The Monkey That Has Blue B a l l s

Vervet monkeys have small cute faces and bright blue ᴛᴇsᴛɪᴄʟᴇs.

Male Vervet Monkeys have blue ᴛᴇsᴛɪᴄʟᴇs. And if the number of google images with them staring in bewilderment at their own nuts is any indication, they are as disconcerted by them as we are.

Despite the beautiful blue of their ᴛᴇsᴛɪᴄʟᴇs, life is kind of rough and transitory for male vervet monkeys. Their mothers kick them out of the group once they reach s.ᴇ xual maturity so they won’t inbreed with their sisters, and after that they have to switch social groups multiple times in their lives. They tend to move approxiᴍᴀᴛᴇly every two years, because that’s when their female offspring reach s.ᴇ xual maturity – they gotta go before they try to impregnate their own daughters.

But why the blue balls??

First things first: These blue ᴛᴇsᴛɪᴄʟᴇs are not due to s.ᴇ xual frustration.

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

Though the blue ᴘɪɢᴍᴇɴᴛs are not completely understood, they’re likely linked to s.ᴇ xual selection, Bercovitch says, though in mandrills, color has been linked to social status.

Male mandrills, native to rain forests of equtorial Africa, have vivid red and blue facial colors that match the eye-catching colors on their hindquarters. The brighter the face, ʀᴜᴍᴘ, and ɢᴇɴɪᴛᴀʟia, the higher the male’s rank, which a 2005 study showed could sometimes help avoid costly ᴄᴏɴfʟɪᴄᴛ. 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 ᴀɢɢʀᴇssɪᴠᴇ 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 priᴍᴀᴛᴇ 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 ᴛᴇsᴛɪᴄʟᴇs can grow to twice their size during ᴍᴀᴛɪɴɢ season.

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