It didn't exactly break the Internet, but there is no denying that it's an eye-catching photo: a smiling man holding a yam that is about 3 feet long.

"That's the biggest one I've seen from that particular species," says Paul Wilkin, the head of natural capital and plant health at the United Kingdom's Royal Botanic Gardens, Kew.

Mamy Tiana Rajaonah, who lives in Antananarivo, Madagascar, took the photo and tweeted it out to the world on Nov. 10. Rajaonah has been studying yams for more than 15 years and is the yam project manager for the Kew Madagascar Conservation Centre.

Why so much interest in yams? They're a major food source on Madagascar, which has some 40 species, of which more than 30 are found only on the island. And the Malagasy people definitely benefit by eating the tubers.

"You dig up one of those tubers, you've got food for a family," says Wilkin, who has been studying yams for decades.

They're healthy, too — a good source of fiber and potassium, for example.

That is a boon in a country where more than half the children under age 5 are chronically malnourished, according to the World Bank.

And yams definitely fill you up. People in Madagascar generally eat rice to feel full, Rajaonah says. But if there isn't enough rice around, a roasted or boiled yam helps. "When we eat yams, we don't need rice after," says Rajaonah.

But the island's yams are in a jam. Their habitat is disappearing, partly because of a rise in peanut farming, Wilkin says. And yams, which are the edible root of climbing vines, are being overharvested.

"I've seen 15 or so adult plants surrounded by 100 holes where yams have been extracted," Wilkin says. "That's not sustainable for very long."

Bako — the superlong yam from the tweet — is one of the affected species.

"People say it used to be much easier to find them. Now they have to walk a lot further to find them," Wilkin says.

"The rate of exploitation is scary. That's why the prices have gone through the roof," he says.

The price of bako in local markets has more than quintupled since they first identified it in a 2008 paper. It went from about $1.25 for a large piece of the tuber to more than $6 for a smaller piece, Wilkin says.

To make sure locals can get plenty of yams, the yam project distributes "seed yams" — cut-up pieces of a bigger tuber or yams that aren't very big and can be used for replanting. The goal is to encourage people to grow their own wild yams.

Although, it isn't necessarily easy. "Wild plants don't always take to being cultivated," says Wilkin. "We're just starting to learn how to work with them."

An additional problem is yam swiping. People will sneak into these yam gardens and "try to steal the yams cultivated by the other [people]," says Rajaonah.

Meanwhile, on this Thanksgiving holiday, we should note that yams are not the same as sweet potatoes. While both are tubers that grow underground, sweet potatoes are part of the morning glory family while yams are related to grasses and lilies.

The yams are starchier and drier than sweet potatoes, and the Madagascan varieties have white or purple flesh.

If you're curious to taste a yam, you may have to do a little tuber shopping. In the U.S., according to the Library of Congress, most tubers that are labeled "yams" are actually sweet potatoes — so check the fine print on the sticker.

But if you see something that looks like a sweet potato and is 3 feet long — that could be a Madagascan yam.

Courtney Columbus is a multimedia journalist based in the Washington, D.C., area. She covers science, global health and consumer health. Her past work has appeared in theArizona Republic and on Arizona PBS. Contact her@cmcolumbus11.

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