Gorillas on a diet

Toronto Zoo gorillas are eating more but losing weight. Could a high-fibre diet do the same for people?

Gorillas on a diet

Photograph by Andrew Tolson

Charles used to have big rolls of fat around his neck, and chunky legs and thighs. These days, he’s much less pudgy. Charles dropped 19 lb. from June to October, which—for a 400-lb. male silverback gorilla—is no small feat. “The zoo regulars have been commenting on how good the gorillas look lately,” says Heidi Manicki Claffey, who’s been a gorilla handler at the Toronto Zoo for the past 25 years. Charles, 39, still has a big pot belly, she says, “but that’s normal for a gorilla.”

Charles is on a diet of sorts. Instead of the high-sugar, high-starch foods that zoos have fed gorillas for decades, he and the six others at the Toronto Zoo are munching on parsnips, cabbage, nuts and tofu. (As a treat, they get cinnamon-flavoured herbal tea.) This winter, for the first time, they’re also being given “browse” each day: branches they strip of edible leaves and bark, as they would in the wild. These gorillas are actually being given a greater volume of food, but they still seem to be shedding flab. From June to October, Josephine, a 40-year-old mother gorilla, dropped 22 lb.

Obesity is a growing health problem for virtually all zoo animals, from elephants to dolphins, and certainly for gorillas. In zoos, “about 40 per cent of adult male gorilla deaths are from heart disease,” which is now the number one killer of male western lowland gorillas, the only species in North American zoos, says Elena Hoellein Less, who’s finishing a Ph.D. in biology at Case Western Reserve University. Less, who works at the Cleveland Metroparks Zoo, is leading an effort to bring gorilla diets closer to what the animals eat in nature. Several zoos, including Toronto, are collaborating with her, feeding gorillas a greater volume of food—and more calories—than before, with a focus on high-fibre vegetables and browse, instead of starch and carbohydrates. This work could have implications for our own obesity epidemic, too.

Zoo gorillas live longer than those in the wild because they don’t have predators, parasites and other threats to worry about. And they have a steady source of nutrient-rich food, but it’s not what they’d eat in the jungle. Zoo gorillas are typically fed a diet of special gorilla biscuits, which can account for 15 per cent of their diet, Less says. They also get fruits and vegetables—which still tend to be higher in sugar and starch, and lower in fibre, than anything they’d eat outside the zoo—and sometimes even yogourt. “These foods deliver a lot of nutrients in a short time,” says Jaap Wensvoort, nutritionist at the Toronto Zoo. Even a banana, he says, “is very dense” for a gorilla. He calls it fast food.

Gorillas on a diet

Photograph by Andrew Tolson

But gorillas are, by nature, slow eaters. In the jungle, where calories are harder to come by, they spend up to 70 per cent of the day eating or foraging; in zoos, meals take up less than half that time. To fill the extra hours, zoo gorillas are given toys to play with, but “unless it’s food, they don’t really care,” says Ali Vella-Irving, another Toronto gorilla handler. Boredom could be one reason that about 65 per cent of zoo gorillas will voluntarily regurgitate their food and then eat it again, a behaviour that isn’t seen in the wild.

Even before collaborating on Less’s project, the Toronto Zoo had been tinkering with its gorilla diets. About a decade ago, “we took the yogourt and a lot of the gorilla chow away,” Wensvoort says, and added soy proteins, which protect against heart disease in humans. (Gorillas are vegetarian.) Today, they’re fed five times daily, mainly high-fibre vegetables and browse. They still get some gorilla biscuits and muffins—which deliver specific nutrients, and medication like birth control—but they get very little carbs. Higher-calorie foods, like nuts, tofu and yam, are given to each gorilla individually, so they don’t fight over them; lower-calorie foods like celery, mushrooms and leeks are spread around their enclosure, which keeps them busy.

Browse is left around their enclosure, too. These leafy branches are clipped from nearby parks, orchards, or even zoo grounds. (Gorillas like everything from apple to willow, forsythia and hawthorn.) In the Canadian climate, finding a steady source of browse can be a challenge. “In the winter, it will dry out and the leaves will fall off,” Wensvoort says, which makes it unappetizing. The zoo has solved that problem by creating a new press, the first of its kind in North America, that packs leaves and branches into barrels, where they’re preserved with lactic acid bacteria (like sauerkraut), creating a moist brown silage. Gorillas are particularly fond of silage made from apple trees, which smells sharp and sweet, like apple cider vinegar.

