Why the long face? Doubts over crocodile snouts

An international team of scientists has shown how the shape of a crocodile’s snout could determine its ability to feast on certain types of prey, from large mammals to small fish.

Using computer technology, the researchers subjected the crocodile jaws to the sorts of biting, shaking, and twisting loads they use to feed on large prey.

The American and Australian biologists generated 3D images showing the strain measured on the jaws of seven diverse species of crocodile and compared the jaw strength of different types of crocodiles when feeding on large prey.

They found that the mandibles of short-snouted crocodiles performed well under the loads applied to mimic the feeding behaviour on large prey, but those with elongated jaws were more likely to break under the same loads, showing their limited ability to feed on large prey.

In a paper published in the journal PLoS One, the scientists say that the findings contribute to understanding how the shape of the crocodile’s skull correlates with strength. It is the first study of its kind to investigate the mechanics that underlie the link between the shape of the lower jaw and diet.

The notion that long, narrow snouted crocodiles feed primarily on fish or small prey is well established, but the biomechanics of the crocodiles’ lower jaw, the mandible, have not been previously explored.

To test the jaw biomechanics of large crocodiles, the team used a computational engineering approach, called finite element analysis, that is widely used to design planes, cars, boats, buildings, bridges and many other structures.

They found that mandible shape correlated consistently with jaw biomechanics. This means the lower jaws of long-snouted species were not as strong and were more likely to break during feeding on large prey. It was no surprise that these species tend to concentrate on small, agile, aquatic prey while shorter and more robust-snouted animals are capable of taking much larger prey.

The scientists say the findings are relevant to a broad range of aquatic predators, including dolphins and fossil marine reptiles. The amount of strain a jaw was under was directly proportional to the length of the symphysis, the joint between two halves of the lower jaw, so the animal's biomechanical response to force could be accurately predicted by knowing the length of its chin.

Killer whales, alligators and salt-water crocodiles can all feed on large prey but in all these species the symphysis is a small proportion of the length of the jaw, whereas fish-eating crocodiles and dolphins have long, narrow chins.

The biologists say that further research is needed to explain why crocodiles that feed on small prey had elongated snouts. They suspect the answer lies in the hydrodynamic efficiency of the elongate jaws and they plan to explore this further using other computational engineering techniques.