Episode 8 Field Guide: Crocodiles are the Chomping Champions!
Fossils are the raw materials of paleontology, but if we want to know how an animal moved or ate, paleontologists, like Dr. Paul Gignac, need to study living animals, too. Dr. Gignac studies crocodylians, measuring their bite forces across species and as they grow up to figure out how the strongest bite in nature evolved. Using techniques drawn from mechanical engineering and physiology, Dr. Gignac discovered the relationship between body size and bite force in crocodiles, and developed equations to calculate those forces. Then he used these equations to calculate the bite forces of giant extinct crocodiles like Deinosuchus. He also studied the bite marks left by the sickle-clawed dinosaur Deinonychus to calculate dinosaur bite forces. There are so many questions to ask about extinct animals but there’s also a lot left to learn about their living descendants! The lines between scientific disciplines get blurred and the questions just get more interesting in this episode of Past Time!
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Crocodylia – The name for the group of reptiles that includes crocodiles, alligators, gharials, caimans, and their extinct relatives. There are 23 extant (still living) species of crocodylians. Today, crocodylians are mostly amphibious animals that spend most of the day hunting in the water and walking onto land to bask and lay eggs. But extinct crocodylians were even more diverse than the modern forms . There were long-legged galloping species, and squat herbivorous species. Check out the episode on the fossils of Madagascar for more on these weird extinct crocodylians.
Deinosuchus – A giant crocodylian from the Late Cretaceous (~70 million years ago) of North America that had the largest bite force ever calculated.
Deinonychus antirrhopus – A mid-sized theropod dinosaur from the Early Cretaceous (~100 million years ago) of North America. A close relative of Velociraptor, Deinonychus was the physical model for the animals in Jurassic Park, but the filmmakers thought the name Velociraptor sounded cooler (fair point).
Gharial – A slender-snouted crocodylian that lives in India. Its needle-like teeth and delicate nose are characteristic of a fish-eating (piscivorous) specialist.
Caimans – Crocodylians from Central and South America that are closely related to alligators. They tend to be small, and have taller skulls with eyes slid more to the side, giving them a passing resemblance to dinosaurs, the distant cousins of crocodiles.
Maximum bite force – The total force that can possibly be generated by the jaw “adductors”, the muscles responsible for pulling the jaw (mandible) closed. Many animals may increase the force they exert on prey by pulling with their neck muscles or forelimbs, but maximum bite force is all about the jaws. Humans have a bite force of about 200 pounds, lions have a maximum bite force of about 900 pounds, and the largest bite force ever measured in a living animal was 4,000 pounds, the deadly chomp of a huge salt water crocodile!
Wrestling Crocodiles for Science
Dr. Paul Gignac is an assistant professor of Anatomy at Oklahoma State University. He describes himself as an integrative or evolutionary biologist. Crocodiles have the strongest bite force in nature, and Dr. Gignac has devoted most of his research career to understanding how these iconic reptiles generate these powerful forces. He has dissected the jaw-closing muscles of most of the known species of crocodylians to understand the inner workings of a crocodylian’s bite. He has also worked with a team of like-minded researchers at Florida State University to collect bite force data from living animals. Because he knew the mass of each animal he studied, he discovered that there was a tight connection between body mass and bite force. Dr. Gignac then took one abundant (especially in Florida) crocodylian, Alligator mississippiensis (the American alligator), and studied the development of their snouts, teeth, and bite forces as the animals matured from tiny, lizard-sized hatchlings to harder-to-handle adults. If you’re interested in the science Dr. Gignac does, his department is recruiting graduate students for next year’s class. Contact us for his e-mail address!
There are twenty three or twenty four species of crocodylians that fall into four major groups. There are the crocodiles (Crocodyloidea), such as the Nile crocodile of Africa and the salt water crocodile from Australia which tend to have wedge-shaped heads. The alligators (Alligatorinae) which are found in North America and China and tend to have more blunted snouts. The caimans (Caimaninae), actually a subfamily of the alligators, are found in Central and South America. They have taller skulls than most crocodylians and their eyes are placed more on the side of the skull. The final group contains the gharials (Gavialidae), animals with very long, slender snouts and needle-like teeth.
