Orangutans Fruit Scarcity

Wild Bornean orangutans (Pongo pygmaeus) catabolize muscle tissue when the availability of fruit is low, according to new research led by Rutgers University scientists; that’s remarkable because orangutans are thought to be especially good at storing and using fat for energy.

A male Bornean orangutan (Pongo pygmaeus) eating non-fruit vegetation instead of the fruit orangutans prefer on the island of Borneo. Image credit: Kristana Parinters Makur / Tuanan Orangutan Research Project.

Orangutans weigh up to about 80 kg and live up to 55 years in the wild. They are the most solitary of the great apes, spending almost all of their time in trees.

Bornean orangutans also spend some time on the ground. Deforestation linked to logging, the production of palm oil and paper pulp, and hunting all pose threats to orangutans, whose populations have plummeted in recent decades.

These animals also face great challenges in meeting their nutritional needs. With low and unpredictable fruit availability in their Southeast Asian forest habitats, they often struggle to eat enough to avoid calorie deficits and losing weight.

Because orangutans are critically endangered, scientists need to explore new ways to monitor their health without triggering more stress in them.

“Conservation plans must consider the availability of fruit in forest patches or corridors that orangutans may need to occupy as deforestation continues across their range,” said Dr. Caitlin O’Connell, a postdoctoral researcher in the Department of Anthropology at Rutgers University.

In the study, Dr. O’Connell and colleagues measured creatinine, a waste product formed when muscle breaks down, in wild orangutan urine to estimate how much muscle the primates had when fruit was scarce versus when it was abundant.

In humans, burning through muscle as the main source of energy marks the third and final phase of starvation, which occurs after stores of body fat are greatly reduced.

So, the researchers were surprised to find that both males and females of all ages had reduced muscle mass when fruit availability was low compared with when it was high, meaning they had burned through most of their fat reserves and resorted to burning muscle mass.

“Orangutans seem to go through cycles of building fat and possibly muscle mass and then using fat and muscle for energy when preferred fruits are scarce and caloric intake is greatly reduced,” said Professor Erin Vogel, also from the Department of Anthropology at Rutgers University.

“Our team plans to investigate how other non-invasive measures of health vary with muscle mass and how the increasingly severe wildfires on Borneo might contribute to muscle loss and other negative health impacts.”

The results were published in the journal Scientific Reports.

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C.A. O’Connell et al. 2021. Wild Bornean orangutans experience muscle catabolism during episodes of fruit scarcity. Sci Rep 11, 10185; doi: 10.1038/s41598-021-89186-4