Researchers believe they have discovered one of the reasons why most shark attacks in Australian waters are aborted after a single bite.
A new study by team of scientists from Australia and the USA, published in the Journal of Biomechanics, shows that although the jaw muscles of adolescent great white sharks can exercise incredible bite forces, the jaws themselves may be too weak to capture and kill large animals, including humans.
Co-author Dr Vic Peddemors, of the Cronulla Fisheries Research Centre, says the study may explain why many of the shark attacks off the NSW coast are aborted after a single exploratory bite.
"The great whites involved are usually juveniles that might sustain jaw injury if they persevered with the attack," he says.
Most shark attacks are carried out by young sharks that are still testing their environment and aren't yet sure what they can eat. But why they usually swim away after taking a quick bite hadn't been clear.
3-D computer simulation
The study, which looked at the teeth, jaws, and jaw adductor muscles of two endangered species, the great white (Cacharadon carcharias) and the harmless grey nurse, or sandtiger (Carcharias Taurus) showed that sharks, unlike mammals, can maintain high bite forces no matter how wide their jaws are opened.
The main author, PhD student Toni Ferrara, at the University of New South Wales Faculty of Science, says this is thanks to a unique jaw muscle arrangement.
"The jaw muscles of sharks are divided into upper and lower parts by a tendon", she says. "As the mouth opens this tendon pulls the muscle fibres perpendicularly. Why is this important? Well imagine you want to move a refrigerator — you don't push it at an angle — you would push it with your arms perpendicular to it, to generate the most force."
This study is the first of its kind to use sophisticated three-dimensional computer simulations and advanced engineering techniques to examine how the sharks hunt and kill prey.
Using a 2.5 metre great white shark which was caught by the NSW Bather Protection Program, Ferrara and colleagues set out to see if there was more to the theory that you can tell what a shark eats by just looking at its teeth.
Digital models revealed that the jaws of grey nurse sharks are spring-loaded for a rapid strike on small, fast-moving fish, while those of great whites are better suited for a powerful bite on prey anywhere up to the size of large marine mammals.
"So if you compare a grey nurse with a great white", Ferrara says, "the grey nurse has needle-shaped teeth which are very good for sort of stabbing and swallowing small fast-moving prey, whereas the great whites have a more triangular saw-like dentition which is really for cutting through tissue."
"They're just awkward teenagers"
The researchers were surprised that although the teeth and jaws of a sub-adult great white looked the part, and the necessary muscle structure was in place, the jaws themselves weren't up to the job.
Ferarra says the sharks add additional mineral layers onto the outside of their jaws.
"It basically reinforces them and builds a stronger layer of cartilage", she says. "What our data suggests is that until they start doing that, which they do when they get to around three meters or so, they really can't grab anything that big because their jaws are still weak."
This explains why the stomachs of juvenile and adolescent great whites mostly contained smaller fish, while mature sharks fed mainly on larger marine mammals.
"It is hard to believe, but at this size great whites are basically just awkward teenagers that can't hunt large prey very effectively," says Ferrara.
"It seems paradoxical that the iconic jaws of great white sharks are actually rather vulnerable when these sharks are young. Great white sharks are not born super-predators, they take years to become formidable hunters."