The Four-Headed Wonder
Male echidnas have a four-headed penis, but only use two heads at a time, alternating sides with each mating.
The echidna, one of only five surviving species of egg-laying mammals (monotremes), has one of the strangest penises in the mammal world: it has four heads.
The penis has a single shaft that splits into four separate heads (or "rosettes") at the tip. During mating, only two heads are used at a time — the other two shut down. Researchers believe echidnas alternate between the two pairs, possibly to give each side time to recover between matings.
This unusual anatomy likely evolved to match the female echidna's two-branched reproductive tract. By using two heads at once, the male can deliver sperm to both branches simultaneously.
Adding to the weirdness: during the mating season, up to 10 male echidnas will follow a single female in what's called an "echidna train," waiting their turn to mate. This can go on for weeks.
Echidnas also have some of the lowest body temperatures of any mammal, and their sperm has evolved to work at these cooler temperatures.