The skull of an American alligator.
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Photo by Didier Descouens, CC BY 3.0.
The skull of an American alligator.

Bitten Bones Show Alligators are Rough Feeders

What happens when you throw bones to a group of American alligators? This isn’t a question for late night horror movies, but for science.

Standing behind a safety barrier at Florida’s St. Augustine Alligator Farm, University of Tennessee paleontologist Stephanie Drumheller tossed skeletonized cow legs to a crush of curious alligators. Over and over again, the armored archosaurs rushed in to snap at the morsels, and with every bite they left the predatory hallmarks in the form of punctures and scrapes. These traces were what Drumheller was after. Through understanding the damage modern alligators leave on bones, Drumheller and other paleontologists can follow the depredations of alligators and their croc cousins through time.

Even though the idea that alligators and crocodiles are “living fossils” is total bunk, there has nevertheless been a long series of croc-shaped aquatic ambush predators over the past 220 million years. The first were the phytosaurs, gharial-ish Triassic carnivores that had their nostrils up close to their eyes, and an array of crocodyliformes later took up the same lurking lifestyle over and over again. Today, only the crocodylians – gharials, alligators, and crocodiles – remain.

Whatever lineage they belonged to, though, these varied, sneaky hunters often left bite marks on bones, usually recognizable as circular punctures or furrows created by conical teeth. Bite marks on 75 million year old bones have shown that dinosaurs sometimes wound up as meals for Cretaceous crocs, for example, and distinctive toothmarks on bones found from Olduvai Gorge have shown that ancient crocodiles dined on early humans. In fact, crocodylians and croc-like animals might have helped create parts of the fossil record by dragging unlucky animals into watery habitats where the leftover bones were more likely to be preserved.

Despite the long history of toothy ambush predators, though, scientists know relatively little about the kinds of damage modern crocodylians leave on bones. Scientists have only run a handful of experiments, all within the last decade, to ascertain the traces crocodiles leave while they feed, with the focus on species like the Nile and saltwater crocodiles. Now, in a new Ichnos paper, Drumheller and co-author Christopher Brochu have detailed the damage American alligators create when they chomp down on skeletons.

American alligators were an obvious choice for Drumheller and Brochu. They’re numerous, well-studied animals, and since they last shared a common ancestor with Nile and saltwater crocodiles in the Cretaceous, the bite marks they create can help elucidate the evolution of feeding behavior in a lineage of crocodylian that hasn’t been studied this way before. To do this, the researchers got partially-butchered pig femora and cow legs to present to the St. Augustine gators in two different experiment setups.

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An American alligator. Photo by Sara Viernum, CC BY-NC-ND 2.0.

In the first experiment, 14 individual alligators of different sizes – including one named “Fluffy” – were given pig bones or cow legs, depending on the gator’s size. (This sometimes required the assistance of a handler who had to sit on the alligator’s back and hold the predator by the base of the skull, which is surely in the running for “Most Unenviable Job, Ever.”) The second experiment made gathering data a little more difficult. The researchers threw 27 cow legs to adult alligators held in two different enclosures. The bones stayed with the alligators for hours to days, depending on when the alligators could be moved out of the enclosures to retrieve the remains, although a small number were lost or totally eaten by the alligators.

After cleaning the retrieved bones with Borax and distinguishing between tooth traces and clean cuts made by butcher’s blades, Drumheller and Brochu found that every single bone in the sample was pocked with bite marks. In fact, the recovered bones from the second experiment showed 3,831 bite marks in all – an exceptionally high number of traces including pits, punctures, scores, furrows, and fractures. Some of these are identifiable to certain types of anatomy of behavior. “Bisected marks”, for example, are made by fresh teeth, while “hook scores” are L- and J-shaped marks made when alligators repositioned bones in their mouths.

The two experiments turned up different types of traces. The individual alligator experiment showed a large number of bisected marks, with fracturing and punctures appearing to relate to size and, therefore, stronger bite forces. The bones from the group feeding experiments, on the other hand,  showed a high proportion of scores with a smaller number of pits and other sorts of damage, and the sheer number of bite marks obviously related to the fact that the cow legs were thrown to a gaggle of gators that competing to bite the bones. But it wasn’t just that. Compared to Nile crocodiles, American alligators left more fractures, furrows, and hook scores on the bones – all signs of a harsher method of stripping flesh from skeletons and crushing bone. “It seems that A. mississippiensis is simply a more violent feeder than C. niloticus,” Drumheller and Brochu write, “utilizing its ability to crush similar bones to greater effect.”

It’s not altogether surprising that Nile crocodiles and American alligators feed in different ways. Aside from different snout shapes – alligators more rounded, crocodiles more pointed – alligators evolved from ancestors adapted to crush hard-shelled prey to generalist feeders capable of taking on most anything, while crocodiles have been dietary generalists for the past 66 million years. Perhaps the rough treatment of the bones by the alligators is a remnant of their deep past. But the study also turned up a key similarity. Both alligators and crocodiles made “bisected marks”, which has only been associated with crocs and croc-like predators. This means that bisected marks are a diagnostic form of damage that paleontologists can use to confidently determine the carnivorous calling card of crocs.

There’s still a great deal of work to do in parsing these traces, though. Drumheller and Brochu mention the need to study how wild alligators rip apart and modify skeletons, as well as possible comparisons with the traces left by thin-snouted crocodylians, such as gharials. Such studies will expand and refine paleontologists’ search image for bite traces. In order to see the signs of predators past, scientists like Drumheller and Brochu will have to keep throwing modern crocs a bone.