Adding a pop of color to a neutral outfit can really brighten it up.

It also helps to know where to put it.

Why So Blue?

First things first—these blue testicles are not due to sexual frustration.

The color is also not caused by hormonal shifts, as in the case of the red genitalia seen in baboons and other primates, Fred Bercovitch, a wildlife biologist at Kyoto University in Japan, says via email.

Though the blue pigments are not completely understood, they're likely linked to sexual selection, Bercovitch says, though in mandrills, color has been linked to social status.

View Images Vervets, which live throughout sub-Saharan Africa, are the most wide-ranging of the African monkeys. Photograph by Goncalo Diniz, Alamy Stock Photo

Male vervets of East Africa that have more intense blue scrota are "more likely to be aggressive with and bully juvenile males," says Jennifer Danzy Cramer, a biological anthropologist at American Public University in Charles Town, West Virginia.

Vervets also like to show off their bonnie blues, adds Kyoto University's Bercovitch, unlike mandrills and patas, a primate native to the central African grasslands.

Overall, Bercovitch says greater contrast and larger size "are probaby alluring traits" (think the eye-catching peacock's tail) so those males with the most vibrant and biggest scrota attract females. For instance, patas testicles can grow to twice their size during mating season.

Family Jewels

So how do the primates actually get their blue junk? At a molecular level, the color originates from the Tyndall effect, the scattering of light by the skin itself, Bercovitch says. The skin of blue-hued monkeys also has unusually neat and orderly collagen fibers, according to a 2004 study. They're so well organized that a change of as little as a millionth of an inch in size of or distances between would produce a different color.

Speaking of, the monkeys can take on different hues.

The lesula, an African primate, only formally discovered in 2007, has a bright blue scrotum and buttocks that turns white when the animal dies.

Patas have aquamarine scrota, while many adult vervets' are more turquoise.

All male vervet monkeys "start life [with] dull, dark, dusty blue" scrota—it's only in adolescence that differences emerge, Cramer says.

For instance, the green monkey, a vervet native to West Africa and the West Indies, has genitalia turns a pale blue or even white in adulthood.

Quite the colorful family jewels.