1 In different animals
1.1.2 Other vertebrates
3 See also
5 External links
In different animals
Further information: Sexual reproduction in male mammals
For the male human sexual organ, see Human penis.
Penis of an Asian elephant.
As with any other bodily attribute, the length and girth of the penis can be highly variable between individuals of the same species. In many animals, especially mammals, the size of a flaccid penis is smaller than its erect size. A bone called the baculum or os penis is present in most mammals but absent in humans and horses. In mammals the penis is divided into three parts:
Roots (crura): these begin at the caudal border of the pelvic ischial arch. Body: the part of the penis extending from the roots.
Glans: the free end of the penis.
The internal structures of the penis consist mainly of cavernous, erectile tissue, which is a collection of blood sinusoids separated by sheets of connective tissue (trabeculae). Some mammals have a lot of erectile tissue relative to connective tissue, for example horses. Because of this a horse's penis can enlarge more than a bull's penis. The urethra is on the ventral side of the body of the penis. Stallions have a vascular penis. When non-erect, it is quite flaccid and contained within the prepuce (foreskin, or sheath). The retractor penis muscle is relatively underdeveloped. Erection and protrusion take place gradually, by the increasing tumescence of the erectile vascular tissue in the corpus cavernosum penis. Bulls, rams and boars have an S-shaped penis with a sigmoid flexure which straightens out during erection. Bulls have a fibro-elastic penis. Given the small amount of erectile tissue, there is little enlargement after erection. The penis is quite rigid when non-erect, and becomes even more rigid during erection. Protrusion is not affected much by erection, but more by relaxation of the retractor penis muscle and straightening of the sigmoid flexure. Canids, including dogs, have a bulbus glandis at the base of their penis. During coitus the bulbus glandis swells up and results in a 'tie' (the male and female dogs being tied together). Muscles in the vagina of the female assist the retention by contracting. Cats have barbed penises, with about 120–150 one millimeter long backwards-pointing spines. Upon withdrawal of the penis, the spines rake the walls of the female's vagina, which is a trigger for ovulation. As a general rule, a mammal's penis is proportional to its body size, but this varies greatly between species – even between closely related ones. For example, an adult gorilla's erect penis is about 4 cm (1.5 in) in...