Crocs have super-sensitive jaws

American alligator Small black dots are sensitive spots on alligator jaws

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Thick-skinned crocodilians are actually more sensitive to touch than humans, according to scientists.

US researchers investigated the dome-shaped dots along the jaws of alligators and crocodiles.

They discovered the bumps were made of specialised cells and were more sensitive than human fingertips.

The neuroscientists suggest the sensitive spots play a major part in the aquatic reptiles' impressive reaction times when hunting.

The findings are published in the Journal of Experimental Biology.

Co-author Duncan Leitch from Vanderbilt University, Nashville, US, commented: "The jaws of crocodiles seem to be unique" and can almost be said to perform some of the tactile functions of human hands.

Mr Leitch explained that because crocodilians' forelimbs are too short to grasp anything, they use their jaws in maternal care - gently cracking open eggs and transporting their young inside their mouths.

Good croc, bad croc

Nile crocodile

This behaviour hints that, in addition to their well-known biting force of up to 2,000 psi, the jaws of crocodiles, alligators and other members of the family are capable of very controlled movements.

The jaws of crocodilian species are dotted with black bumps but until now scientists have been unable to explain the purpose of these features.

The bumps have previously been theorised to be electrical or magnetic field receptors, pores for secreting fluid or salinity detectors.

In the lab, Mr Leitch tested how the features reacted to a variety of stimuli. He found no reaction to salinity or electrical fields but touch was a key trigger.

"When I used a calibrated series of fibres to touch or tickle the [bumps], I found that they were responsive to forces finer than our own fingertips - a sensory system widely studied for its own sensitivity," said Mr Leitch.

Analysis of the internal structure of the bumps revealed how they worked.

"I found that there were many specialised cell receptor types - many of which are very similar to those found in human skin," said Mr Leitch.

Under a microscope, he found that there were many free nerve-endings near the surface of the bumps. Lower in the skin he found pressure and vibration-sensitive structures.

High speed video at 500 frames per second shows the nocturnal animal's reactions.

"I was very surprised at these results, especially considering how armoured and scaly the crocodiles and alligators appear," he told BBC Nature.

"However, it seems to makes sense that an animal that might need to carefully discriminate between inedible objects and food, especially in dark or nocturnal environments, would be well-served by having an exquisite sense of touch."

Evolutionary jigsaw

Mr Leitch traced the nerve structures and found that the Nile crocodiles and American alligators studied had a delicate network of nerves throughout their jaws, which threaded through the skull before ending in the bumps.

He suggested that this layout, with the network largely encased within the skull, may help to protect the nerves during the crocodilians' familiar displays of aggression when hunting or protecting themselves.

"Although crocodilians are certainly not the ancestors to humans, it is interesting to see how different parts of their forebrain may have evolved to process different sensations," Mr Leitch said.

"One goal with a lot of this research is gaining a better understanding of how very different nervous systems have evolved to solve similar problems."

Mr Leitch added that, as so-called "living fossils" that have survived mass extinction events, crocodilians represent a key part of the evolutionary puzzle.

"It is interesting to consider what adaptations, including possibly sensory capabilities, have made them such robust creatures."

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