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Posted:

18th February, 2008


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The plugged-in platypus

On a recent visit, my 30-year-old daughter Kim was bemoaning her lack of first-hand experience with a wild platypus. Only two days later, while paying a visit to one of the most beautiful spots in the whole world, the Caveside picnic area, she almost fainted with excitement when we spotted a duckbill (colloquial for platypus) darting and flashing and scrabbling in the little stream that flows by. And then, just a few days ago, I read about the stir caused by a report in a January edition of the "Proceedings of the National Academy of Sciences of the USA" on the discovery of a species of extinct platypus going back 112 million years. (See "Fossil jaw speaks against evolution".) Researchers were amazed to discover that the ancient beast, just like its modern counterpart, could do something quite remarkable - locate lunch "electrically". Time to do a little investigation into this sensory tour de force, methought.

Ornithorhynchus anatinus has evoked wonder, not to mention controversy, ever since the first dried skin from Australia arrived in "the homeland" in 1798. Scientists at first thought it was a fake, one of those clever Chinese stitch-jobs that have fooled people time and again. (I must confess to staring for a long time at a preserved specimen of a half-baby-half-fish at the Alligator Farm in Hot Springs, Arkansas.) It has also provoked one of the shortest telegrams known. In 1884, a young Cambridge biologist shot off a message to the Annual Meeting of the British Association in Montreal that read simply, "Monotremes oviparous, ovum meroblastic". His learned friends knew what he meant.

In 1927 Australian naturalist Harry Burrell made a prediction that was not finally confirmed until the late 1980s. Observing its capacity to locate prey underwater with eyes, ears and nostrils closed, he suggested the platypus must have a "sixth sense". In 1977, University of New South Wales investigators discovered that the part of the brain processing information from the bill is much larger than the parts coping with nerve inputs from eyes, ears, and limbs combined. That the bill was packed with nerves and was sensitive to touch had been known for a long time. But sensitivity to touch alone could not explain the size of the brain portion devoted to the bill. Something more was awaiting discovery. In 1985, Australian and German biologiss figured it out. Following a hunch, they placed one male and three female platypuses in a pool containing a 1.5 volt

alkaline battery. The results were conclusive; patrolling platypuses inevitably switched behavior to search phase when they came near the battery. Further,

Control experiments offering a choice between a battery, a piece of shrimp tail or a dead battery placed 10 cm apart from each other established a clear preference for the active battery ( Scheich and others 1986, Electroreception and electrolocation in platypus, Nature, 319:401-402).

They concluded that the bill of the platypus, previously considered to be exclusively sensitive to touch, is exquisitely attuned to weak electrical fields, just the ticket needed to home in on prey hidden under rocks and submerged logs. They could even distinguish between a.c. and d.c., dexterously locating and avoiding objects on the basis of the very weak d.c. fields they generated when water passed over them while zeroing in on the a.c. fields generated by animal muscle activity. Forty thousand tiny electroreceptors in the bill detect the latter from 10 cm away and send pulses of information to the brain where they are interpreted. The movements that create an electrical field also create mechanical waves that travel through the water and are picked up by touch sensitive receptors in the bill:

These mechanical waves arrive after the electrical activity from the same prey, as a function of distance. Bimodal cortical neurones, sensitive to combined mechanical and electrical stimulation, with a delay, can thus signal directly the absolute distance of the prey. Combined with the directional information provided by signal processing of the thousands of receptors on the bill surface, the stripe-like cortical array enables the platypus to use two different sensory systems in its bill to achieve a complete, three-dimensional "fix" on its underwater prey (Pettigrew and others 1998, The Sensory World of the Platypus, Philosophical Transactions of the Royal Society of London, B, 353: 1199-1210).

Much, much more could be said, but I think you get the idea; sophisticated, complicated, complex, ingenious, brilliant design. But what else should one expect when one remembers "Whodunit"?


Platypus bill's two kinds of pore. The electrosensitive pore (far left) is the opening of a mucus gland duct while the touch-sensitive pore contains a pushrod device that triggers nerves when compressed by a mechanical wave in the water or when it comes into contact with an object (Australian Geographic).

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