About 50 different types of animals at the zoo receive browse, including these gorillas, says Wensvoort, who prepares 450 different animal menus every day. “Elephants love browse,” he says. “Katydids just eat the leaves. With Indian rhinos, you put big sticks in there, and it’s like a wood chipper.” Browse left in the gorillas’ enclosure keeps them busy for hours as they chew bark from each branch, working on it with their lips and teeth, like eating the meat off a bone. “Browse stimulates their natural foraging behaviour,” Vella-Irving says. Since introducing more browse into their diet, the gorillas’ voluntary regurgitation has all but disappeared. Bark and leaves are so fibrous, they’re probably hard to spit up; and now, these gorillas have another food-related activity to keep them busy.


The impact of these dietary changes still isn’t totally understood. In humans, obesity can lead to all sorts of problems, but in gorillas, “we don’t really know what ‘obese’ is,” Less says. She and others are taking gorillas’ body mass indexes, or BMIs—a measurement based on weight and height—to determine what the BMI of an overweight gorilla might be. (In humans, a BMI of 25 or higher is considered overweight; 30 or higher is considered obese.) They’re looking at biomarkers of inflammation and insulin resistance, too.

And in Toronto, Manicki Claffey and Vella-Irving have trained the gorillas to accept a mock probe with contact gel to their chests, so the zoo can eventually take ultrasounds of their hearts. (They recently acquired an ultrasound machine.) These images, along with blood samples and maybe even blood pressure readings, will give a much better idea of the gorillas’ cardiac health.

Gorillas on a diet

Photograph by Andrew Tolson

Their findings might change the way we think about our own nutrition. “We’re only about three per cent different genetically from other great apes,” and share a common ancestor, says Dr. David Jenkins of the University of Toronto, Canada Research Chair in nutrition and metabolism. In a study in The Journal of Nutrition, Jenkins and his team crafted a human adaptation of the gorilla diet—low in fat and very high in fibre, mainly from vegetables—and fed it to young, healthy volunteers for two weeks. “You had to spend eight hours a day eating,” he says. “They ate the food under duress. It wasn’t that it was unpleasant; it was too much.” But their cholesterol levels, he says, “plummeted into their boots,” an effect comparable to what’s achieved with statins (cholesterol-lowering drugs).

In another study, published in Metabolism in 2001, Jenkins and co-authors noted that modern efforts to boost humans’ fibre consumption have focused on cereal fibres like wheat bran, but we’ve only consumed cereal for a relatively short period of time, so it’s probably had very little impact on our evolution. They looked at three diets—a very high-fibre diet based on vegetables, fruits and nuts; a starch-based diet with cereals and legumes; and a low-fat diet—and concluded that the first one had the biggest positive impact on cholesterol levels. Jenkins plans to collaborate with Wensvoort on more “cross-species research” soon. “Humans are not so different from the rest of our primate cousins,” he says, “so we’d better look after them, and learn from them, too.”

Back at the Toronto Zoo, the seven gorillas seem to be flourishing. “Since the browse was introduced, it’s night and day,” Manicki Claffey says. Wensvoort is hoping to create a dedicated plantation to produce a steady supply of browse for zoo animals, gorillas included. “With slow foods, they express their natural behaviours,” he says.

On a recent weekday, Wensvoort and the two gorilla handlers stood in the African Pavilion—where the gorillas spend much of the winter months—and watched as the gorillas took their noontime meal. Charles grabbed a handful of silage from an open bucket, lowered himself to the ground, and munched it by the handful. Josephine observed a branch in her hands, carefully peeling back its leaves and folding them into her mouth. And Shalia walked around bipedally, carrying armfuls of silage, probably looking for a place to sit. Baby Nassir, who’s only about 18 months old, held on to mom Ngozi as she prowled through the enclosure.

“They’re not skinny, but they’re active,” Wensvoort says, “They’re a normal family group.” Manicki Claffey laughs. “I should be on the gorilla diet,” she says.