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Scientific VIEW

The Platypus: An Electrifying Discovery

March 12, 1986|BETTY ANN KEVLES

Perhaps Rudolf should be replaced by a platypus at the head of Santa's team. In the trend to make folklore more consistent with our knowledge of real animals, a lead platypus would have more verisimilitude. The electrically sensitive nose that glowed on the mythical reindeer does not exist, but researchers in Australia and West Germany have just discovered an electrical component in the strange duckbill of the platypus.

They ascertained in a series of four experiments in an artificial pond that the platypus's bill could locate both fresh-water shrimp and a miniature 1.5-volt alkaline battery. The 1 1/2-foot-long mammal searched for food with its eyes, ear canals and nares (nasal openings) closed, and a forefoot always in contact with the wall.

The experiments confirmed the researchers' suspicions that the platypus is able to detect and respond to very subtle DC and AC electrical fields underwater. This helps them navigate around obstacles, and as predators to hone in on the electrical fields of shrimp, frogs and fishes.

After the researchers had ascertained that the duckbill is the platypus' electroreceptive organ, they discovered with the aid of an electron-microscope that within the large bill the platypus has unique gland duct receptors that connect that gland to the mucous glands on the surface of the platypus's skin.

Until the publication of this discovery last month in Nature, electroreceptors had been found only in fish such as sharks, skates and rays. However, as long ago as 1917 observers had noted that fish, such as the North American catfish, had some electrical sensitivity. Zoologists began serious research into the electrical sensitivity of the dogfish, a small shark, in 1934. Forty years later they had documented both the techniques that dogfish use to detect the electric fields of food fish such as flounder, and the mechanism within the shark that provided the electroreceptors.

Sharks, rays and skates carry these receptors in cells called the Ampullae of Lorenzini. These are cells that are found in the head and in lines along the sides of the fish and developed from the acoustic-lateralis system. They apparently evolved from the sensory hairs that have the same root as the hairs within the human inner ear that help us retain our equilibrium.

The receptors in the platypus have a different evolutionary history. The platypus, though comfortable in the water, is only semi-aquatic. It sleeps and nests on land where it raises its young. The glands in the bill that are electroreceptors fill the double function of preventing the bill from drying out when it is out of the water. Thus it seems clear that while the electroreceptors in platypuses and in sharks function alike in detecting underwater food, they evolved independently in the very different species.

But why did they evolve at all? Most animals find their nourishment by using their eyes, ears or noses. What is it about platypuses and sharks that selected for a sixth sense, the ability to detect very low-voltage electric fields?

All organic life has an electrical component. Within our own bodies messages move electrically from cell to cell. However, electricity is even more efficient in water. It is not surprising that among animals that feed in water, electrical sensitivity should be selected as a more efficient way to hunt. Smells travel well in water, as does light, and in some species those senses also are especially acute.

Sharks and platypuses are nocturnal. At night in the water, sight is not very useful. So in both of these not at all kindred night-feeding aquatic species, a sixth sense compensates for the dark.

The discovery of yet another curious talent in the platypus, an egg-laying mammal unique to Australia that lives on land but like the otter, feeds in water, is another instance of the phenomenon of convergent evolution. This refers to unrelated species that evolve analogous organs to take advantage of the peculiar habitats they find themselves in. In this way, totally different creatures such as platypuses and sharks that have separate geographical and evolutionary histories have the same ability to detect subtle electric currents from live prey. Electroreceptivity is apparently a successful way in which different species that feed under similar conditions have solved the problem of prey detection.

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