Related to the platypus, this nocturnal worm-eater is the largest egg-laying mammal in the world. An echidna can weigh up to 36 pounds. Photograph by Tim Laman It may be the strangest mammal in the world—spiky hairs, pointy beak, no nipples, four-headed penis. The long-beaked echidna, found in the rain forest of New Guinea’s Foja mountains, has adapted in remarkable ways. A monotreme, from a group of egg-laying mammals that have been around since the time of the dinosaurs, this primitive animal serves as a living link between mammals and reptiles. Short-beaked echidnas and platypuses are the only other living...