"Some Monkeys Have Blue Testicles—Here's Why" by Liz Langley

[malymin]

Well. I don't like thinking about genitals, animal or otherwise, but this is relevant to my interests in animal coloration and so I must make it available to The People.

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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.

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, 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.

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 that turns a pale blue or even white in adulthood.

Quite the colorful family jewels.

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Comments

[gremdark]

Ooh! I can share a relevant science fact! In Costa Rica, I saw a troop of mantled howler monkeys. The monkeys are black all over except for their scrota, which are white. My guide told us a local folktale that [paraphrased] is about a howler monkey (all-white in those days) who climbed to the top of the tallest tree in the forest in the middle of a thunderstorm. Seeing that lightning was about to strike, he hurriedly shielded his balls, leaving them white while the rest of him was singed black.

[malymin]

Oh, that's such a funny folktale! XD

There's similar stories explaining animals being "scorched black" from some previous all-white or rainbow state in other mythologies. (There's examples for ravens in both the Greco-Roman tradition and in Pacific Northwest indigenous creation myths, for example.) But the idea of a monkey seeing a threat to its life and going "oh no I have to protect MY BALLS" is just really comedic.

[lb_lee]

We appreciate the absurd specificity of this post, but the Oakland Zoo link for the African green money is sadly broken. :(

[malymin]

Thanks!

I copied the article basically as-is, so I didn't have an opportunity to check all the links. The Oakland Zoo link has been replaced with a link to a Wayback archive of the original page, so you should be able to read it now. If any other links are broken, let me know! ^_^

[lb_lee]

Sneak: Yay, thank you! For completely unrelated reasons, we desperately wanted to see an African green monkey. o_o

[malymin]

It's funny, they're not that green in the grand scheme of things, but we call olives green, and they are kind of an olive color from a distance? Just like the olive baboon. Not green as in "blue + yellow" but green as in "yellow + black." In both species, the "green" effect comes from the hairs being a banded pattern of black and pale yellow.

I mean, even this color on an RGB monitor is just "dark yellow", or "yellow mixed with black", but it looks more "green" than "yellow" to the average person, right?

If you want to make something look "dark yellow", you mix it with red or brown, not with black... it's an odd quirk, isn't it?

[lb_lee]

I credit it to the ur-yellow being the sun, which glows, so adding darkness to it breaks our mental categorization of the color.