How the Gator Lures Lunch

Scientists discover crocodilians use enticing twigs to attract birds.

December 14, 2013, 4:59 PM
PHOTO: Crocodiles and alligators have been found to balance sticks on their snouts as a way to lure nest-building birds into reach of their jaws.
Crocodiles and alligators have been found to balance sticks on their snouts as a way to lure nest-building birds into reach of their jaws.
Vladimir Dinets, University of Tennessee, Knoxville

Sept. 15, 2013— -- Not too many years ago we were all taught that only humans used tools. Now, of course, we know that was rubbish.

Scientists have documented the use of tools by all sorts of animals, ranging from chimps to birds and even one species of fish. But it turns out that's not all of the story. One animal has been found to use tools not just as tools, but as lures to attract his prey into his happy hunting grounds.

And surprisingly, that animal turns out to be crocodiles and their close cousins, alligators, known collectively as crocodilians.

The tools?

Common twigs floating in swamps beneath trees that provide nesting areas for birds, including herons and egrets.

Here's how it works, according to Vladimir Dinets, an animal behaviorist in the department of psychology at the University of Tennessee, Knoxville, who has studied the feeding habits of crocodilians in both India and the United States.

Twigs are essential for birds to build their nests, and they are so scarce in some swamps that the birds fight furiously among themselves over just a few sticks. The birds build their nests in the tops of trees that grow in the middle of lakes because that protects their young from snakes and other animals that prey on the chicks.

So they have a symbiotic relationship with the crocs. The swamps provide a safe harbor for their nestlings, but if one falls into the lake, it's a small but easy snack for the big, powerful reptiles below.

According to Dinets, who has seen this many times, the crocks swim slowly beneath the twigs and then surface, carefully keeping the twigs on their snouts.

"It takes a balancing act to keep them there," Dinets said in a telephone interview.

The croc then remains motionless, just beneath the surface, waiting for a bird to try to grab one of the twigs. When the bird gets close enough, the croc uses its powerful tail to fling itself out of the water and snatch the bird.

"It all happens in less than a second," Dinets said. "It's a big splash, a snap, and a lot of feathers."

The first time he saw it, he said, he thought it was accidental -- maybe he just happened to be there when a bird tried to get a twig that had floated onto the nose of a crocodilian. But it happened too often to be an accident.

Dinets and his colleagues, J. C. and J. D. Brueggen, who run the St. Augustine Alligator Farm Zoological Park in St. Augustine, Fla., set out to document the activity at four sites in Louisiana, resulting in a study published in the peer reviewed journal Ethology, Ecology and Evolution. Two of the sites were breeding grounds for birds, and two were not.

The scientists saw a huge increase in the number of alligators with sticks on their snouts at the rookery sites where birds were building their nests, from March to May.

The researchers saw the drama unfold many times every day, but almost exclusively during the time that the birds were building nests. It rarely happened after the nesting season was over, leading the scientists to conclude that the sticks were indeed being used by the reptiles to lure the birds to their deaths.

"The present paper is the first report of tool use by any reptiles, and also the first known case of predators timing the use of lures to a seasonal behavior of the prey," the study concludes.

The scientists think they have produced evidence that crocodilians are just a lot cleverer than anyone had though.

"These discoveries are interesting not just because they show how easy it is to underestimate the intelligence of even relatively familiar animals, but also because crocodilians are a sister taxon of dinosaurs and flying reptiles," the study adds. This suggests that even dinosaurs were "likely very complex as well."

This, and related research, indicates that scientists are finally catching up with a story-teller who regaled audiences in ancient Greece during the fifth century, BC.

In one of Aesop's fables, "The Crow and the Pitcher," a crow used stones to raise the level of water in a pitcher so it could quench its thirst.That story fascinated scientists at the University of Cambridge (where else?) so much that they decided to find out if Aesop was on to something.

They demonstrated that a rook, a member of the corvid family of bright birds that includes crows, would do just as Aesop had suggested if that was the only way he could snatch a worm floating on the top of the water in a pitcher.

The crow not only used stones to raise the water level so it could reach the worm, it also figured out that it could do it a lot quicker if it used a few larger rocks instead of a lot of smaller ones. And it didn't check to see if it could get the worm after it dropped each stone. When the water was high enough, it grabbed the snack.

That won't surprise many bird lovers, because corvids, like chimps and apes and various other animals, have been documented using tools to extract lunch from places they could not otherwise reach. Crocodilians are the latest now to join that exclusive club, but maybe not the most surprising.

At least one crow in Russia has taken it one step farther. As seen in a video posted to YouTube last winter, the crow used a plastic disc as a sled to repeatedly slide down a snow-covered roof. She used a tool not for food, but for fun.

Scientists at the University of California, Santa Cruz, have even found a fish that uses a tool to prepare its meal. While studying the orange-dotted turkfish near the island of Palau in the western Pacific, evolutionary biologist Giacomo Bernardi made a startling discovery.

Repeatedly, he watched the turkfish dig a clam out of the mud and then throw it against a rock over and over again, until the clam shattered, yielding up the fish's lunch.

Bernardi, by the way, thinks there may be many species of fish that use tools. It's just pretty hard to see them in the depths of the ocean.

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