Crocodiles are sometimes called “living fossils” because the extinct relatives of many modern crocodylians looked –and probably acted – a lot like their modern descendants. Species with similar skulls were thought to be closely related and most fossils found with a crocodile-like profile were lumped into Crocodile. But recent work on their family tree has uncovered a much more complicated story with closely related species having very different skull shapes and bizarre fossil crocodiles have demonstrated the cousins of dinosaurs moved in on dinosaur-like niches, especially on the southern Gondwanan continents (Africa+South America+India+Madagascar+Australia+Antarctica) during the Mesozoic Era (Age of Dinosaurs). Instead of staying in an evolutionary rut, paleontologists like Dr. Gignac have revealed crocodiles are a dynamic lineage, responding to new resources and environments for millions of years.
Recording the strongest bites
When Dr. Gignac was working on his PhD at Florida State University, he worked with Dr. Greg Erickson and a team of other crocodylian researchers to collect information on the biology – and bites – of crocodiles. An alligator farm in Florida houses living representatives of each species of crocs alive today. There, the team could study the full diversity of crocodile biting in all different species and at different body sizes. They would work together to restrain the animals so they couldn’t use their bodies to influence the total force measured by the force transducers, small pressure gauges mounted on the ends of long poles that the crocs would be encouraged to chomp down. That’s right, they would be holding onto an animal with the some of the strongest jaws known in nature, encouraging it to bite as hard as they could…for science.
What they found contradicted a century of thought on crocodile diversity. Scientists had assumed snout shape was closely related to diet and bite force. Since animals like freshwater crocodiles of Australia and didn’t need to lurk at the water’s edge, waiting to latch onto a large mammal, their skinny, apparently delicate snouts apparently didn’t need to withstand much force. But when Dr. Gignac got the results of the big, skinny-snout crocodylians’ bites, he was surprised to find that they bit with the same force as a big alligator or Nile crocodile, animals with broader snouts that seem better adapted to withstand large forces. Discovering the simple relationship between body size and bite force in crocodylians allowed Dr. Gignac to move into the fossil record to study the behavior of extinct animals like…
Deinosuchus: Chomp Champ
Deinosuchus lurked in the waterways of North America between 70 and 80 million years ago. The “terrible crocodile” (“Deino” is spelled differently, but it’s the same Greek root that gives us “dinosaurs” or “terrible lizards”) was a massive relative of modern alligators with stout teeth and a dinosaur-sized gape reaching lengths estimated between 10 and 15 meters (about 30 to 50 feet). Because Deinosuchus is closely related to modern crocodylians it was a question of simple math to plug in the estimated body size and get a bite force for Deinosuchus at a staggering 25,000 pounds! The “terrible crocodile” earned its title. But what about applying the bite force equations to crocodile cousins like dinosaurs?
Deinonychus: The “Terrible Claw” had terrifying teeth
The problem with applying the bite force equations to dinosaurs is the equations work well for animals with jaw muscles arranged like crocodile jaw muscles. The more the skull changes from the modern croc shape, the harder it is to trust the numbers. Carnivorous dinosaurs likely had muscles that were very similar to the jaw muscles found in crocodiles, but a glance at a T. rex skull will tell you the sizes and shape of the muscles acting on the taller, narrower skull would be tough to compare to a croc. So, calculating dinosaur bite forces need to be grounded in evidence of dinosaurs using their jaws. This means paleontologists need to find evidence of dinosaur bites recorded in their prey.
Paleontologists from the Chicago Field Museum were working in Wyoming and noticed the bones of an herbivorous dinosaur called Tenontosaurus had deep bite marks that could be attributed to a mid-sized carnivore. The only candidate was Deinonychus, a name that means “terrible claw”. A glance at its feet will tell you how it earned that name.
