According to Darwin, why is the area around the anus, scrotum, vulva, and nipples darker than the skin on the rest of the body

Don't know the answer

Melanin. The scrotum and vulva and anus have areas of thickened and wrinkled skin folds-so the skin may appear darker-good question though. The nipples colouration I think has more to do with the type of erectile tissue rather than standard breast tissue.

x-ray man is right
But there can be many other reasons as well
As the skin has got more melanin than the other parts of skin and ,oreover these areas are delicate too so more melanin gives better prtotection

Other reason can be that the areola (skin around nipples) are bit dark and softer than other skin as well, this is a gift from nature for the suckings so that they can sucj well and easily if this part was hard and tuff imagine how babies could suck milk.


Another things might be , its sensitivity and softer texture as the anatomy of the skin here differs from that of the other skin .due to more suply of blood when required.

As far as anal section is concerned I think it got sphincters that varies in softness and hardness as well depending on the function performed at the time of excretion or...

I hope it helped.