Crocodile Faces Are More Sensitive Than Human Fingertips
Of all the adjectives you could use to describe a crocodile’s face, “sensitive” might not be an obvious one. But their huge jaws, pointed teeth and armoured scales belie a surprising secret. Their faces, and possibly their entire bodies, are covered with tiny bumps that are far more sensitive than our own fingertips.
The bumps are obvious if you look carefully. Each one is a small dome, barely a millimetre wide, surrounded by a groove. There are around 4,000 of them on an alligator’s jaws and inside its mouth. Crocodiles and gharials also have the bumps on virtually every scale of their bodies, giving a total of around 9,000. (All of these animals are called crocodilians.)
They were first described in 1895, and scientists have used them to identify different farmed species of crocodilians from endangered poached ones. But no one knew what the bumps did. Perhaps they secrete waterproofing oils? Maybe they help the croc to detect changes in saltiness, electricity, magnetic fields, or water pressure? There were plenty of hypotheses, but few actual studies.
In 2002, Daphne Soares from the University of Maryland showed that American alligators could orient themselves to the ripples created by a single drop of water, even in complete darkness. The bumps on their faces seems to be involved. It was a brief study, but it intrigued Duncan Leitch from Vanderbilt University, who wanted to know what the bump were doing. Now, with Ken Catania, he has shown that they’re touch sensors, which make a crocodilian’s face exquisitely sensitive despite its hard unyielding armour.
Leitch analysed the bumps, which he calls “integumentary sensory organs” or ISOs, on 18 young American alligators and 4 Nile crocodiles. When he cut away the domes, he uncovered a vast network of nerve endings. They’re all branches of the big trigeminal nerve that carries sensations from the animal’s face to its brain. This splits into three main branches – one going over the croc’s eye, one running through each jaw. They branch and branch again, finally passing through holes in the animals’ skulls to reach into the ISOs themselves.
What type of sensation are these nerve endings conveying? Leitch ruled out a few possibilities. He dipped the animals’ legs in varying degrees of saltwater, and found that the nerves in their ISOs didn’t react. He put a 9-volt battery in the water to create electric fields. Nothing.
Instead, Leitch found that the nerves feeding into the ISOs end in a variety of touch sensors. Some finish in the topmost layers of skin, which is exceptionally thin over the bumps and can compress at the slightest force. Others end in structures called lamellar corpuscles, which are sensitive to vibrations, or Merkel discs, which detect pressure.
The ISOs are extraordinarily sensitive. Leitch recorded the activity of their nerves using electrodes, and stimulated them with von Frey hairs – nylon filaments that can be pressed against a piece of skin with a precise amount of force. He found that the most sensitive ISOs can sense forces as tiny as 78 millionths of a Newton—that’s ten times more sensitive than the most sensitive parts of our fingertips. If you compress the bumps by just 4 millionths of a metre, the nerves within them would fire.
To see this sense in action, Leitch dropped pellets of food into his crocodilians’ tanks, and filmed them in pitch-blackness using infrared cameras. They would rapidly turn towards the ripples and sweep their heads from side to side. Once their skin touched the food, they snapped it up within 50-70 milliseconds. They would also wait submerged until a fish swam past their open jaws. Again, as soon as the prey made contact, the croc snapped shut.
Leitch and Catania think that the ISOs help crocodilians to capture their prey by either sensing ripples in the water, or feeling the direct touch of food. They probably also use the sensors to distinguish between different objects in their jaws, which is why the bumps are most concentrated around their teeth. Mother crocs will often carry their eggs and babies in their jaws, so their finely-tuned sense of touch may help them to distinguish between prey (Bite!) and their own young (Don’t bite!).
But why are they all over a crocodile’s body and only on an alligator’s face? “This is a big mystery,” says Leitch. He saw no differences between the ISOs on different parts of his crocodiles. Some people have speculated that the alligators have body ISOs buried deeper in their skin, but Leitch saw no evidence of this. “Another possible idea is that the ancestors to modern crocodilians had them all over the body and head, but over time, the body ISOs might have been lost in the alligator lineage,” he says. “It definitely is still a puzzle.”
Reference: Leitch & Catania. 2012. Structure, innervation and response properties of integumentary sensory organs in crocodilians. J Exp Biol http://dx.doi.org/10.1242/jeb.076836
Related Topics
Go Further
Animals
- These 'trash fish' are among Earth's most primitive animalsThese 'trash fish' are among Earth's most primitive animals
- These photos are works of art—and the artists are bugsThese photos are works of art—and the artists are bugs
- The epic migration of a 6-foot long, 200-pound catfishThe epic migration of a 6-foot long, 200-pound catfish
- Frans de Waal, biologist who studied animal emotion, dies at 75Frans de Waal, biologist who studied animal emotion, dies at 75
Environment
- Are synthetic diamonds really better for the planet? The answer isn't clear-cut.Are synthetic diamonds really better for the planet? The answer isn't clear-cut.
- This year's cherry blossom peak bloom was a warning signThis year's cherry blossom peak bloom was a warning sign
- The U.S. just announced an asbestos ban. What took so long?The U.S. just announced an asbestos ban. What took so long?
- The most dangerous job? Inside the world of underwater weldersThe most dangerous job? Inside the world of underwater welders
- The harrowing flight that wild whooping cranes make to surviveThe harrowing flight that wild whooping cranes make to survive
History & Culture
- Meet the powerful yokai that inspired the demon king in ‘Demon Slayer’Meet the powerful yokai that inspired the demon king in ‘Demon Slayer’
- A surprising must-wear for European monarchs? Weasels.A surprising must-wear for European monarchs? Weasels.
- Meet the woman who made Polaroid into a cultural iconMeet the woman who made Polaroid into a cultural icon
- Inside the observatory that birthed modern astrophysicsInside the observatory that birthed modern astrophysics
- See how Muslims celebrate Eid al-Fitr around the worldSee how Muslims celebrate Eid al-Fitr around the world
Science
- NASA smashed an asteroid with a rocket. The debris could hit Mars.NASA smashed an asteroid with a rocket. The debris could hit Mars.
- Humans really can have superpowers—scientists are studying themHumans really can have superpowers—scientists are studying them
- Why engineers are concerned about aging infrastructureWhy engineers are concerned about aging infrastructure
- The benefits of eating the peels on your fruits and veggiesThe benefits of eating the peels on your fruits and veggies
- The island of Santorini is hiding an explosive secretThe island of Santorini is hiding an explosive secret
Travel
- Play and stay in the mountains of eastern Nevada
- Paid Content
Play and stay in the mountains of eastern Nevada - This couple quit the city to grow wasabi in Japan's mountainsThis couple quit the city to grow wasabi in Japan's mountains
- This Swedish overnight train takes you straight to the northern lightsThis Swedish overnight train takes you straight to the northern lights