This chicken has black bones, organs, and meat: Here’s why.

The Ayam cemani chicken may be the most deeply pigmented creature on earth. Not only are the bird’s feathers, beak, comb, tongue, and toes a striking, blue-ish black, but so are its bones.

Even the chicken’s meat looks like it has been marinated in squid ink.

Interestingly, the cemani, which is found in Indonesia, is just the most extreme example of what scientists call dermal hyperpigmentation. Another breed, known as the silkie because of its soft, hair-like feathers, also sports hyperpigmented skin and tissues, as do the black H’Mong chickens of Vietnam and the svarthöna of Sweden.

Scientists call the condition fibromelanosis.

“We have evidence that it is a complex rearrangement in the genome,” says Leif Andersson, a geneticist at Uppsala University in Sweden who studies the genetics of domestic animals.

What’s more, Andersson says all of these chickens can trace their mutation back to a single bird that may have lived hundreds or even thousands of years ago.

“The mutation underlying fibromelanosis is very peculiar, so we are sure that it occurred once,” says Andersson.

Who wants some dark meat?

While the internet has seen many different kinds of animals exhibiting melanism, from black panthers and servals to a melanistic flamingo, black-scaled geckos, and snakes, the chickens Andersson studies take this pigmentation to an entirely new level.

Here’s how it works.

Most vertebrates have a gene known as endothelin 3, or EDN3, which, among other things, controls skin color. And when a normal chicken is developing, certain cells, like those in the skin and feather follicles, express EDN3, which triggers the migration of melanoblasts, or the cells that go on to create color.

However, in the hyperpigmented chickens, virtually all of the body’s cells express EDN3, creating up to 10 times as many melanoblasts and leading to bones and innards that look like they’ve been dipped in tar.

“It’s a mis-migration,” says Andersson. “If you express too much endothelin 3, and in the wrong places, the pigment cells migrate to the wrong place.”

Fortunately, the mutation doesn’t seem to have any ill effects for the birds, health-wise.

In fact, it’s quite the opposite—the dark color of these breeds has made them more valuable in the eyes of breeders and gourmands, who say the off-color meat and bones possess a unique and rich flavor.

From oddity to best in show

Even though scientists now understand what makes these chickens special, the history of these breeds is still a matter of mystery.

Many consider a few words penned by Marco Polo to be the first reference to black-boned chickens. In 1298, while traveling in Asia, the explorer wrote of a breed of chickens that “have hair like cats, are black, and lay the best of eggs.” No one can say for sure, but the description sure sounds like a Silkie.

From there, Andersson says the mutation was most likely spread around the world by livestock owners who appreciated the novelty of the birds’ coloration. There’s even an anecdote about a sailor bringing a black chicken back from an East Asian trade route, which could explain how the Svarthöna wound up in Europe.

“I think it’s quite clear that humans like diversity among domestic animals,” says Andersson, who has also researched the genetic origin of the silkie’s feathers and is currently studying how the chickens develop their crests.

Even if the breeds have been around for several centuries, the animals are still relatively rare.

For instance, of the four breeds, only the silkie has been given its own standard of perfection by the American Poultry Association, which means it can compete in shows.

According to John Monaco, who is president of the APA, the process of obtaining a standard can take years.

“The Cemanis haven’t been around that long, and people are really just starting to work with them,” says Monaco. “But Silkies are everywhere. There are a lot of varieties, and they’ve actually won shows as champions.” (See seven regal photos that show the elegance of chickens.)

Of course, all the black chicken breeds are winners in Andersson’s book, simply because their coloration is so unlikely, genetically speaking.

“It’s more common to see defective pigmentation—white spots or lack of pigmentation— because it’s easier to disrupt genes than activate genes in the way that happened here,” he says.

It was chance that made the black chickens possible. But it was choice that led humans to breed them and spread them across the planet.

“That’s what is really fascinating,” says Andersson.