It's a dog-eat-dog world out there, or in the case of the southeastern United States, it's an alligator-eat-shark kind of a world, according to an unsettling new study from Kansas State University scientists.
An American Alligator chomping down on a nurse shark. (Image: U.S. Fish and Wildlife Service J.N. "Ding" Darling National Wildlife Refuge)
The American alligator, also known as Alligator mississippiensis, is an "opportunistic feeder," which is exactly what it sounds like. These gators prefer to munch on crustaceans, snails, and fish, but they won't turn down an exotic meal if one suddenly presents itself.
As new research published in Southeastern Naturalist shows, American alligators — in an example of predator-on-predator action — periodically snatch and consume sharks. The new study, authored by Kansas State University researcher James Nifong and IMSS wildlife biologist Russell Lowers, is the first to provide scientific evidence showing that this behaviour is real, and that widespread interaction exists between the two species.
For the study, Nifong temporarily captured 500 living alligators and pumped their stomachs to see what they'd been feasting on (so, you wanna be a scientist?...). He and Lowers documented four different species of sharks, including the nurse shark, and even stingrays (another previously undocumented prey animal). These sharks, or elasmobranchs, may not be as big as many of the ones featured on Shark Week, but they're sharks nonetheless.
This all seems wildly improbable given that alligators are a freshwater species, and sharks a saltwater species, but it's not uncommon for sharks and rays to venture into freshwater areas. At the same time, alligators are capable of travelling between freshwater and marine habitats; gators don't have salt glands, which filter salt water, like true crocodiles, but that doesn't prevent them from making these journeys.
"Alligators seek out fresh water in high-salinity environments," said Nifong in a statement. "When it rains really hard, they can actually sip fresh water off the surface of the salt water. That can prolong the time they can stay in a saltwater environment."
The researchers also equipped the gators with GPS transmitters to track their movements. They watched as the reptiles travelled between freshwater sources and estuaries, i.e. a partially enclosed coastal water body where freshwater and saltwater mix. It just so happens that estuaries host shark nurseries — which hungry alligators find irresistible.
Prior to this new study, only anecdotal accounts of such feeding habits existed, including some observations from an island off the Georgia coast. But now there's evidence of these interactions from the Atlantic coast of Georgia around the Florida peninsula to the Gulf Coast and Florida panhandle. Naturalists and zoologists are now coming to grips with the fact that sharks and rays are more important to the gator's diet than previously realised.
Interestingly, this is fascinating example of "reciprocal predation." These two predators are now known to hunt each other.
"The frequency of one predator eating the other is really about size dynamic," Nifong said "If a small shark swims by an alligator and the alligator feels like it can take the shark down, it will, but we also reviewed some old stories about larger sharks eating smaller alligators."
Specifically, Nifong dug up news reports from the 19th century describing crazy scenes in which larges masses of sharks and alligators were engaged in pitched battles after flooding and high tides brought the two predators together. In one instance, sharks were attracted to blood from gators feeding on fish, but when the alligators were washed out to sea, the sharks attacked.
Wow, can nature ever be brutal.