Researchers were surprised to find that crocodiles — despite their thick skin and "heavily armoured" bodies — are more sensitive to touch than even human fingertips.

In fact, the dangerous reptiles have one of the most acute senses of touch in the animal kingdom, suggests a study published Thursday in the Journal of Experimental Biology. So do alligators, but only around their deadly jaws.

This heightened sensitivity is part of the reason they're so deadly, the study concludes.

A crocodile's sense of touch, it turns out, is located in pigmented domes all over their skin called integumentary sensor organs (ISOs). Alligators have them, too, but only around their mouths.

For years, biologist weren't sure what these spots were for. Some speculated they were oily secretions that kept the animals clean. Others thought they measured water salinity or detected electric fields.

"We didn't expect these spots to be so sensitive because the animals are so heavily armoured," said Duncan Leitch, the graduate student who performed the studies under the supervision of Ken Catania, lead author and biology professor at Vanderbilt University in Nashville, Tenn.

In 2002, a University of Maryland biologist found that alligators would turn to face single water droplets even in complete darkness with white noise blocking their ability to hear.

"This intriguing finding inspired us to look further," Catania said. "For a variety of reasons, including the way that the spots are distributed around their body, we thought that the ISOs might be more than water ripple sensors."

So Leitch examined the ISOs and their neural connections. He was able to rule out most previous theories: They don't produce oil and they don't react to electric fields or salt.

Rather, he found they are connected to mechanoreceptors, a bundle of nerves that respond to pressure and sensation. Not only could they detect faint water ripples, but they responded to levels of pressure that even a human fingertip can't detect.

That why they're such excellent killers, the study suggests. Their ISOs help them identify a prey's exact location in a body of water because of ripples.

What's more, because these ISOs are so heavily featured around the jaw and mouth, crocs and gators can likely identify exactly what it is they're devouring.

It also explains why female alligators can break open their eggs and carry their young in their extremely powerful jaws without hurting them.