Australian researchers discover why only two of echidna’s four penis heads become erect at one time

The major blood vessel of the penis splits into two main branches which each supply two of the four penile heads

The penis of an echidna has four heads but only two become erect at any one time. Now, Australian researchers have uncovered why.

Scientists discovered the marsupial has unusual reproductive anatomy that causes male echidnas to ejaculate from only two of their four penile heads at one time.