OBJECT OF THE MONTH - November, 2009

Hemipenes of snakes and lizards

Figures A-D: hemipenes of 4 different species of North American watersnakes: (A) Plain-bellied Watersnake Nerodia erythrogaster, (B) Diamondback Watersnake Nerodia rhombifer, (C) Glossy Crayfish Snake Regina rigida, (D) Graham's Crayfish Snake Regina grahamii. A blue dye is inserted into each hemipenis in order to provide contrast for certain morphological structures. Photo © 2009 Robert C. Jadin

A hemipenis is the male sexual organ of snakes and lizards, which are collectively known as squamates. An individual male squamate has two hemipenes, which they use alternately when mating with females. Hemipenes are stored inside the tail, which cause male tails to be a different shape than females for many species. By carefully examining the shape of the tail, you can tell males and females apart for this group of reptiles.

The shapes of the hemipenes differ greatly among groups of squamates, ranging from bilobed (= two-lobes) (Figure A) to cylindrical (Figure D). The structures on the hemipenes are also amazingly diverse, from spines (Figure A and D) to ridges, along with other characteristics. The line down the middle of the hemipenis is called "sulcus spermaticus," and it carries sperm during mating. The function of all these structures is still being investigated, but some structures, such as the spines, are clearly used to ensure that mating lasts long enough for egg fertilization to occur.
Many of these structural features of the hemepenes are specific to particular taxonomic groups of snakes and lizards, while others are more general. These structures are often considered to be under less selective pressure than external body characters, which are influenced more heavily by the environment. Researchers found that hemipenal characters could better explain some evolutionary relationships among squamate species than other more prominent morphological characters, like tail length or head shape.