PARIS (AFP) — Hawks and eagles change the shape of their wings as they dart after a prey while predatory fish use their fins to close on their quarry -- but how do alligators do it?
The answer: the fearsome reptiles use their lungs as movable internal flotation devices, enabling them to dive, veer and barrell roll through water with nary a ripple.
Biologists at the University of Utah, in a study released Friday, discovered that American alligators manoeuvre with stealth and speed thanks to diaphragm, pelvic, abdominal and rib muscles that shift air-filled lungs inside the body cavity.
"Sitting on the surface of the water, they fire these muscles to dive, pulling their lungs back toward the tail," the lead author, Todd Uriona, told AFP in an interview.
Pushing the organs toward the head helps them resurface and shifting them sideways enables the reptiles to lunge laterally.
"It allows them to navigate a watery environment without creating a lot of disturbance," he said. "This is probably really important while they are trying to sneak up on an animal but don't want to create ripples."
Uriona and co-author Colleen Farmer are confident that other semi-aquatic animals -- including the crocodile, some salamanders and turtles -- similarly use the mobile buoyancy of their lungs to slice through water.
The findings challenge the theory that alligators evolved a powerful diaphragm muscle to help them breathe and run at the same time.
"It may be that instead of these muscles arising for breathing, they arose for moving around in the water and later were co-opted for breathing," said Uriona.
The study, published in a British review, The Journal of Experimental Biology, also suggests the muscle ability developed only after crocodilians became amphibious during the Cretaceous Period, which began some 145 million years ago.
During the Triassic Period, which started at least 100 million years earlier, the ancestors of modern alligators and crocodiles were cat-sized animals that lived only on land.
To test their hypothesis, the researchers conducted two sets of experiments.
In one, electrodes were planted on five sets of muscles in two-year old gators (Alligator mississippiensis) as they swam in a huge fish tank. The specimens were 38 to 50 centimetres long (15 to 20 inches), compared to adults that can grow to more than four metres (15 feet).
The scientists monitored which muscles were activated during different movements.
The diaphragm muscle, which in gators runs parallel to the belly and encases the stomach and intestines, pulls the liver back toward the pelvis and tail when the animal inhales.
The lungs, attached to the liver, are pulled back at the same time, helping the gator plunge into the depths. Another set of muscles on either side of the pubis, the ischiopubis, contributes to the same result.
An alligator's "six-pack" abdominal muscles do not maintain good posture, as they do in humans. But they do help the reptile exhale by pushing the gut inward against the lungs.
Finally, the internal muscles between the ribs cause the rib cage to expand and contract.
All four of these muscle groups work together to give the alligators the gift of aquatic ballet.
In a second experiment, small weights equal to 2.5 percent of the animal mass were attached either to the snout or the tail, and the creatures were able to adjust their muscles accordingly to meet the challenge.
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