Author Spotlight: The Astonishing Bites of Crocodilians and How They Do It

15 06 2016

Ontogenetic bite-force modeling of Alligator mississippiensis: implications for dietary transitions in a large-bodied vertebrate and the evolution of crocodylian feeding

P. M. Gignac and G. M. Erickson

Alligators and crocodiles are impressive predators that can generate up to 3,000 and 4,000-pound bite forces, which they use to subdue large prey such as deer and wildebeests. To put that much force into perspective, being captured in the jaws of a large crocodilian is comparable to being pinned beneath a 2016 Ford Mustang, or having Dwayne “The Rock” Johnson stand on you with another dozen Dwayne “The Rock” Johnsons standing on him. You may have heard before that these animals are the living kings of chomp, brandishing the title for the most forceful bites measured in any animal to date. Not only is that true, but today’s species are not even the largest crocodilians to have ever lived—not by far.

So, how is it even possible for an animal to generate this much force?

Gignac_Alligator_BiteForce

Paul Gignac holding the head of a 12-foot American alligator during a bite force test. Photo by Joern Hurum

That is a super question, and it is the central focus of our new study. Our research team has been working with these animals for more than a decade, trying to understand their unique lifestyles and how they make a living. Before this research we measured the bites of every living crocodilian species, and we were astonished to discover how much force is behind them. In our new Journal of Zoology study we dissected a growth series of American alligators to examine how the jaw musculature changes across development. We documented muscle position, arrangement, and—particularly—fascicle orientations, which allowed us to build mathematical models of theoretical bite forces that incorporated muscle morphology, position, physiology, and activation. We tested these models against the bite forces we had measured directly (in vivo) for similarly sized animals, and our calculations turned out not to differ from the in vivo values.

We then went back into the experimentally validated models and looked at how muscle positions, sizes, and force outputs changed from hatchlings through large-bodied adults. We found that all muscles had meaningful contributions to those 3,000 and 4,000-pound bite forces, but one muscle in particular called the “ventral pterygoideus” was the major player. If you have ever seen a crocodilian for yourself, you might have thought that it had oddly thick neck. Well, it turns out, that is not all neck! The ventral pterygoideus muscle is so large that it spill out behind the head and wraps around the jaw, attaching from below and behind. This muscle makes up about half of the overall muscle mass that goes into generating a bite, but it contributes more than 60% of the bite force in most individuals and an astonishing 70% in exceptionally large adults.

This one muscle is so large that it could not fit inside the head, where the other jaw muscles are housed, even if it was the only one. Instead, it seems that over time this muscle was pushed backwards towards the animal’s neck, which allowed it to become massive while also maintaining the fairly low-profile skull that is typical of crocodilians. As a result, these near-shore predators can hide in shallow water to ambush animals that come too close for a drink, while also being capable of sustaining the thousands of pounds of bite force necessary to capture and consume such prey.

We also discovered a few particulars about this large muscle that are important. For instance, it has fascicles arranged in a chevron pattern (like a “V”), similar to those in your calf muscles or deltoids, which is a great way to pack a lot of force into a given volume of space. So not only is the ventral pterygoideus muscle enormous, but it is also particularly good at generating force, even for its large size.

It is this combination of traits that have helped crocodilians become masters of their domain since the age of the dinosaurs. Our next step is to figure out how they got to be this way by plotting out the evolution of the crocodilian jaw system across 240 million years of its evolutionary history from a gracile and svelte morphology to the intimidating and powerful one we commonly think of today.

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