The platypus is a unique and enigmatic mammal, native to eastern Australia and Tasmania. This semi-aquatic creature exhibits a blend of characteristics that initially baffled early naturalists. As a monotreme, it lays eggs, unlike most other mammals. Its distinctive appearance, including a duck-like bill, webbed feet, and a beaver-like tail, highlights its evolutionary distinctiveness.
The Unique Case of Platypus Hearing
The platypus has an auditory system, though it lacks external ear flaps, or pinnae, common to most mammals. Its ear openings are discreetly located within a muscular groove behind the bill. When the platypus dives underwater, this groove automatically pinches shut, sealing off the ear canal, eyes, and nostrils. This adaptation protects these sensory organs from the aquatic environment, meaning the platypus relies on other senses for underwater navigation and foraging.
Despite lacking external ears, the platypus’s internal ear structures are well-developed for hearing on land. Like other mammals, it has three middle ear bones, though its cochlea, the spiral-shaped part of the inner ear, is less coiled than in placental mammals. The cochlea contains inner and outer hair cells, with outer hair cells adapted for detecting high frequencies, a trait shared with placental mammals. Platypus hearing is most sensitive to sound frequencies around 4 kilohertz, similar to human hearing, and can detect frequencies up to 15 kilohertz. The ear region is encased in cartilage rather than bone, reflecting more ancient mammalian characteristics.
Beyond Hearing: Other Remarkable Platypus Senses
When submerged, the platypus compensates for its closed eyes, ears, and nostrils by relying on its sensitive bill. This specialized appendage is densely packed with tens of thousands of sensory receptors, making it the primary tool for navigating and foraging underwater. The bill contains two main types of receptors: electroreceptors and mechanoreceptors.
Electroreceptors, numbering around 40,000, are arranged in striped patterns across the bill’s surface and detect minute electrical currents generated by muscle contractions of aquatic prey. These receptors are sensitive, capable of detecting signals as faint as 20 microvolts per square centimeter, or even 50 nanovolts. The platypus uses a characteristic side-to-side head motion while swimming to interpret these electrical signals, allowing it to pinpoint the location and direction of hidden invertebrates in murky water. This ability, known as electroreception, enables effective hunting without sight or traditional hearing.
Complementing electroreception are the bill’s mechanoreceptors, numbering approximately 60,000 and often referred to as “push rods.” These receptors are sensitive to subtle pressure changes and water movements, detecting displacements as small as 20 microns. Together, electroreceptors and mechanoreceptors provide the platypus with a detailed sensory map of its underwater environment. The brain integrates information from both systems, with electrical signals arriving slightly before mechanical cues, allowing the platypus to accurately judge the distance to its prey. While their eyes are small and typically close underwater for protection, they are used on the surface for detecting potential predators.