the joint. Rotating the vertebrae by 90deg increments to calculate the joint's stiffness as if it was wiggling side to side or bending up and down, Molnar repeated the procedure for all eight spinal joints along each crocodile's torso.
However, when she compared the stiffness of the joints along the back, Molnar was intrigued to find that the joints in the lower back – the lumbar region – were stiffer than she had expected. ‘In a typical mammal they seem to have very stiff thoracic joints and mobile lumbar joints and the lumbosacral joint is often the most compliant’, explains Molnar. However, the crocodile lumbar vertebral joints were much stiffer than the thoracic joints in all directions. ‘It was a big surprise for us’, says Molnar. Also, the spine as a whole was more flexible from side to side, which is the opposite of the mammalian spine, but possibly provides the crocodiles with the flexibility required for their sprawled gait.
Molnar admits that the team was amazed that the crocodile's spine was so different from the mammals'. She explains that smaller crocodiles are even able to bound and gallop like modern upright mammals, so the team had thought that the crocodile's spinal column might be more similar to those of modern upright mammals. However, when Molnar scanned the literature to see whether there were any other species that had a similar stiffness distribution along the torso's vertebral joints, she realised that dolphins also have relatively inflexible lower spines, suggesting that the pattern could be associated with a return to the water.
‘This is one of those studies that says it is more complicated than we thought’, laughs Molnar, who is keen to find out how other structures in the animal's back might contribute to its stiffness.