Platypus bills integrate signals from two different sensory systems to better identify and track prey.
Introduction
In 1798, when George Shaw at the British Museum examined the first specimen of a platypus sent to England from Australia, he thought it could be a prank. Did some jokester attach a duck’s beak to an otter’s body?
The platypus’s odd appearance was just the start. The animals just get curiouser and curiouser. They’re the only mammals that lay external eggs (echidnas lay them into their pouches). They have fur and webbed feet and flat bills. And when they dive into murky rivers, skin flaps cover their eyes, ears, and nostrils to seal out water. So how do they find prey without sight, sound, or smell?
Two centuries after the first specimen arrived in England, the platypus’s unique and unlikely duckbill surprised and astounded scientists once again. They discovered that it acts like an underwater antenna, allowing platypuses to navigate and hunt with a sense that humans do not possess.
The Strategy
Platypus bills are highly sensitive organs. They are lined, top and bottom, with tens of thousands of specialized skin cells. One type of cell (mechanoreceptors) can detect subtle movements of water produced by the insects, crustaceans, worms, mollusks, and larvae that platypuses eat. Other types of cell (electroreceptors within mucous glands) can detect extremely slight electric fields with strengths as low as 20 microvolts per square centimeter. That means they could detect a signal less than a millionth of the voltage of one AA battery.
Most living things generate electric fields when brain cells signal muscles to contract. Even single-celled algae emit a little electricity. Those electric currents travel swiftly through water to platypuses’ electroreceptors. Meanwhile, their mechanoreceptors detect pressure from water pushed outward by a fish fin, for example.
Views of some of the striking parts of platypus anatomy were included in "The Naturalist’s Miscellany" written by George Shaw and illustrated by Frederick Polydore Nodder, published in twenty-two volumes between 1789 and 1813.
Unlike electric fish, which receive signals on stationary receptors on their bodies, platypuses tilt their heads up and down and side to side as they swim, actively scanning for electrical or motion signals from various directions in a wide arc of surrounding water. They are continually repositioning their bills so that the largest number of receptors receives signals.
The mechanoreceptors and electroreceptors are interspersed throughout their bills, and neighboring receptors are connected to the same nerve cells that transmit signals to the brain. Thus, the two different kinds of receptors receive and transmit signals nearly simultaneously. The two different systems “crosstalk,” rapidly integrating signals from both incoming sources to distinguish potential prey, determine its direction and distance, and home in.
The Potential
Understanding the biological structures and processes that give platypuses their electroreception ability could reveal new ways to create materials, devices, and systems to detect subtle electrical signals. These could be used, as platypus do, to monitor and explore the depths—to study underwater organisms and ecosystems or to surveil ships for commercial or military purposes.
Scientists are investigating using electroreception to create another method, along with X-rays, magnetic resonance and others, to produce images of body tissues (electrical impedance tomography). Other proposed applications are medical sensors that detect and analyze electrical signals in the body. Advances in this field would be applied to the growing market of wearable electronics—devices and clothing that measure vital signs to monitor health or measure fitness levels.
