Hairy butts! The official name is the Nada Fighting Festival, perhaps because that is what the participants are wearing — nada! No, these guys are butt-naked! Butt-nakedness is actually a tradition of some Japanese Shinto festivals.
Kenka Matsuri: death and hairy butts | The Japan Times
Like many physical traits, the amount of hair on the buttocks varies from person to person. For the most part, hair on the buttocks is more of a cosmetic feature than a medical one. Your genes determine how much body hair you have, as well as what kind of hair you have, such as color and texture. So, if you have an abundance of hair on your buttocks, your parents most likely passed this trait on to you. Adrenal glands produce androgens. These hormones can trigger the growth of body hair on both females and males.