Orangutans can’t master their complex diets without cultural knowledge

When a wild orangutan leaves its mother after spending many years by her side, it has a mental catalog of almost 250 edible plants and animals, and the knowledge of how to acquire and process them.

A new study in Nature Human Behaviour reveals that no lone orangutan could build this encyclopedic knowledge through trial and error. Instead, this knowledge forms a “culturally-dependent repertoire”— a diverse set of knowledge that is only attainable through years of watching and exploring alongside others.

As humans, we must learn broad repertoires of knowledge to survive and thrive—ranging from local customs, to the skills to engineer new innovations like fishing spears and iPhones. Much of this cultural knowledge is too broad or complex for any single human to innovate from scratch in their lifetime. Rather, culture accumulates from the innovations of many individuals. Until now, it has been unclear whether similar processes are at play for wild non-human species. An international team of researchers has now investigated whether the breadth of wild orangutans’ diets exceeds what any one individual could acquire on their own within a relevant time frame.

“We provide convincing evidence that culture enables wild orangutans to construct repertoires of knowledge that are much broader than they could otherwise learn independently,” says first author Dr Elliot Howard- Spink, postdoctoral researcher from the Max Planck Institute of Animal Behavior, now a researcher at the University of Zürich.

“These diets must be the product of experiences and innovations of many other individuals, which have accumulated over time,” adds coauthor Dr Claudio Tennie, University of Tübingen. “The roots of humans’ cultural accumulation may therefore reach back at least 13 million years to our last common ancestor with great apes.”

Simulating how orangutans learn what to eat

The team wanted to know if young orangutans can independently learn their full set of edible plant and animal species before they become self-sufficient adults at around 15 years old—or if they need to learn this information from others. The researchers used extensive data collected on wild Sumatran orangutans living in the swamp forests of Suaq Balimbing, Indonesia. This included 12 years of daily observations, where the behaviors of orangutans were recorded every few minutes.

But this data set alone was not enough. The team needed to create scenarios in which young orangutans were cut off from different types of social interactions as they grew. “We would never do this to wild orangutans,” says Howard-Spink. Also, it was impossible for the scientists to follow orangutans every single day over the many years it takes the animals to grow up, while also recording all their learning opportunities. So, the scientists devised another way.

Using daily snapshots of real-life data, Howard-Spink built a simulation model that reenacted orangutans’ lives from birth to maturity at fifteen years old. The model incorporated three key social behaviors predicted to influence how the diet of orangutans develop: close-range observation of others while they ate foods in the forest (a behavior called ‘peering’); being in very close proximity to other orangutans who were feeding (which made them more likely to explore similar foods); or, simply being guided to suitable feeding sites, without any further social contact.

“Every single parameter of this model is based on our long-term data from wild orangutans,” says Dr Caroline Schuppli, who lead the study and is a group leader at MPI-AB. “It allows us to pinpoint which types of social interactions help young orangutans learn what to eat, and even to rank their importance.”

When all three types of social learning were available (the condition most similar to wild individuals), simulated orangutans cultivated adult-like diets—about 224 food types—at around the same age as wild orangutans. These similarities between the model and the wild confirmed the simulation’s accuracy and real-world applicability, the authors say.

“The fact that our simulation matched wild individuals’ development so closely is due to the extensive and uniquely detailed data collected from the wild at Suaq, and the hard work of a large team involved,” says Howard-Spink.

Discovering orangutans’ “cultural cuisine”

Howard-Spink then began cutting the simulated orangutans off from different social interactions. Just cutting off close-range observations (peering), had an effect: simulated orangutans had slower diet development and reached only 85% of the full wild diet repertoire by adulthood. But removing both peering and close-proximity associations left simulated apes with drastically narrower diets. These diets never approached the breadths possessed by wild adults, and essentially stopped developing well before the end of immaturity.

“Socially-isolated, simulated orangutans still had hundreds of thousands of opportunities to encounter food items during development,” says Howard-Spink. “But even massive amounts of exposure to food could not replace what was lost when they couldn’t engage in these social interactions.”

Says coauthor Andrew Whiten, University of St Andrews: “We’re seeing the strongest evidence yet that orangutan diets are culturally accumulated over many generations.”

The next step is to understand how this culturally-accumulated knowledge influences orangutans’ energy intake, survival, and success. “Given how much diet development suffers without social inputs, the effect of culture on orangutans’ daily lives is potentially profound,” adds Whiten.

The team will address this question as part of a further study. “We will again use empirically-validated simulations to understand how reliant orangutans are on cultural knowledge to survive and thrive in wild habitats,” says Schuppli.

Conserving accumulated cultures

Adult orangutans are generally solitary, making their long childhoods a precious window for cultural transmission. “In the wild, the constant presence of a mother, and fleeting associations with other individuals, are critical for orangutan learning and development during the early years,” says Schuppli. “It offers a crucial apprenticeship that paves the path to independence.”

With orangutan populations dwindling, this study has practical urgency. Orphaned apes, reintroduced without the full breadth of a wild diet, or introduced in different environments, may face starvation or poisoning from unfamiliar plants. “Reintroduction programs already teach orangutans to feed themselves outside captivity,” adds Schuppli. “Our study emphasizes how important this is to pass on their full cultural menu, so that these animals have the greatest chance of success in the wild.”