Watching a sauntering American alligator (Alligator mississippiensis) is like stepping back to the Jurassic period. Slapping the soles of their feet down, their stately flat-footed posture provides stability, whether their legs are splayed wide or holding the body up. Yet, their prehistoric gait has more in common with the flat-footed strides of bears and humans. How alligators and their crocodile close cousins walk has intrigued scientists for almost a century, but no one had paid much attention to what goes on below the ankle. ‘How do the bones in the feet move to help alligators maintain foot-ground contact across postural extremes from belly sprawls to high walks and sharp turns?’, asks Morgan Turner from Brown University, USA. To learn more about how the bones in the soles of the feet – the metatarsals – adapt as these creatures stride and turn, Turner and colleague Stephen Gatesy needed the reptiles to walk and manoeuvre in front of a pair of X-ray cameras to reveal shifts in the bones in 3D.
As adult American alligators are large and unpredictable, Turner and Gatesy collected three young females from an alligator colony at California State University, San Bernardino, USA, to put through their paces. Back at Brown University, the duo implanted minute metal markers at both ends of the first and fourth metatarsals of the alligators’ hind feet; ‘the metal markers and bones are clearly visible in the X-rays and are both important for how we reconstruct movement of the skeleton’, Turner explains. Then, she encouraged the reptiles to saunter on a treadmill at a speed they chose, in the hope of getting them to walk with straight and splayed legs. In addition, she recorded the foot bones’ shadowy movements as the alligators explored an enclosure. ‘The animals could easily move out of the soccer ball-sized X-ray camera field of view, so we used dark spaces, which they liked to move towards, along with the walls of the arena and my physical position, to try to anticipate where a manoeuvre would happen’, Turner recalls. Then, she painstakingly identified 24 clear strides, as well as 13 manoeuvres – turns and pivots from both left and right feet – before reconstructing the 3D motions of the four metatarsals.
Viewing the metatarsal reconstructions as the alligators walked on straight legs, the duo saw the tips of the splayed bones spread by as much as 200% when the foot was in contact with the ground, collapsing together as the foot lifted off. And when Turner monitored the arrangement of the metatarsals relative to each other, they remained fairly flat and parallel to the floor while in contact with the ground, only tipping down as the heel lifted from the ground halfway through the stride.
In contrast, the bones shifted dramatically relative to one another as the alligators turned and pivoted, even though externally the foot appeared to remain flat on the floor. As well as splaying by up to 256%, the bones rotated around their attachment to the heel, with the outermost metatarsals rotating upward or downward as the alligator pushed off in different directions.
‘We were surprised to see just how mobile the metatarsals were, even when in full contact with the ground. Contrary to a simple depiction of a flat, immobile metatarsus, the four weight-bearing metatarsals are able to “skew” and break out of a planar configuration’, Turner says. And now she is turning her attention to the animals’ ankles. ‘Alligators have a distinct peg-and-socket ankle joint that has defined a major group of extinct crocodile relatives for 250 million years’, she says, hoping that these living fossils can teach us how their forebears roamed the planet.