Orangutans’ complex diets are down to cultural knowledge, new research suggests

New Research from the University of St Andrews has revealed just how much wild orangutans depend on social learning to build diets spanning hundreds of foods.In a paper published in Nature Human Behaviour, researchers combined 12 years of meticulous fieldwork with computer modelling.

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

This new study reveals that no lone orangutan could build this encyclopaedic 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 throughout their lifetime. Rather, culture accumulates from the innovations of many individuals. A human’s capacity to accumulate broad cultural repertoires, to breadths no individual could produce alone, is potentially a capacity that evolved at least 13 million years ago, in our common ancestor with great apes.

Until now though, it has been unclear whether similar processes are at play for wild non-human species too.

The team, made up of international researchers from the Universities of St Andrews, Zurich and Tübingen, and led by the Max Planck Institute, has now investigated whether the breadth of wild orangutans’ diets exceeds what any one individual could acquire on their own.

Using thousands of snapshots of real-life data, researchers built a computer simulation model that reenacted orangutans lives from birth to maturity at fifteen years old. The model incorporated three social behaviours predicted to influence how the diet of orangutans develop: close range observations of others while they ate foods in the forest (a behaviour 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.

This allowed researchers 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, simulated orangutans cultivated adult like diets, around 224 food types, at around the same age as wild orangutans. These similarities between the model and the wild confirmed the simulation accuracy and real-world validity.

The researchers 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 diets 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.

Professor Andrew Whiten from the School of Psychology and Neuroscience, who co- authored the study, said: “We’re seeing the strongest evidence yet that orangutan diets are culturally accumulated over many generations. Given how much diet development suffers without social inputs, the effect of culture on orangutans’ daily lives is clearly quite profound.”

The next step is to understand how this culturally accumulated knowledge influences orangutan’s energy intake, survival and success, questions the team will address as part of a further study.