Be it feast or famine, orangutans adapt with flexible diets

Humans could learn a thing or two from orangutans when it comes to maintaining a balanced, protein-filled diet.

Great apes native to the rainforests of Indonesia and Malaysia, orangutans are marvels of adaptation to the vagaries of food supply in the wild, according to an international team of researchers led by a Rutgers University-New Brunswick scientist. The critically endangered primates outshine modern humans in avoiding obesity through their balanced choices of food and exercise, the scientists found.

The researchers reported their findings, based on 15 years of firsthand observations of wild orangutans in the jungles of Borneo, in Science Advances.

“These findings show how wild Bornean orangutans adapt to changes in their environment by adjusting their nutrient intake, behavior and energy use,” said Erin Vogel, the Henry Rutgers Term Chair Professor in the Department of Anthropology in the School of Arts and Sciences, who led the study. “The work highlights the importance of understanding natural dietary patterns and their impact on health, both for orangutans and humans.”

Orangutans are one of the closest living relatives to humans, sharing a common ancestor, Vogel said. This evolutionary relationship means that orangutans and humans have similar physiological and metabolic processes, dietary needs and behavioral adaptations. Studying orangutans can provide insights into the evolutionary adaptations that might also be relevant to humans, she said.

Humans also exhibit metabolic flexibility, Vogel said, but modern diets high in processed foods can disrupt this balance, leading to metabolic disorders such as diabetes.

While orangutans reduce physical activity during low fruit periods to conserve energy, Vogel said, humans, especially those with sedentary lifestyles, may not adjust their energy expenditure to match their caloric intake, leading to weight gain and associated health issues.

“Understanding these adaptations can help us learn more about how humans can manage their diets and health,” Vogel said. “It also highlights the importance of conserving orangutan habitats to ensure their survival.”

The research was conducted at the Tuanan Orangutan Research Station in the Mawas Conservation Area in Central Kalimantan, Indonesia, on the island of Borneo. The conservation area, a peat swamp forest, protects about 764,000 acres, an area roughly the size of Rhode Island. Peat forests are richly biodiverse, ancient ecosystems with landscapes dominated by waterlogged trees that grow on layers of dead leaves and plant material.

Understanding the dietary strategies of orangutans can inform better nutritional practices for humans, said Vogel, who also is director of the Center for Human Evolutionary Studies at Rutgers.

“In essence, the research on orangutans underscores the importance of dietary balance and metabolic flexibility, which are crucial for maintaining health in both orangutans and humans,” Vogel said. “It suggests that modern dietary habits, characterized by high consumption of processed foods rich in sugars and fats, can lead to metabolic imbalances and health issues.”

In earlier studies, Vogel and an international team of colleagues established the patterns by which orangutans fed. Orangutans prefer to eat fruit because it is rich in carbohydrates, but when fruit is scarce, they switch to eating more leaves, bark and other foods that can provide more protein but fewer sugary carbohydrates. In times of high fruit availability, orangutans still consume protein but get most of their energy from carbohydrates and fats in the fruit.

“We wanted to find out how their bodies handle these changes,” Vogel said. “We tested how the availability of fruit affects their diet and how their bodies adapt to avoid energy imbalance. We looked at how they switch between different types of fuel – like fats and proteins – when preferred food availability changes.”

To conduct the study, Vogel, research colleagues, students and a staff that mostly included field technicians indigenous to the island of Borneo collected data for more than a decade on what the orangutans ate daily and analyzed their urine to see how their bodies responded to any nutritional changes. This required staying in close proximity to the ape in the equatorial, humid jungle from dawn until night.

The scientists made a number of key findings:

• Orangutans avoid obesity as part of a response to the significant fluctuations – in both magnitude and duration – in fruit availability in their natural habitat. Unlike humans in Western culture, who have constant access to high-calorie foods, orangutans experience periods of both abundance and scarcity. The periods of scarcity and resulting low caloric intake, similar to humans’ intermittent fasting, may help maintain their health by reducing oxidative stress.
• During periods of fruit scarcity, orangutans exhibit metabolic flexibility, switching to using stored body fat and muscle protein for energy. This allows them to survive when food is scarce.
• During periods of fruit scarcity, orangutans exhibit behavioral adaptability, relying on reduced physical activity as well as stored energy and muscles to conserve energy. They rest more, go to sleep earlier, travel less and spend less time with other orangutans. This flexibility enables them to use body fat and protein for fuel when needed. They rebuild fat reserves and muscle when fruit availability is high.
• The orangutan diet also prioritizes a consistent level of protein, which contrasts with a modern Western diet, which often can be rich in low-cost, energy-dense, protein-poor foods. Those choices contribute to obesity and metabolic diseases in humans.

This research builds on a report published earlier this year in The American Journal of Biological Anthropology, led by doctoral student Will Aguado, as the first author. This study found that orangutans at Tuanan get most of their protein from the leaves and seeds of just one out of nearly 200 species in the diet -- a vine called Bowringia callicarpa. The protein in this plant fuels orangutans through seasons of fruit scarcity and likely allows orangutans at Tuanan to persist and for their population to grow.

Other scientists on the study from Rutgers included Malcolm Watford, a professor in the Department of Nutritional Sciences, Rutgers School of Environmental and Biological Sciences; and former Rutgers doctoral student Rebecca Brittain, Tatang Mitra-Setia and Sri Suci Utami from Universitas Nasional in Indonesia, graduate students William Aguado, Astri Zulfa and Alysse Moldawer, all with the Department of Anthropology in the School of Arts and Sciences. Former graduate student Timothy Bransford, who also contributed to the study, is now at Eckerd College, St. Petersburg, Fla.

Researchers from the following institutions also contributed to the study: The Max Planck Institute of Animal Behavior and the University of Konstanz in Germany; Yale University; Jagiellonian University in Krakow, Poland; the University of Cincinnati; the University of Colorado; Eckerd College in St. Petersburg, Fla.; Universitas Nasional in Jakarta, Indonesia; National Research and Innovation Agency in Cibinong-Bogor, Indonesia; University of Zurich in Switzerland; Hunter College of the City University of New York; and the University of Sydney in Australia.

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