Reptiles have unusual reproductive equipment. For example, female snakes and lizards have two clitorises. Meanwhile, the male tuatara has no penis. "The male simply mounts the female and places the opening of his cloaca—-the cavity where the intestinal, genital, and urinary tracts meet in reptiles—-over hers," writes Tina Deines in a National Geographic article. From Nat Geo:
The hemipenes(double penis) of lizards and snakes sport tiny spikes and hooks.
Scientists have a few ideas about why hemipenes exhibit this sort of ornamentation. According to one hypothesis, male and female genital form has adapted so that mating can occur only between a male and female of the same species. The genitals of males and females of the same species fit together, and the spikes and hooks could help the male keep his hemipenis in place during mating.
One study found that those spikes and hooks may also increase the duration of copulation, thereby increasing mating success.
"I would stress that the female genitalia need to be studied in order to fully understand the function of elaboration of the male genitalia," says (University of Queensland zoologist Christopher) Friesen.