>>2412
As a general rule, an animal's penis is proportional to its body size, but this varies greatly between species — even between closely related species. For example, an adult gorilla's erect penis is about 4 cm (1.5 in) in length; an adult chimpanzee, significantly smaller (in body size) than a gorilla, has a penis size about double that of the gorilla. In comparison, the human penis is larger than that of any other primate, both in proportion to body size and in absolute terms.
While results vary across studies, the consensus is that the average human penis is approximately 12.7-15 cm (5-5.9 in) in length[4][5][6] and 12.3 cm (4.85 in) in circumference when fully erect. The average penis size is slightly larger than the median size. Most of these studies were performed on subjects of primarily European descent. Worldwide averages may vary.
A research project, summarizing dozens of published studies conducted by physicians of different nationalities, shows that worldwide, erect-penis size averages vary between 9.6 cm (3.7 in) and 16 cm (6.2 in). It has been suggested that this difference is caused not only by genetics, but also by environmental factors such as culture, diet, chemical/pollution exposure[7][8], etc.
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 much smaller than its erect size. In humans and some other species, flaccid vs. erect penis size varies greatly between individuals, such that penis size when flaccid is not a reliable indicator of size when erect.
Except for extreme cases at either end of the size spectrum, penis size does not correspond strongly with reproductive ability in almost any species.