Previous studies of the composition of dinosaur bone, similar to the work Dr. Mike D’Emic did on the microstructure of dinosaur bone for studying growth rates, showed that thick cow bone was a pretty good model for dinosaur bone. Dr. Gignac just needed to recreate the bite marks using molds of Deinonychus teeth puncturing cow bone. He measured the amount of force needed to make the marks and found a bite force of 1,000 pounds, on par with modern hyenas, the strongest biters among mammals. The bites left on the Tenontosaurus bone were probably from the front of the jaw where bite forces are usually not as strong as they are at the back of the jaw, so the dinosaur’s bite may have been even stronger. Deinonychus certainly had a strong bite for a dinosaur, but the giant carnivores like tyrannosaurs were much more powerful.
Growing Into Big Bites
Mammalian carnivores have the luxury of waiting for their meat-shearing teeth to grow into their adult-sized jaws by delaying the eruption of adult teeth. They can delay because for some period of time at the beginning of every mammal’s life, they subsist on mother’s milk rather than mother’s food. She uses her adult teeth and guts to process plant matter if she’s an herbivore and prey if she is a carnivore. Some of the calories she ingests are invested in producing the milk. Her infant doesn’t need to worry about getting its own food because mom is keeping baby supplied while it grows into its own specialized feeding niche.
Crocodylians aren’t so lucky. Growing up in alligators is probably best understood. Baby alligators are incredibly vulnerable the minute they leave the egg. Mom may carry them to the water from the nest, but she can’t catch the little insects that the babies need to eat. They need to take care of that on their own with stumpy snouts and more-pointed teeth. This insect-eating niche is really crowded and the baby alligators are competing with lizards, fish, birds, and small mammals for insects, small vertebrates, and fish, while avoiding becoming prey. They also have to compete with all of their brothers and sisters at the same time, and an estimated 80% of baby alligators don’t make it through their first year. As they get larger, their teeth become wider, their snouts become longer, and their bites become stronger to help them compete with each other and with other species. Only 5% of hatchlings make it to adulthood.
Dr. Gignac has gathered evidence supporting the hypothesis that the crocodylian bite is the product of this growth trajectory. There are advantages to strong bite forces at small body sizes in establishing social hierarchies and outcompeting the other alligators. If an alligator keeps growing, it’s going to take its strong jaws along for the ride and maybe use them to latch onto something large and hapless that can be dragged into the water to be rolled and drowned.
But what about baby gharials? Do they go through the same process as they mature, moving through food niches and suffering massive mortality rates as a group of hatchlings matures? What about caimans? Or dwarf crocodiles? We don’t know. Most crocodylians haven’t been studied in the wild either because they’re tough to find, extremely endangered, or there just aren’t enough scientists to take on these big questions. But paleontologists like Dr. Gignac are pressing forward with croc questions that reveal new insights about the living crocs and their fossil ancestors!
Further reading for a general audience:
Crocodiles and Alligators – A perfect reference for croc aficionados and beginners alike, this book features introductions to the living species, their habitats, and their behaviors. Also a beautiful book to look at, with tons of full-color photographs of all of the species.
King of the Crocodylians – It’s a bit of a technical book, but King of the Crocodylians tells the story of the mightiest croc that ever munched on dinosaurs: Deinosuchus. Adam read it, and he considers it turtle-munching, globe-trotting, croc-loving fun…but then again, he would think that…
Eyelids of Morning: The Mingled Destinies of Crocodiles and Men – A very powerful book and definitely one for adults, Eyelids of Morning tells of an expedition to a remote African lake to study a population of Nile crocodiles. This is an excellent place to learn about a real field study of these giant reptiles, but it’s also good for anyone who wants to learn about Africa and it’s peoples.
For those hoping for something a bit more technical, you can check out the article by Dr. Gignac and his colleagues where they describe their crocodile bite studies. If you’d like to read about Dr. Gignac’s work on dinosaur bites, send us a message and we’ll get you his contact information.
Finally, be sure to check out your closest zoo if you want to see real crocodylians! If you’re in Florida, you can visit the St. Augustine Alligator Farm, where almost all of the living crocodylian species can be seen!