The star on the star-nosed mole may look freakish and useless, but in fact it gives the animal the ability to sense electric fields.
For the most part, the star-nosed mole looks normal enough. Stocky and dark and about six inches long, it has a body like that of a garden-variety mole and the same powerful digging hands and poorly developed eyes. But then there is the matter of its nose. Clustered around its nostrils, like petals on a nightmarish flower, are 22 writhing, probing, fleshy tentacles. It is not obvious what evolution had in mind in designing such an organ, and Edwin Gould, a zoologist at the National Zoological Park in Washington, D.C., has been wondering about that question for a decade. But now he thinks he has the answer. Those tentacles, Gould’s research indicates, enable the star-nosed mole to detect the electric fields of its prey.
Sweat and mucus are rich in electrically charged molecules, or ions, which can set up an electric field around an animal. A smattering of creatures can detect such fields by means of receptors in the skin-- essentially nerve endings tipped with specialized antennalike cells. Until recently the animals reported to do this were mostly ancient lineages of fish such as sharks and rays. Electroreception seemed to be an evolutionary vestige, a sixth sense that had been eclipsed as the other five came into their own.
In 1986, however, Henning Scheich, of the Technical University Darmstadt in Germany, and his colleagues reported electroreception for the first time in a mammal: the platypus. It has electroreceptors on its bill, the researchers announced, and it uses them to hunt tadpoles and crayfish on the dimly lit, muddy bottoms of streams and ponds. Yet the discovery still left electroreception with a primitive luster, since platypuses, which are egg-laying monotremes, are living fossils par excellence.
I heard about Scheich’s work on the radio while I was driving, Gould remembers. At the time he had been puzzling over the star-nosed mole for four years. Like other moles, it digs tunnels, but its tunnels often lead into streams and ponds: the star-nosed mole is an excellent swimmer. Gould had learned to respect the mole’s mysterious ability, given its small eyes, to hunt worms in muddy water. And like biologists before him, he was intrigued and baffled by its star. The tentacles, he knew, are covered with nerve endings--might they be doing more than just touching things?
Sitting there in the car, I began to wonder whether star-nosed moles were like the platypus, says Gould. After all, they hunt for food in the same conditions. He soon began a series of experiments to look for electroreception. He constructed a burrow for the moles by filling an aquarium with peat and letting them scamper down a plastic tube to a second aquarium with earthworms writhing in a few inches of water. Then he substituted two penlight batteries for the worms--one completely wrapped in plastic to mask its electric field, the other with its poles left uncovered. Seven out of ten moles picked the uncovered battery to investigate.
Scheich had used the same technique to test his platypuses, but some question it, since a battery’s electric field is much stronger than a worm’s. So in his next experiments Gould measured the electric field of real worms. He found that it was strongest near the middle of their body, where reproductive glands secrete mucus. If the moles were indifferent to a worm’s electric field, they would attack it at random points. But Gould found that they attacked the worm’s midsection four times more frequently than expected. To make sure the moles weren’t smelling the mucus, Gould built artificial, odorless worms, consisting of an electrode and a bunch of conducting tubes that together produced a wormlike electric field. Six out of seven moles preferred the worm that was switched on over the one that wasn’t.
Gould now suggests that the nerve endings in the star’s tentacles are indeed electroreceptors and that the moles move them around constantly to sample the strength of the fields at many places as they search for prey. He and other researchers still have to figure out exactly how the electroreceptors work. But his findings so far, besides explaining one of the strangest of all noses, shed new light on the evolution of electroreception. Far from being a primitive vestige, it seems to be a tool that animals reinvent when their surroundings--such as muddy waters--demand it.