Some Monkeys Have Blue Testicles—Here's Why
Adding a pop of color to a neutral outfit can really brighten it up. It also helps to know where to put it. Male old World monkeys—including patas, mandrills, vervets, talapoins, and lesula—sport their, um, accessories in an unusual place. This prompted Weird Animal Question of the Week to ask: “Why do some monkeys have blue scrota?”
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.
Male mandrills, native to rain forests of equatorial Africa, have vivid red and blue facial colors that match the eye-catching colors on their hindquarters. The brighter the face, rump, and genitalia, the higher the male’s rank, which a 2005 study showed could sometimes help avoid costly conflict. What's more, female mandrills prefer males with more vibrant colors.
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.
http://news.nationalgeographic.com/2016/09/primates-monkeys-blue-scrota-genitalia/
