More menu choices: Migrant orangutans learn a lot about food by watching the locals

Orangutans are dependent on their mothers longer than any other non-human animal, nursing until they are at least six years old and living with her for up to three more years, learning how to find, choose, and process the exceedingly varied range of foods they eat. But how do orangutans that have left their mothers and now live far from their natal ranges, where the available foods may be very different, decide what to eat and figure out how to eat it? Now, an international team of authors has shown that in such cases, migrants follow the rule 'observe, and do as the locals do'.

"Here we show evidence that migrant orangutan males use observational social learning to learn new ecological knowledge from local individuals after dispersing to a new area," says Julia Mörchen, a doctoral student at the Max Planck Institute for Evolutionary Anthropology and Leipzig University, in Germany, and the study's lead author. "Our results suggest that migrant males not only learn where to find food and what to feed on from locals, but also continue to learn how to process these new foods."

Mörchen and colleagues showed that migrant males learn this information through a behaviour called 'peering': intensely observing for at least five seconds and from within two metres at a role model. Typically, peering orangutans faced the role model and showed signs of following his or her actions with head movements, indicating attentive interest.

Male orangutans migrate to another area after becoming independent, while females tend to settle close to their natal home range. "What we don't yet know is how far orangutan males disperse, or where they disperse to. But it's possible to make informed guesses: genetic data and observations of orangutans crossing physical barriers such as rivers and mountains suggest long-distance dispersal, likely over tens of kilometres," says Mörchen. "This implies that during migration, males likely come across several habitat types and thus experience a variety of faunistic compositions, especially when crossing through habitats of different altitudes. Over evolutionary time, being able to quickly adapt to novel environments by attending to crucial information from locals, likely provided individuals with a survival advantage. As a result, this ability is likely ancestral in our hominin lineage, reaching back between at least 12 and 14 million years to the last common ancestor we share with orangutans."

Peering by males was observed 534 times, occurring in 207 (5.2%) of these associations. In Suaq Balimbing, males most frequently peered at local females followed by local juveniles, and least at adult males. In the less sociable population of Tuanan, the opposite held: males most frequently peered at adult males, followed by immature orangutans, and least at adult females. Migrant males at Tuanan may lack opportunities to peer at local females, as females are known to avoid long associations with them in this population. Migrant males then interacted more frequently with the peered-at food afterwards, putting into practice what they had learned through peering.

"Our detailed analyses further showed that the migrant orangutan males in our study peered most frequently at food items that are difficult to process, or which are only rarely eaten by the locals: including foods that were only recorded being eaten for a few minutes over the entire study period," says Dr Anja Widdig, a professor at Leipzig University and co-senior author of the study.

The authors cautioned that it is still unknown how many times adult orangutans need to peer at a particular behaviour before mastering it themselves. Observations suggest that depending on the complexity or novelty of the learned skill, adults may still use explorative behaviours on certain food items they first learned about through peering -- possibly to figure out more details, strengthen and memorize the new information, or to compare the latter with previous knowledge.