An illustration of a monkey holding a banana amid soda bottles.
Madison McVeigh/CityLab

Put down that banana! Eating human food is making the world’s city-dwelling simians sick.

Earlier this year, a story about a morbidly obese wild monkey made the rounds on the internet. “Uncle Fat,” as locals called the Bangkok macaque, had eaten so much junk food—think soda and chips—that he tipped the scales at 60 pounds, three times his healthy weight. Uncle Fat had accumulated a massive mound of flesh that dangled from his midsection, which interfered with his ability to move with ease. But no matter: The stout simian did not need to search for food. As the leader of his pack, minion monkeys regularly supplied him with more human treats.

Thai wildlife officials intervened. Uncle Fat is now under the care of a veterinarian, and is on a strict diet of lean protein, fruits, and vegetables—closer to what he would eat in the wild. The hope is that he’ll be able to be released soon.

Uncle Fat makes an entertaining story, but his sorry state is indicative of a surprisingly serious problem found across the globe. Cities such as Bangkok, New Delhi, and Kathmandu have large populations of simian residents, and they increasingly find themselves needing to discourage people—mostly tourists—from stuffing their monkeys with human food that isn’t good for them.

This means junk food, but even the bananas we eat are like cake to a monkey. “[Bananas that are cultivated for humans] have lots of calories—and contain much more sugar that’s bad for their teeth and can lead to diabetes and similar conditions,” Amy Plowman, a nutrition expert at England’s Paignton Zoo explained in a press release.

In Medellín, Colombia, a team headed by University of Antioquia conservation biologist Iván Dario Soto studied the effects of human food on the city’s white-footed tamarin monkeys. Soto told New Scientist that “people have fun feeding them things like cane sugar, cookies, marshmallows, and bananas.” Medellín’s tamarins also have no predators, and have easy access to the city’s guava and mango trees. Hence, they’ve become human-like in their inactivity. “[City] tamarins probably don’t have to move as much as rural tamarins,” Soto said.

Soto’s team compared the health of 16 adult urban tamarins to 20 adults in rural rainforests by measuring them and testing their blood and feces several times over two years. They found that the urban tamarins were overweight, with a “significantly larger” body mass than their rural counterparts, and had a 38 percent higher level of cholesterol.

The Bangkok macaque dubbed “Uncle Fat” by locals is now following a strict diet. (Sakchai Lalit/AP)

Mukesh Kumar Chalise, an associate professor of zoology at Tribhuvan University in Kathmandu, has been studying the city’s macaques for more than 20 years, and he’s observed how fatty human food tends to give the animals skin diseases and diarrhea. “The monkeys can also catch coughs and other respiratory viruses from humans when humans feed them,” he says. (This can occur if, for instance, a person drinks from a soda bottle and then gives it to a monkey.)

The urban monkey obesity crisis has another undesired side effect: too many monkeys. Chalise told the Kathmandu Post that in an area around the Swayambhu Buddhist temple complex west of the city, there are 10 times the number of monkeys than should be. Such overpopulation, he says, has led to more aggressive behavior among the macaques (with each other but also with people), as there are a larger number of them competing for food, as well as abnormal breeding habits such as too-frequent pregnancies.

The numerous health problems stemming from human food result in a shortened lifespan: Urban monkeys, says Chalise, only live up to 10 to 12 years, rather than the more usual maximum of 20 years.   

Researchers and organizations such as the Moroccan Primate Conservation Foundation regularly call for restrictions on feeding monkeys as well as public education on why it’s hazardous. The Moroccan foundation’s 2012 action plan for saving the country’s Barbary macaques, for example, advocates for such education.

Some cities are also taking an active role. Singapore, for example, fines monkey feeders $370. Yet Chalise says that on the whole cities need to do more than try to limit feeding. “Cities should only allow the number of monkeys that would naturally survive in urban green spaces,” he says, “and then they should transfer the excess healthy population to rural areas.”

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