How do you tell an alligator from a crocodile?

The most obvious way to discern the two reptiles is to stare down their sinister snouts. Alligators have Ushaped faces that are wide and short, while crocodiles have slender, almost V-shaped muzzles. And if you’re daring enough, take a gander at their chompers. When an alligator closes its mouth, you tend to see only its upper teeth. Crocodiles on the other hand flash a toothy grin with their top and bottom teeth interlacing.

Many of the differences between the two centre on their heads and mouths. Now, researchers from Japan have identified what they believe to be another feature that sets the reptiles apart: alligators tend to have shorter humerus bones in their forelimbs and shorter femurs in their hind limbs than crocodiles.

“This information could help explain differences in their ecology and locomotion, including the strange fact that, while small crocodiles have been observed to bound and gallop, alligators have not,” says Julia Molnar, an evolutionary biologist from the New York Institute of Technology College of Osteopathic Medicine who was not involved in the study.

The differences are small, but the finding may provide insights into the ways in which the two reptiles move.

Masaya Iijima, a vertebrate paleontologist from Hokkaido University in Japan and lead author on the study, measured more than 120 alligator and crocodile skeletons from nearly a dozen museums across the world. Then he analysed the results using a statistical model. The specimens mostly belonged to extinct crocodilians, which is the supergroup that encompasses both alligators and crocodiles, as well as caimans and gharials.

Alligators and crocodiles diverged evolutionarily during the Late Cretaceous period some 80 million years ago. To put that into context, humans and chimpanzees split ways about 7 million years ago. Both reptiles also survived the mass extinction event that wiped out the dinosaurs, and since then have remained relatively unchanged. That includes the differences seen in their limb proportions, according to Iijima.