Crocodiles are masters of deception. I’m not talking about their ability to perfectly conceal themselves until the moment they burst from the water, jaws agape. I mean that they’re not as sluggish as they might seem. Yes, crocodiles often drag their bellies along the riverbank, but they get around in other ways. Not only can they push themselves into a scaly strut called a “high walk”, crocodiles can bound and gallop.
For experts who study animal mechanics, “bound” and “gallop” have specific meanings. A crocodilian bound is distinguished by the animal’s forelimbs hitting the ground simultaneously with the hindlimbs pushing off quickly after, while a gallop is a four-beat sequence in which the fore and hindlimbs touch off in turn.
Only little crocodiles – under six and a half feet long – have been seen to take off bounding and galloping. But the behaviors have been seen across five different crocodile species, as well as in their distant cousin the gharial. And that’s what makes alligators seem strange by comparison. With the exception of one instance when researchers thought they saw an American alligator possibly trying to gallop for a split-second in the middle of a stride, no one has ever seen alligators take off the way their cousins do. Why?
Muscles may be the answer. After studying 40 alligators and crocodiles representing six different species, Vivian Allen and his colleagues at the Royal Veterinary College’s Structure & Motion Lab found that crocodiles have bundles of muscle fibers better suited to bounding and galloping. Their muscle bundles are longer and thinner than those of alligators, especially in the forelimbs and chest. This means that crocodiles can cycle their limbs quicker and get a little more reach with each step, offering the flexibility to bound and gallop.
Of course, someone may one day observe a little alligator galloping merrily along. But barring such a discovery, the muscular differences seem to account for why various crocodiles have been seen to bound and gallop and the well-studied American alligator never has. And the new study also cuts down to an often-forgotten truth about these reptiles. Alligators and crocodiles are not near-identical holdovers from prehistory. Alligators and crocodiles are as different from each other as we are from our closest primate relatives, if not more so, and we are only just beginning to look beyond our mammalian blinders to see the reptiles as they truly are.
Allen, V., Molnar, J., Parker, W., Pollard, A., Hutchinson, J. 2014. Comparative architectural properties of limb muscles in Crocodylidae and Alligatoridae and their relevance to divergent use of asymmetrical gaits in extant Crocodylia. Journal of Anatomy. 225 (6): 569-582. doi: 10.1111/joa.12245