
SEEING ELECTRICITY AND PRESSURE?
The monotremes, the echidnas and platypus, are the only living mammals that are known to have an ability to sense electricity. The platypus bill is something unique in nature, so sophisticated and advanced that no one can call the platypus "primitive." The skin surface of the bill contains 40,000 tiny electroreceptors, or specialized sensory nerve endings, arranged in rows along the length of the bill. These detect tiny, underwater bursts of electricity from the muscles of swimming creatures that the platypus hunts. The electroreceptors are intermingled with 60,000 mechanoreceptors, nerves ending at the skin in tiny "push rods" that respond to small pressure changes and detect the movements of prey animals underwater and on the bottoms of streams and ponds. Together, the two senses allow the platypus to home in on prey.
In addition to detecting bursts of electricity from prey animals, the electroreceptors in a platypus's bill can probably detect the tiny electric currents made by water flowing over and around rocks and sunken logs, thus producing a three-dimensional map of a river or lake bottom within the platypus brain. In the platypus, the combined abilities of electroreception and mechanoreception are so sensitive and detailed that they have become something like vision, providing a three-dimensional "view" of the platypus's underwater world, and enabling the platypus to pinpoint, in all three dimensions, the exact locations of its prey.
The mother, having no nipples, nurses the young with milk that comes directly from her belly skin.