Because the best conditions for mating are not necessarily the best for ovulation and gestation, female snakes can store sperm for a long time, up to 5 years and possibly longer. But unlike in humans, female snakes have a lot of control over whether or not they get pregnant after mating. They spend some time together, intertwine their tails, and the male inserts one hemipenis so that his sperm find their way safely from cloaca to cloaca. The amazingly variable structure of the hemipenes has often been used in snake taxonomy for this reason.īut why two? Wouldn't one penis do just as well, since male snakes only use one at a time anyway? Let's take a quick look at the timeline of snake reproduction. Hemipenes often have various spines, knobs, branches, and other projections, which typically correspond with the cloacal anatomy of female snakes of the same species, forming a sort of 'lock-and-key' mechanism that isolates species by discouraging mating among unrelated individuals. Think of a canal rather than a pipe, although during mating the wall of the female's reproductive tract forms the other part of the tube that we mammals have. In general, snake hemipenes are endowed with a groove, called the sulcus spermaticus, down which the sperm runs. Hemipenes are one of the shared derived characters of squamates (snakes and lizards), distinguishing them from other reptiles ( tuataras, turtles, crocodilians, and birds), all of which have either a single or no penis. Some examples of snake hemipenes photo by Robert Jadin
Sexual dimorphism is rare in snakes, except that male snakes almost always have longer, thicker tails than females, because they need someplace to store their hemipenes. During mating, one hemipenis or the other is everted in a manner similar to taking off a sock. This is shown nicely in the above diagram. Hemipenes are normally stored inside out in the base of the tail, forming a pocket into which a probe can be, well, probed to check the sex of a lizard or snake. Each hemipenis is associated with a single testis, meaning that sperm produced in the right testis are ejaculated through the right hemipenis, and those produced on the left come out of the left. Together, the two penises of squamates (snakes and lizards) are called hemipenes, and each individually is called a hemipenis. I did know that they only use one at a time, and I had once heard that it was so that they could copulate with a female no matter which side she was on, but that doesn't really seem to make sense to me any more, especially considering that lizards also have two penises.
Recently somebody asked me "Why do snakes have two penises?" When I tried to answer, I realized that I didn't really know.
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