Bird Sex, Part II

A couple of weeks ago we began a series on avian reproduction with Yeat’s retelling of the myth of Leda and the Swan. We ended with the rhetorical question of why Zeus chose to become a Swan instead of some other bird before raping Leda. That post is here. Today, we supply the answer.

Male swans are among the few birds that have penises.

In fact, only male waterfowl and male flightless birds – ostriches, rheas, emus, cassowaries, and kiwis – have phalluses. All other birds evolved a different method of sexual reproduction that does not require a penis. We’ll start with them.

Because birds fly, they must be as light as possible. One of their many adaptations to that need are cloacas which both male and female possess. A cloaca combines the functions of a bladder, waste receptacle, excretory organ, anus and sex organ into one anatomical feature resulting in lighter weight. Another such adaptation is that most birds’ ovaries and testes shrink during the non-breeding season which further streamlines them for flight and reduces the sex drive in both, freeing up energy for flying, migration and staying warm.

When the breeding season is upon them, female birds’ ovaries enlarge as do male testes. The males produce sperm which travels from their internal testes (more streamlined arrangement than mammalian external male gonads) to the male cloaca which then is extruded from the bird’s body and swells with semen. After whatever breeding rituals particular to its species, the male mounts the female and pushes his cloaca against hers. In a matter of seconds the semen is transferred by this touch, known as a “cloaca kiss.” With the possible exception of some papillaries which may actually reach inside the outer edge of the female cloaca, no penetration takes place. In fact, birds really don’t copulate; they inseminate.

After receiving the semen, the females get to decide what to do with it. They can use it to fertilize eggs, save it for up to a month or get rid of it. Some females can lay numerous fertile eggs after only one sexual contact. Other species engage in much more copulatory contact, several times daily for as much as a month. Scientists suspect; however, that birds which copulate often do it for the same reason mammals do it: To maintain a pair bond. In other words, they use sex – as do humans – for more than reproduction. They do it to stay together. Lots of birds bond for life and it is no easier for birds than for us to stay together for an entire adult life. Plus, it probably feels good to them too.

But male waterfowls have a cloacal phallus. Sort of a corkscrew actually. Shaped like a ram’s horns. Technically it isn’t really a penis because it has no ureter inside it. No urine in birds. Urine requires too much water and, as any hiker of the Grand Canyon can tell you, water is heavy. No bird could fly with that much water on board. But that means there is no tube down which the waterfowl’s semen can pass. Rather, it is transported on the surface of the erect cloacal phallus. (Made erect, not by blood as in mammals, but by lymphatic pressure. More efficient, less weight.) The penis actually corkscrews into the female cloaca so waterfowls do “copulate” or, if you please, “screw.” Almost always on water and, for the female, frequently under water.

In many waterfowl species the males also engage in what scientists euphemistically call “Forced Extra Pair Copulations.” (FEPC) If a human male did it, we would call it “rape.” Not only did Zeus need a bird with a phallus to disguise himself, he needed an avian rapist in case Leda wasn’t interested in mating with a swan.

Imagine if Leda – a human female – could have rejected Zeus’s sperm. No Helen to burn Troy.

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Here is Part I of this series and here is part III.

Tags: bird sex, cloacal kiss, Leda and the Swan, swans