Hemipenis

They differ from the intromittent organs of most other amniotes such as mammals, archosaurs and turtles that have a single genital tubercle, as squamates have the paired genitalia remaining separate.

[7] This developmentally significant difference suggests that the two types of penises could have distinct homologies, and it is thought that this could be attributed to variance of signaling genes during embryological development.

[9] Such studies can help researchers understand adaptive radiation and recover phylogenetic relationships, especially between species that are morphologically very similar.

The surface of hemipenes is one of the most interesting and unique features, and is often covered in sharp spines and spicules that are organized in formations called rosettes.

For example, the hemipenis of the Siamese spitting cobra (Naja siamensis) is smooth with blunt ends, while that of the many-spotted cat-eyed snake (Boiga multomaculata) is entirely covered in hooked spines and spicules.

Rather, it is believed that hemipenes found in the squamate world exhibit such diverse designs to facilitate mating compatibility amongst individuals of the same species, a theory that is referred to as the “lock-and-key mechanism”.

The idea was first postulated in 1844 by French entomologist, Leon Dufour, who observed the diversity of genital morphology among Dipteran flies.

In snakes and lizards, morphological differences in the reproductive organs are believed to exist to help the male copulate with the female.

Spikes and hooks are thought to assist the male in fixing the hemipenis in place during mating, and are made specifically compatible to the female of the species.

This huge variety among the reproductive organs of Squamata is of interest to taxonomists, as it may be able to shed light on evolutionary relationships among reptiles.

While the exact mechanism of how the female controls the stored sperm to fertilize her eggs remains unclear, it is believed that the specialized pockets found in the reproductive tract play a key role.

Most research in the area of squamate reproductive organs has focused on the male hemipenis, but recent studies have investigated the homologous paired erectile structure in females, dubbed "hemiclitoris" (pl.

[20][21] Among vertebrates, penises can be found in a variety of shapes, sizes and structures, such as the lymphatic erection mechanism of ratites and the single vascular erectile body of turtles.