why is the area around the anus darker

according to Darwin, why is the area around the anus, scrotum, vulva, and nipples darker than the skin on the rest of the body
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.