Sumatran orangutans (Pongo abelii) have one of the longest periods of maternal care in the animal kingdom. Offspring are only weaned at between six and nine years of age, and spend all their time prior to weaning in constant, close contact with their mothers. Since they will be solitary as adults, young orangutans have to acquire all their feeding skills before they are weaned.
In order to become independent, immature orangutans must learn to locate, identify and process more than 200 different potential food items, many of which require specific skills in order to process and consume. During this time, mother orangutans are the role models, although the exact extent to which they actively teach their offspring these skills has not been established.
An immature orangutan learns its foraging skills from its mother in two main ways. It ‘peers’ at its mother while she feeds, observing her every movement from a close range. Offspring tend to peer more intently and for longer when the mother is dealing with more complex food items.
Secondly, an offspring also solicits, or begs, food items from its mother, after which she may share the food or keep it to herself. In both of these orangutan social learning pathways the mother serves as the primary role model but appears to take no active part in teaching the offspring.
In a new study, however, a team of scientists uncovered the first evidence yet of active involvement by orangutan mothers in their offspring’s acquisition of feeding skills. The researchers analyzed the behavior and interactions between 21 immature Sumatran orangutans and their mothers, collected over 13 years, in the Suaq Balimbing forest in Sumatra, Indonesia. They observed 1390 instances of food solicitation and were able to examine the degree to which these interactions represented active teaching by the mother.
“It was puzzling that mothers always seemed so passive during these feeding interactions,” said Caroline Schuppli from the Max Planck Institute of Animal Behavior, who led the study. “Mothers have so much time with their offspring, and maintain such a close connection, but they never appeared to be actively involved in the skill acquisition of their young.” The mystery was compounded by a lack of data. “Past studies had always examined skill learning from the point of view of the offspring, so we didn’t know the role of the role model.”
For the study, Schuppli teamed up with researchers from the University of Zurich in Switzerland, the Universitas Nasional in Indonesia, and the Max Planck Institute for Evolutionary Anthropology in Germany to investigate the role of food solicitation in the acquisition of foraging skills by immature orangutans. Field researchers recorded each solicitation event and noted whether the mother let the immature take the food from her or not. They then correlated this behavior with information on the offspring’s age and the properties of the food items.
Some food items are easy for orangutans to eat, such as leaves and flowers that require no processing. Others, such as bark, require several steps before consumption – it must be loosened from the tree and have the pith scraped with the teeth to remove the nutritional parts. The most difficult foods require tools, such as sticks that are converted into brushes for excavating honey from bee hives.
The results showed that orangutan mothers do not always respond positively to their offspring’s food solicitation. Mothers adjust their responses depending on the age of the offspring and the skills required for processing the food item.
Mothers are most likely to transfer food when the offspring is younger and at the age during which food recognition and food processing skills are being acquired. The older the offspring, the less likely it is that its begging will be successful. By doing this, mothers encourage their offspring to find and process their own food, as their competence levels increase.
In addition, orangutan mothers were most likely to transfer food items that were difficult to process, and they continued to do this for the longest time. For items that require tool use, for example, the mothers showed the highest tolerance for begging and remained tolerant throughout the dependency period of their offspring.
On the other hand, for leaves, which can just be picked and ingested whole, they showed much lower levels of tolerance to begging and stop sharing these food items when the offspring reached a certain age.
These findings, published in the journal Scientific Reports, show that mother orangutans adjust their behavior as their offspring become more skilled at finding and processing their own food.
“Our findings suggest that orangutan mothers are actively involved in their offspring’s skill learning,” said Schuppli. “However, they do this in a reactive, rather than proactive way. Interestingly, there were very few incidents of active food sharing only. This means that orangutan immatures need to take the initiative during learning. This is very different from humans, where active teaching plays an important role and role models are much more proactive. It is also different from chimpanzees, where mothers seem to be more proactive.”
Whether or not these behavioral adjustments by orangutan mothers classify as functional teaching is still unknown. While the study’s results fulfil some criteria of the teaching definition, other criteria cannot be tested with the data set.
“These findings give us a special insight into the factors that lead to the evolution of teaching,” said Schuppli. “While teaching is quite rare in the animal kingdom, it occurs in widely separated species. Our study shows that these orangutans have at least some, and perhaps all, of the cognitive, ecological, and social conditions to support teaching ability.”