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They are said to include at least three "supercentenarians" -- the name given to those aged over 110 -- out of fewer than 400 thought to exist worldwide.
Li Aizhu, whose government-issued residence permit says she was born in 1900, hobbles daily from her tiny concrete bedroom, where an electric fan beats back the intense heat, to watch quacking ducks waddle past her family farmhouse.
"Who are these people?" Li asked relatives when visitors arrived, a shock of white hair framing her wrinkled face.
She aimed the same question at government officials who arrived with a metal plaque declaring her to be a "longevity celebrity", her family said. It now hangs above a hard wooden bench in her simple living room, entitling her to a 500 yuan ($81) monthly payment, and free medical treatment.
Other clusters of longevity are said to include Cuba, islands off Greece and Japan, and a peninsula in Costa Rica, with researchers finding common characteristics including an emphasis on family, lifestyles requiring physical activity, and a plant-based diet.
was swept by dramatic changes -- from Japanese invasion, to the Communist victory in the civil war, and a transition from a planned economy to the market -- most Chengmai residents carried on doing what they had always done, growing crops.
"I've never done any exercise, except hard farmwork," said 86 year-old Wang Kailu, who lives in a simple concrete shack with his wife Wu Aihe. The couple said they married the day after Japan's World War Two surrender, 68 years ago.
Their one-storey dwelling is barely furnished and Wang draws water from a well to spread on his small vegetable plot.
Experts on ageing who have travelled to the area say several factors could be involved in the phenomenon.
Jennifer Holdaway, who runs the China Environment and Health Initiative of the U.S.-based Social Science Research Council and visited Chengmai for a government-funded conference last year, pointed out that its economy was centred around agriculture.
"There is not a lot of industry, the climate is good, they can get exercise easily, and the diet is healthy; they have plenty of fresh fruit and vegetables, and the soil is also naturally high in selenium", an essential nutrient, she said.
Locals, though, claimed a healthy reliance on alcohol was vital. Xu Yuhe, whose residency permit lists her as 104, told AFP that she takes daily shots of "Three Coconut Spring" a local grain spirit.
"I drink alcohol every evening, just a little bit, it helps you feel warm," added Sheng She, an 80-year-old who says she has 31 children and grandchildren.
Experts say an active social life is also a key to longevity, and each morning dozens of Chengmai's elderly pack out benches in a teahouse, with men at the back playing cards and women at the front, chatting and listening to clattering opera recordings.
"I come here every morning to exercise, watch opera and drink tea," said Sheng, before darting out to test her strength on a yellow-painted metal exercise machine.
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A study commissioned by Chengmai's Communist Party committee put its residents' lifespans down to their being "diligent, simple-minded and magnanimous, mainly taking a vegetarian diet, eating sensibly, early to bed early to rise."
The county has corralled its centenarians into singing concerts and paraded them for the media as it seeks to promote itself as a center of long life.
Property companies -- which have developed large swathes of Hainan in recent years -- also hope to cash in, with the island a hotspot for retirees from across China.
Soaring clumps of newly-built apartment blocks dot the county, with one named "Longevity City" trumpeting the ageing population in its advertising.
But while Chengmai has stepped up investment in elderly care -- AFP visited a newly built set of huts for housing those whose families cannot look after them -- facilities for the aged themselves remain basic.
Experts say longevity zones around the world are rarely in rich areas.
"If you're living in a climate like that you don't need very much to be comfortable: simple housing, a mosquito net, some rattan chairs and other people to hang out with," said Holdaway.
"It's a different matter if you live in the North where you have to spend a lot of time indoors and fresh food is expensive in the winter."
Li Aizhu, though, has a simpler explanation. "We asked her once," said the 113-year-old's great-granddaughter, Yi Mei. "She said it's because she eats a lot of peanut oil, that's her secret."