More (Inner) Bark Than Bite: Studying Orangutan Diets

Nutrition Program Director Barbara Toddes has managed the nutrition program at the Philadelphia Zoo since 1984. As part of her job, she ensures every calorie our 1,700 animals consume daily is packed with nutrients appropriate for their species.

That’s why she and her nutrition department have teamed up with researchers who study orangutans in the wild to compare the nutrients in the diets we offer at the Zoo to what the orangutans eat in the wild.

Researchers Dr. Andrea DiGiogio (Princeton), Dr. Liz Ballare (Rutgers), and Dr. Erin Vogel (Rutgers) recently visited the Zoo to teach our browse workers how to collect phloem samples to send for analysis—the same method they use in the field. Phloem is the vascular tissue in plants that transports nutrients to the leaves. It is commonly referred to as the “inner bark.”

“Wild orangutans prefer the inner bark of their browse and we have found our orangutans do also and extract it in the same fashion as their wild counterparts,” said Toddes. “We plan to analyze the inner bark from the browse our orangutans eat and do some feeding rate comparisons to those Dr. DiGiogio and her associates have collected on wild orangutans.”

Toddes and DiGiorgio, a biological anthropologist whose work is focused on biological conditions that directly affect primates, have been working together for more than a year and met through their mutual interest in orangutan conservation. Since meeting, they have worked together to improve both diets for the orangutans at the Zoo as well as diets for animals at rehabilitation centers in Indonesia.

Imitating the Wild

The digestive tract of an orangutan is designed for very high fiber, low sugar foods. Wild orangutans eat forest fruits when they are available, which is only about three months out of the year. Additionally, these fruits are lower in sugar than the ones we humans consume. For example, something like a mango or banana will contain ~16% sugar, while an eggplant or cucumber that are much more similar to forest fruit have sugar contents closer to 2%. When the forest isn’t fruiting, the orangutans will consume more browse and leafy plants. Toddes formulates diets that replicate these patterns in the wild.

To begin the research at the Zoo, the team visited PECO Primate Reserve to watch our orangutans, male Sugi and female Tua, eat some mulberry browse, which is a favorite of theirs and many animals across the Zoo. Thanks to our partnership with PECO, donated fresh mulberry browse is delivered to the Zoo from PECOs preventative tree trimming program twice weekly June through September.

As Sugi and Tua began to eat, our browse workers noted how they ate. The workers recorded how much inner bark they extracted and ate in one bite and how much inner bark they extracted and ate from individual branches before moving onto the next branch.

After observing the animals, Dr. DiGiogio then showed our browse workers how to collect the inner bark and prepare it for analysis. This involved cutting samples of inner bark into approximate orangutan bite size pieces, then measuring, weighing and labeling each sample. The samples were then frozen in preparation for transport to a nutrition laboratory for nutrient analysis.

Toddes says this research benefits both orangutans in zoos and in the wild.

“Orangutans are designed to live in very harsh climates and withstand long intervals on what we would consider low quality foods – helping our keepers, curators and veterinary staff better understand the actual dietary needs of the animals helps us better care for the animals and promote physical and behavioral health,” she said. “Conversely, our ability to modify foods and environments for our animals with positive effect can help the field workers better understand and work with rehabbers in Indonesia to promote the best animal care and health allowing for successful reintroduction into the wild.”