With its mash-up of traits, the platypus may be one of the strangest mammals on Earth today.
Along with echidnas, this semi-aquatic animalis one of only five mammalian species that lays eggs. These monotremes, as egg-laying mammals are known, share another characteristic. They have a so-called sixth sense: electroreception.
Duck-Billed Detector
Swimming in the rivers and streams of its native Australia after dusk, the platypus closes its eyes, nose, and ears when it dives in search of dinner—bottom-dwelling invertebrates such as insect larvae, crustaceans, worms, and mollusks.
To find these meals in the mud, it relies on its bill instead. This super-sensory organ is packed with three distinct receptor cells that help the platypus detect movements and subtle electric fields produced by its prey.
Push-rod mechanoreceptors on the bill detect changes in pressure and motion, while two types of electroreceptors track the electrical signals produced by the muscular contractions of the small prey. Using a side-to-side motion of its head, the platypus gauges the direction and distance of its next meal by collecting, and combining, these flows of sensory information.
Top of the Monotremes
The platypus may not be the only monotreme with electroreception, but its sensory structures are the most complex.
About 40,000 specialized electroreceptor skin cells are arranged in stripes on the top and underside of its bill. Echidna species have anywhere from 2,000 to as few as 400, as is the case with the short-billed echidna. This species, which is found in dry habitats, has what researchers think is “no more than a remnant of this sensory system.”