This seems to be based on mate choice, rather than physiology.

They also point to other sources of variation in mate choice.

Higher ranking males may also be a more attractive mate choice.

"I think mate choice is probably a lot more complicated, particularly in humans."

Currently, there are five mechanisms that explain the evolution of mate choice.

The color changes are more dramatic in males, due to sexual selection and female mate choice.

Sometimes facial color or leg brightness can play a role in mate choice.

Other studies have approached mate choice based on odor preference.

When there is more than one male available, females will exhibit mate choice.

The relevance of a running mate choice, he said, was that it "may tell the American people something more about the two major candidates."