Field Museum scientists describe "loop and pull" in Nature
CHICAGO – Snakes are known to swallow their prey whole, which limits
the size of what they can eat. But now scientists have discovered that
a species of snake can tear apart its prey. This snake loops its body
around a crab to hold one end while using its mouth to pull off legs or
rip the crab’s body into pieces.
This "loop and pull" method allows a snake to eat crabs that are
relatively huge – far too large to swallow whole. Even more astounding,
this appears to be solely based on a unique behavior rather than newly
derived physical traits.
The research will be published in Nature June 11, 2002.
"This rather ordinary looking snake is breaking all the rules,"
says Harold Voris, co-author and curator of amphibians and reptiles at
The Field Museum. "Gerarda prevostiana is the only snake to tear
oversized prey apart, yet you could not have predicted this on the
basis of its morphology [form and structure]. It appears to have
accomplished this feat solely through a unique behavior, not
In fact, the novel behavior has overcome the limitations that
limb loss and body form changes imposed on snakes early in their
evolution, Voris says. "These results serve notice that behavioral
changes alone may allow for major and exceptional changes in life
The snakes and crabs were collected in a mangrove forest of
Singapore. The unique feeding behavior was recorded at night in a dark
room with infrared video cameras. In 85% of the trials, the snake used
"loop and pull."
The scientists captured two G. prevostiana that had consumed
pieces of crabs much larger than any used in the lab trials. This
verified that the feeding behavior was not limited to the lab.
A sister species, Fordonia leucobalia, was also found to pull
apart oversized prey, but it has hypertrophied cranial musculature and
short, blunt teeth that facilitate this behavior.
Although the two snake species consume the same species of
crabs, they share this food source by specializing on different stages.
G. prevostiana specializes in soft, newly molted crabs while F.
leucobalia eats the crabs after their shells get hard. Another snake in
the same mangroves eat only snapping shrimp while a fourth eats only
"These findings represent an interesting example of
evolutionary divergence of behavior between closely related species,"
Voris says. "This is only one of many examples of how the tropics are
uniquely suited for gaining insights into evolution, diversity and
Field Museum. July 2002.