Thinking about sexual dimorphism (again)

Well, here is a particularly dramatic example of sexual dimorphism, a new species of monkey identified in the Congo (and described in the NYT) and an interesting quote, "And adult males have a huge bare patch of skin in the buttocks, testicles and perianal area,” said John A. Hart, the researcher who described the coloring. “It’s a brilliant blue, really pretty spectacular.”" Now I wonder why?