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Crocodile chin length gives away its diet

Twist and rip The length of a crocodile's chin can be used to predict whether or not it is up to tackling large prey like pigs and buffalos, say researchers.

Dr Colin McHenry, of Monash University's Functional Anatomy and Biomechanics Lab, and colleagues report their findings in a recent issue of PLOS ONE.

Scientists have long noted crocodiles that feed on small prey like fish, tend to have very long snouts, while those that feed on big prey tend to have short snouts.

In long-snouted animals, the mandibular symphysis - the part of the jaw where the bones join to form the chin - is very long. In Indian gharials for example, the chin is more than 50 per cent length of the jaws.

Short-snouted animals, like salt-water crocodiles, on the other hand have a very short chin.

"There's something about the length of the symphysis that correlates very well with the length of the jaws," says McHenry.

He and colleagues, including PhD sudent Chris Walmsley, suspected there would be a biomechanical explanation for this, given what crocodiles do when faced with prey too large to swallow.

"If a croc catches a pig it will hold it up in the air and shake it vigorously, just like the way a dog shakes a rabbit, and it will shake it until that animal starts falling apart," says McHenry.

"If the croc is faced with some very big prey like a buffalo, it will twist into an infamous death roll."

Engineering rules

McHenry and colleagues used engineering first principles to reason that the length of a crocodile's jaw must influence the strength of its jaws when the animal shakes and twists its food.

"In order to be able to withstand the loads associated with shaking and twisting, we predict the crocodile should have a short symphysis (chin)," says McHenry.

By contrast, they reasoned, chin length should not affect jaw strength when the crocodile is simply just biting its prey.

The researchers took CT scans of skulls from seven different species of crocodiles with chins that were anything from 10 per cent to 50 per cent of their total jaw lengths.

From the scans they made high-resolution 3D models of the skulls then simulated the strain on the bones when different types of loads were applied to them.

"The lower the strain, the stronger the jaw," says McHenry.

The researchers looked at how close to breaking the skulls came when loads equivalent to biting, shaking and twisting were applied.

They found that while the length of the whole jaw was the best predictor of strain in biting, chin length was the best predictor of strain during shaking and twisting.

"The behaviour of these structures is actually what you'd predict from engineering first principles and they are much more complex than any shape any engineer has analysed," says McHenry.

Wider application

McHenry says the findings can be applied to other aquatic animals including dolphins, and will help palaeontologists work out the diet of an ancient marine reptile.

"As long as you can work out the length of the symphysis, you could start to say something about the likely diet of that animal," he says.

"That's gold for palaeontology because most of the time we're dealing with fragmentary fossils."

As to why fish-eating crocodiles tend to have long thin chins remains a mystery, says McHenry, but his working hypothesis is that this helps streamline the animal as it tries to catch its prey.

"Fish are really agile, really fast and they're pretty good at avoiding predators, so catching fish is really quite a challenge," he says.

McHenry and colleagues plan to test this hypothesis in the future by using computational fluid dynamics to calculate drag caused by different shaped jaws as fluid flows across them.