In birds fertilisation is internal, meaning that the sperm needs to be inserted into the body of the female by the male. However, only a handful of species have a penis to insert into the female’s cloaca during copulation. The cloaca is a chamber in the lower part of the gut where the digestive, urinary and reproductive tracts end. Almost all other birds do not possess a penis. Now why is this so?
Copulation involves the male balancing on the female’s back to pass on his sperms to the female. For land birds like ostriches LINK, emus and curassows, this is not a problem (above). The female can crouch on the ground and remain steady while the male is on her back during copulation. Thus these land birds, including storks and a few others have a corkscrew penis that is an erectile expansion of the cloacal wall (below, Ostrich with corkscrew penis of the male).
Waterfowls like ducks and geese similarly have a penis. This ensures the male’s sperms are deposited deep inside the oviduct. Otherwise the sperms may be washed away during copulation when both birds are in the water.
On the other hand, with the majority of birds that perch on branches where balancing poses a major problem, evolution has seen to the disappearance of the penis so that these birds have more control over reproduction.
In a study on how birds lost their penises LINK, it was shown that chickens have rudimentary penises during the early embryos stage. However, subsequent development results in these budding penises failing to grow. Apparently there is a gene called Bmp4 that controls penis development. In birds that have a penis, this gene is switched off to allow for its continued development. In chicken for example, this gene is switched on resulting in shrinking of the male genital.
So how do birds without a penis copulate? They indulge on the so-called “cloacal kiss” LINK. The cloaca of both the male and the female bird can turn inside out during this “kiss” when sperm is transferred in the split of a second (below).
But a “cloacal kiss” or a series of it do not guarantee that the male who contributes the sperm will be the father of the chicks. Sperm takes time to travel up the oviduct before fertilising the egg. Depending on the species, it may take 30 minutes to days or even weeks before fertilisation actually occurs. Thus many males guard their females after copulation to ensure paternity. But there are ways and means to bypass the males’ vigilance if the females so wish…
Credit: YC Wee (text), willis (ostrich image), Adrian Lim (trogon image).