Almost all known snake species eat their prey whole, in a single and sometimes monumental gulp.
But scientists have found an exception to this general rule in the cat-eyed water snake (Gerarda prevostiana), a small serpent native to mangrove swamps throughout Southeast Asia.
A new study has found that these snakes will attack and eat crabs up to five times larger than their jaw can accommodate.
Picky eaters, they only go after freshly molted crabs in the 10-to-15-minute period after the animals shed their old shells, according to the study, published recently in the Biological Journal of the Linnean Society.
"They are quite the little gourmands," says study leader Bruce Jayne, a professor of biology at the University of Cincinnati. (Read about which snake is the fiercest of them all.)
Their strange feeding style begins with a bite, and a tight grasp of the crab's main body, or carapace. They then make a loop with their body, pulling the prey through it repeatedly until the crab is deformed or breaks apart, Jayne explains.
Next they eat the individual pieces, or if the carapace is still too big, sometimes they just pull off the legs and eat them one by one, Jayne says.
"They are doing things that snakes are not supposed to do," says Alan Savitzky, a herpetologist at Utah State University who wasn't involved in the study.
Earning Its Dinner
It took many years for Jayne to record this "most un-snake-like behavior." After capturing cat-eyed water snakes in Malaysia, and later Singapore, Jayne and colleagues took some into the lab and placed them in mud-bottom tanks where the creatures formed tunnels and stayed put.
"They are very shy snakes," Jayne says. When they initially put crabs on top the mud, the animals showed no interest.
Why weren’t the snakes feeding? The colleagues remained stumped until Jayne came upon a clue from an unrelated animal, the queen snake (Regina septemvittata).
This North American creature prefers to prey upon freshly molted crayfish, snacking on the animals when they are softer and easier to swallow.
If cat-eyed water snakes were similar, it would explain why some crabs recovered from inside the stomach of snakes they captured in the wild appeared squishy, perhaps from being newly molted rather than being partially digested.
With the help of Peter Ng, a crab specialist at the University of Singapore, the researchers acquired a large collection of the prey animals, so that they would have some freshly molted specimens. (See our amazing photos of snakes.)
It worked. When they placed freshly molted crabs in the tanks, the snakes would "zoom right out of the burrow, and instead of moving slowly, their heads would trash back and forth," and then they'd attack, Jayne says.
It's a risky lifestyle: "If the snakes are small enough, and attack a hard-shell crab instead of a softshell crab, instead of getting dinner, they may become dinner," he says.
The researchers also examined the eating habits of two related species, the white-bellied mangrove snake (Fordonia leucobalia) and Cantor's water snake (Cantoria violacea), which respectively eat hard-shelled crabs and snapping shrimp.
They found that the white-bellied mangrove snakes also have a very strange attacking method, striking at crabs with a closed mouth, pinning them down into the mud. They then wrap their body around the crab and bite it, sometimes pulling off individual legs, before downing the whole thing. (Read "Deadly Snake Wags Tongue to Lure Its Prey—A First.")
Although it's rare, a few other snakes have been found to eat their prey in parts.
For instance, a recent study in the Bulletin of the Chicago Herpetological Society notes that a Thai snake called the Spencer's mountain keelback (Opisthotropis spenceri) can rip apart freshwater crabs and devour their internal organs.
Such unusual behaviors show how snakes have evolved creative ways to hunt crustaceans, a most unusual prey item with a hard outer body and the ability to fight back.
"It reminds us," Savitzky says, "of how much we still do not know about even the most fundamental aspects of the life history and ecology of fairly abundant animals."