Crocodiles are sensitive animals

Crocodiles have the reputation of being dangerous, armoured to the teeth. Surprisingly, however, crocodiles and alligators are actually very sensitive animals. Their mouths are even more sensitive to the touch than human fingertips.
An alligator has approximately 4000 small, black bumps on its head. These bumps are concentrated on the jaws, but occur in the mouth and between the teeth as well. The crocodile also has these bumps – up to 9000 of them – spread over its entire body. The presence of the bumps has been known for more than a century, but their function has remained a mystery. Scientists suspected that the bumps had something to do with the detection of weak currents given off by prey, or the registration of the water’s salt content. Recent research, however, has revealed that very mild pressure changes can be perceived by these bumps.

A student of Vanderbilt University began his investigation into the mystery of the bumps by collecting test animals. Eighteen American alligators were received from shelters and four Nile crocodiles came from commercial breeders. Small animals were selected in order to make the research easier.

The research showed that the bumps do not react to salt or electricity, but rather that they contain receptors specialised to react to pressure and vibration. The ‘bare’ nerve endings were also found. This was tested with Von Frey filaments: very thin, standardised fibres used to measure pressure. Some of the bumps turned out to be so sensitive that pressure was perceived which could not even be measured with the filaments. The investigators also tested themselves; the pressure was so minimal that it could not be felt with the fingertips.

In dead crocodiles, the nerves leading to the bumps were dyed and traced to the brain, where they emerged in the part of the brain involved in biting, chewing and swallowing.

The investigators next wondered why these crocodiles and alligators would have such incredibly sensitive pressure organs. It is suspected that the animals use their mouths to feel, and then catch, their prey. Female crocodiles also use their mouths to help their young to emerge from their egg shell, or to take the young into their mouths for protection. This, of course, requires a great degree of sensitivity.

Questions for the next round of research include why alligators have these bumps only on their heads, while crocodiles have them all over their bodies.

The results of the research have been published in the Journal of Experimental Biology.