Starving orangutans may explain why humans evolved to become obese

They're not as closely related to us as chimps and gorillas, but we share something crucial with orangutans: we both evolved to survive long periods without decent food. And that shared past could help explain our modern obesity epidemic.

The orangutans on the Indonesian island of Borneo have been isolated for the last 400,000 years. In that time, they've had to adapt to the unique challenges posed by this island ecosystem, which lacks consistently fertile soil and so does not produce reliable crops of plants to eat. Though orangutans prefer to eat fruit, the island only produces large quantities of the stuff every four or five years, and so the apes have to rely on tougher, less nutritious foods like seeds and the starchy tissues found behind tree bark.

It's an unusual situation, as most other primates can rely on consistent access to food. One other exception to that general rule is our own hominid ancestors, who faced similar food shortages in ancient Africa. And according to research by Dartmouth's Nathaniel Dominy, both Borneo orangutans and our hominid ancestors evolved similar solutions to deal with these harsh conditions.

Through collecting orangutan urine on Borneo, Dominy and his team were able to track how the creatures' diets changed over time. When fruits were abundant, the orangutans gorged on their favorite food, building up fat reserves. During the hard times that followed, the urine revealed a spike in dietary markers known as ketones, which are found when the body has to break down those fat reserves for energy. This is a tolerable situation for the orangutans, but once the fat reserves run out, the orangutans' bodies have to start cannibalizing their own muscles to survive.

Dominy says this strategy has allowed the orangutans to survive on Borneo for as long as they have, and another crucial area of adaptation has been in their teeth. Orangutans have developed larger molars and bigger jaws that are useful in crunching through the tough seeds. It's an adaptation that is only occasionally useful — but when it is needed, the orangutans would likely die without it. Early hominids show similar adaptations, suggesting they too had to sometimes rely on this incredibly tough food.

Dominy comments, in a press release:

"Perhaps the hard objects were things they ate only very occasionally under ecological duress. It is not what they ate regularly that matters. It is what they were eating during crunch times. Because they routinely go through these dire times, orangutans may be a good model for what happened to human ancestors in deep time."

So what does this all have to do with obesity? For that, we turn to Rutgers researcher Erin Vogel, who has also been studying the Borneo orangutans. For one thing, this study argues fairly strongly against the idea that high protein diets are useful in weight loss, as orangutans only ever put on fat during periods of high protein and caloric intake, while they burn their fat reserves for energy when they're low on proteins. Vogel explains:

"There is such a large obesity epidemic today and yet we don't really understand the basis of the obesity condition or how these high-protein or low-protein diets work. I think studying the diets of some of our closest living relatives, the great apes; may help us understand issues with our own modern day diets. We discovered through this research that the daily amount of protein the orangutans take in when fruit is not available is inadequate for humans and one-tenth of the intake of mountain gorillas. But it is sufficient to avert a severe protein deficit."

Vogel says the behaviors the orangutans display — gorging on food when it's readily available, burning off all their built-up reserves when it's not — are likely similar to those of our own hominid ancestors, and they may help explain the modern behaviors that underpin both obesity and, on the other side of the spectrum, eating disorders like anorexia.

The orangutans are particularly interesting because they don't enjoy a consistent food source, and the constant need to readjust one's eating habits in response to different environmental conditions may help explain where these disorders evolved. Of course, that's not to downplay the importance of cultural, social, and psychological factors - just to point to a possibly intriguing deeper evolutionary history for these eating disorders.

Via the American Society of Primatologists. Original papers in Biology Letters will be published shortly. Image by Tim Laman.