Lizards are members of the class Reptilia and the order Squamata, along with snakes, worm lizards, and the Tuatara. The lizard suborder Sauria, with 3750 species, is the most successful group of living reptiles. The Sauria are further divided into four infraorders, Iguana (Iguanas), Gekkota (Geckos), Scincomorpha (Skinks), and Anguinomorpha (snake-like lizards), each containing several families.
Lizards have breeding seasons determined by cycles of photoperiod, temperature, rainfall and availability of food. In males, there is a corresponding fluctuation in testicular size. Male iguanas and other lizards are often noted to be more territorial and aggressive during the breeding season. Fertilization is internal. Male lizards have paired hemipenis that are sac-like and lack erectile tissue. They are stored in an inverted position in the base of the tail and may produce noticeable bulges in the lower tail region. Only one hemipenis is used at a time during copulation. Female lizards have paired ovaries and oviducts.
See also: Snakes, Gila Monsters, Dinosaurs, Reptiles