There may be animals with double penises (like lizards, snakes or sharks) out there, others to possess a double-headed penis (like marsupials), but echidna is unique: it has a four-headed penis!

Scientists have been puzzled by this odd anatomy. Echidna does not even use its penis for urination and it gets it out of the body only in erection.

When resting, the penis is kept inside a cloaca. Echidna and its relative, the platypus, form the group of monotremes, the most primitive mammals still living, and displaying many archaic traits inherited from mammals’ reptile ancestor like this cloaca (just like in birds and reptiles), egg laying, weak ability to maintain constant body temperature (which is anyhow lower than in other mammals, at about 32o C, compared to 37o C in humans), tiny brain and many bone characteristics (especially in the ear, jaw and shoulder). Monotremes are living fossils, a link between mammal and reptiles.

Echidnas are also called spiny anteaters, but besides the fact they are indeed spiny, as their sole defense against predators, they have nothing to do with the real anteaters of tropical Americas (except the fact that they are mammals, too, and ingest ants or termites).

Now the mystery of the four-headed penis has been solved, revealing another reptilian trait: male echidnas ejaculate with just two heads (half of the penis) at a time.