Platypus

When hunting underwater, platypuses shut their eyes, ears and nostrils. Their bills contain about 40,000 sensors that detect faint electric fields from prey muscles. They locate food in visual darkness with a sense almost no other mammal possesses.

Mammal milk evolved from skin secretions that originally kept eggs moist. The platypus, the only surviving egg-laying mammal, still coats its soft eggs with secretions to prevent drying, and its hatchlings feed on these secretions for nutrition.