The vast majority of pokemon exhibit a low rate of gender dimorphism. Compared to humans, the female and male of the species generally differ much less in height, have less prominent breasts (if mammalian) and less difference in facial structure: indeed, many pokemon can only be distinguished by the actual genitalia.

One of the most prominent exceptions to this rule is the Nidoran. The female of the species exhibit large whiskers and are light blue, while the males have a spikier body, a purple color and a long horn. Occasionally, intersex Nidoran are found, with the color of one type of Nidoran, but the physical appearance of another.

It is said that the whiskers and rounded ears of a female Nidoran give it senses not only superior to its male counterparts, but among the sharpest in the pokemon world. They are a shy race of pokemon who have learned the fear of man, and it is these senses which allow them to hide so well in the wild. Although they outnumbering the overhunted male of the species by a ratio of about 5-2, most trainers believe the two Nidoran to be of equivalent rarity.