Why does the crocodile have spots?

This isn't a Just So Story. It's a science story — but don't worry, it's still weird. New research suggests that the spotty bumps that pepper the skin of crocodiles and alligators are even more receptive to touch than human fingertips. But how do the spots work — and what purpose do they serve?

You can see these spots flecked along the mouth of the crocodile pictured up top. While biologists have known about these dots for decades, there hasn't been a whole lot of consensus on what they actually do. Some researchers have hypothesized that the bumps might act as prey-sensing electroreceptors, similar to the ampullae of Lorenzini in sharks. Others have suggested they might be used to detect shifts in water salinity, or produce oily secretion not unlike human pores.

But in 2002, researchers had a breakthrough when a study published in Nature showed that these tiny black spots were very sensitive to touch, demonstrating that the sensory organs could "detect small disruptions in the surface of the surrounding water" — a nice skill to have if you're, say, waiting for a meal to swim your way.

That got researchers Duncan Leitch and Kenneth Catania thinking. How sensitive were these bumps, really? And what purpose (or purposes) did they serve? In the latest issue of The Journal of Experimental Biology, the researchers recount how they went about finding out. National Geographic's Shannon Fischer gives a tidy summary of what they found:

The domes didn't respond to salt or electricity, but they did respond to the touch of von Frey filaments-hairlike, standardized wires used to gauge sensation levels. In fact, some of the domes turned out to be so sensitive they could detect pressures too small to measure via the filaments.

"My professor and I didn't believe at first that they could be that reactive," Leitch said. "We closed our eyes and tried to tickle each other with [the filaments] on our fingertips, and neither of us could even feel it."

Later, using croc carcasses, the researchers stained the dome nerves with dye and traced them back to the brain. They turned out to be tied into a system stemming from the trigeminal nerve-associated with biting, chewing, and swallowing.

So yes: there's a decent chance crocodiles do use these sensory organs to detect the movement of prey in the water, or for detecting the precise location of a potential meal by first nudging it with their snout. In fact, the crocodilian touch system is so acute, the researchers think it likely serves a purpose in other, non-food-related tasks. Young-rearing, for instance. Mom crocs often use their mouths to help their young out of their shells, and even cradle them in between their jaws for protection. If your mouth serves the dual purpose of killing machine and baby-coddler, it makes sense to have it rival human touch in the sensitivity department.

[The Journal of Experimental Biology via National Geographic]