In lush times, orangutans on the island of Borneo gorge themselves on forest fruits, packing on extra pounds in preparation for leaner years, when they live off leaves and bark and their own stored fat.
This behavior of overeating is all too common in humans, but rarely seen in nonhuman primates, and studying it may offer some clues about obesity and eating disorders in people, U.S. researchers said on Tuesday.
"Orangutans make very interesting models for studying human obesity because they are really the only apes and potentially the only nonhuman primates in the wild that actually store fat deposits," said Erin Vogel, an evolutionary anthropologist from Rutgers University in New Jersey, whose study appears in the journal Biology Letters.
"It's never been documented in any other species," Vogel said in a telephone interview.
Vogel and colleagues studied urine samples from Bornean orangutans laboriously collected over a period of five years by a team led Dr. Cheryl Knott, a biological anthropologist at Boston University.
"Orangutans living in this really challenging habitat are able to take advantage of these periods of incredible fruit abundance -- these masting periods, where 80 percent of the fruit on the trees are fruiting," Vogel said.
"They eat and eat and eat and they get fat," she said.
Then they go through periods of very low fruit production that can last up to eight years.
In the study, as food stores became more and more scarce, the orangutans shifted to bark and tough leaves to survive. And the team noticed changes in the apes' urine.
First, they saw ketones, a sign that the body was metabolizing fat. "It indicates they are burning this fat for energy," Vogel said.
And then they saw elevated nitrogen isotopes. These indicated that muscle cells were being broken down to obtain protein and energy.
"They have to get energy from somewhere, so they start to digest their body tissue, just like you would find in situations were humans are very impoverished, and in anorexia, where we would potentially see conditions where humans would digest their own muscles," Vogel said.
Vogel credits Knott's team for collecting the urine samples, which was no mean feat.
The team followed the orangutans from the time they woke up in their nest until the time they went to sleep.
"As soon as they wake up, they typically void -- they urinate," Vogel said.
Knott's team would be waiting underneath the tree canopy to collect these samples, either with plastic sheeting or an inverted umbrella held over their heads, which worked as both a collection device and some protection from the shower of urine.
Vogel said the study shows how orangutans have taken advantage of their ability to store fat to increase their chances of survival, but this same ability is a deficit for most humans who do not need to forage for food.
"We have this wonderful ability to store fat, and now most of us wish we didn't have it," she said.
In future studies, Vogel said she plans to look for fluctuations in the hunger-related hormones ghrelin and leptin during periods of food scarcity and abundance, as well as changes in inflammatory cell signaling chemicals known as cytokines, which are thought to play a role in obesity.
Orangutans are endangered. There are only 50,000 individuals remaining in Borneo and 7,300 in Sumatra -- the two places in the world where they can still be found in the wild.