A sixth sense for hunting
When the platypus (Ornithorhynchus anatinus) dives into the freshwater rivers of Tasmania, it enters a world of silence and darkness. Folds of skin cover its eyes and ears, and its nostrils form a waterproof seal. Yet, it hunts with precision. The platypus finds its food, insect larvae, freshwater shrimp, and worms, by detecting the weak electric fields their bodies produce. This remarkable ability is called electroreception, a sense that monotremes, the egg-laying mammals, are the only mammals known to possess, aside from one species of dolphin.
The source is its unique, rubbery bill. The bill's soft surface contains approximately 40,000 specialized electroreceptors, which are modified mucous glands. These receptors are arranged in distinct stripes running down the bill, an array that helps the platypus pinpoint the direction and distance of a signal. When a shrimp flicks its tail, the muscle contraction generates a tiny electrical pulse. The receptors in the bill can detect fields as weak as 20 microvolts per centimeter, prompting the platypus to investigate.
The bill has a dual sensory system. Alongside the 40,000 electroreceptors are about 60,000 mechanoreceptors. These "push-rod" receptors are sensitive to touch and pressure, detecting disturbances in the water made by moving prey from up to 20 centimeters away. Working together, these two systems give the platypus a detailed three-dimensional map of its immediate surroundings.
A monotreme marvel
The platypus perfects its hunt with a characteristic side-to-side sweep of its head as it swims along the riverbed. This scanning motion allows the striped array of receptors to gather precise data on the location of the electrical source. Once it finds prey, it scoops the item up, along with gravel and mud, and stores it in cheek pouches. It surfaces to breathe and grind up its meal, typically diving for less than a minute at a time.
Electroreception is a trait shared by its fellow monotremes, the echidnas, but to a much lesser extent. The short-billed echidna (Tachyglossus aculeatus), which forages in drier environments, has only about 400 electroreceptors on the tip of its snout. The long-billed echidna (Zaglossus bruijnii) has around 2,000. The platypus, with its 40,000 receptors, demonstrates a highly advanced adaptation to its semi-aquatic life. This complex sensory system, evolved independently from that of electroreceptive fish, is a feature of this unique Australian mammal.