Orangutans stand apart from their great ape relatives. Gorillas, chimpanzees, and bonobos live in social groups where playmates are simply part of the landscape. Orangutan mothers, by contrast, spend most of their time alone with their dependent offspring, who remain with them for seven or more years before striking out on their own. Infants have very few chances to meet other youngsters their own age. Yet playing together helps young orangutans build the strength, coordination, and social skills they will need throughout their lives. The question that intrigued researchers was whether those rare meetings were simply chance encounters in the forest canopy, or something far more deliberate.

The answer comes from one of the longest-running behavioral datasets ever assembled for wild orangutans. Since 2003, researchers led by Dr. Maria van Noordwijk, a fellow at the Max Planck Institute of Animal Behavior who conducted the fieldwork while at the University of Zurich, have followed mother-infant pairs at the Tuanan Research Station in Central Kalimantan, Indonesia, observing habituated animals from morning until night. Over fifteen years the team accumulated 32,000 hours of observations spanning 31 mother-infant pairs. Encounters between adult females are rare and cautious affairs in which the mothers usually keep their distance, but their infants behave entirely differently.

As van Noordwijk describes it, the infants zoom toward one another the moment they are within reach, roughhousing, biting, and grappling in bursts of exuberant play, while their mothers watch closely from nearby. She likened the behavior to a play button that only a mother can switch off when she decides it is time to move on. From years in the field she also noticed something else: days with infant play seemed to be long, hard days for the mothers themselves. To test whether this impression reflected a genuine cost, the team needed a rigorous analytical framework, which came from Dr. Odd Jacobson, a postdoctoral researcher at the Max Planck Institute of Animal Behavior and first author of the study.

Jacobson applied a statistical technique that combined the mothers’ patterns of space use with the social characteristics of neighboring mother-infant pairs. The analysis produced a striking result. Mothers whose infants were closer in age shared more of the same forest areas, and those infants also played together more often, even after statistically accounting for factors such as genetic relatedness and fruit availability. In other words, mothers appeared to match their offspring with play partners that were physically and developmentally suitable, rather than simply tolerating whatever company the forest happened to provide.

The playdates also carried a clear and distinctive movement signature. Mothers travelled further not only on the days when play occurred, but on the days surrounding play as well. Starting on the day before a play session, a mother shifted her ranging toward the core area of the other mother, and after the playday she moved back toward her own core area. Because adult females are known to concentrate their activity within stable core areas, these consistent excursions and returns suggest that orangutan playdates are not chance meetings but are actively shaped by maternal ranging decisions. To find a suitable playmate, a mother positions herself spatially to increase the likelihood of encountering another female with a similarly aged offspring.

This targeted navigation implies that orangutan mothers maintain an intimate knowledge of their social landscape. Van Noordwijk describes this as a kind of forest gossip: although orangutans spend most of their day alone, they remain embedded in a social network. They know who their mostly lifelong neighbors are, and they know who gave birth to a baby and when. From their high positions in the trees they can probably, at least on occasion, get a rough sense of where those neighbors are. The researchers argue that this ability draws on well-documented cognitive skills. Scientists have long accepted that orangutans apply spatial navigation and route planning to locate and access food resources, and the new findings show that they combine a cognitive map of both feeding and social resources in order to provide for their young.

All of this effort comes at a measurable price. On days associated with playdates, mothers spent more time travelling and less time feeding, an energetic cost that represents an overlooked form of maternal investment in their offspring’s social development. In species that live in groups, playmates come naturally with communal life, but the play problem arises when a mother spends nearly all her time alone with a single young and rarely comes together with others for feeding, socializing, or protection. How many other solitary species face this same problem, and how they might solve it, remains an open question for future research.

The investment is real but modest in frequency. Playdates occurred on just 6 percent of the days that a mother-infant pair was followed, roughly once every twenty days, and even on days when infant peer play did take place, the infants spent only around twenty minutes playing in total. Those twenty minutes, however, may carry outsized developmental weight. Whether the time and energy mothers spend arranging these meetings actually gives their young an advantage later in life is still unknown, and the authors emphasize that only long-term research of this kind can connect behavior with development, environment, and demography across the lifespan of such long-lived animals.

The study also resonates far beyond the Bornean forest. As coauthor Professor Meg Crofoot, a director at the Max Planck Institute of Animal Behavior, notes, humans in some ways resemble orangutans more than we might like to admit. Our species evolved to raise children in large groups, yet modern life increasingly pushes families into isolated, solitary lifestyles. When opportunities for social contact become scarce, someone always pays the cost, and the orangutan mothers of Tuanan demonstrate what that cost looks like when it must be paid deliberately, one carefully planned journey at a time. For the most solitary of the great apes, ensuring that a young ape gets to play turns out to be a job that only a mother can do.

Subject of Research: Maternal ranging strategies that facilitate offspring social play in wild Bornean orangutans

Article Title: Orangutan mothers seek out playmates for young

Article References: Orangutan mothers seek out playmates for young. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: orangutans, Bornean orangutan, social play, maternal care, animal behavior, primate cognition, ranging behavior, maternal investment, long-term field study, great apes, PNAS, Borneo

The editorial team behind the issue argues that the field has spent decades focusing heavily on emotion recognition: how accurately observers can identify what someone else is feeling from a face, a voice or a posture. The new collection deliberately shifts the lens inward, toward the person doing the expressing. Emotional expressions, the editors propose, should be understood in terms of their origins and development, their continuity across animal species, and the practical roles they play in behaviour and social life. In this framing, emotions become a visible inner world, a living link between brain, body and environment, rather than a private mental state that occasionally leaks to the surface.

Among the most striking contributions is a study co-conducted by Cristina Colonnesi of the University of Amsterdam, which examines how infants in their first year of life actively construct positive moments with the people around them. The researchers observed that eight-month-old babies, markedly more than four-month-olds, initiate episodes of shared positive emotion themselves. A typical sequence involves a baby smiling and then drawing an adult into mutual laughter, creating a loop of shared enjoyment that the infant, not the caregiver, set in motion. Rather than primarily reacting to another person’s smile, older babies increasingly deploy their own positive expressions as social tools, using them to start and sustain warm, enjoyable interactions.

That developmental shift may look subtle, but its implications are considerable. The ability to initiate, rather than merely respond to, shared positive emotion suggests growing flexibility in how infants use their emotional behaviour in social contexts. It hints that the foundations of humour, joint attention and even the social bonding that characterises later friendship may be laid in the first year of life, not through language or instruction, but through the infant’s own expressive behaviour. Shared laughter between a baby and an adult is, in this view, not a charming by-product of caregiving but a developmental achievement produced by the infant’s emerging expressive control.

A second line of work, presented by Milica Nikolić and colleagues, turns to self-conscious emotions: shame, guilt, shyness and pride. These emotions are distinctive because they arise when children begin to view themselves through the eyes of others, and the research shows that they are already visible in a child’s facial expressions, body posture and gaze during the first few years of life. Crucially, they prove to be far more than internal feelings. The studies reviewed in the issue indicate that these emotions have characteristic outward signatures and, more importantly, predictable behavioural consequences that begin shaping a child’s social world very early.

The consequences differ sharply depending on the emotion. When children feel guilty about something that has gone wrong, they are subsequently more likely to try to make amends, whether by helping others, sharing resources or repairing damage they have caused. Shame produces the opposite pattern: children who feel ashamed are more likely to withdraw, avoid eye contact and steer clear of the situations that triggered the feeling. Nikolić and her colleagues are careful to note that the evidence base is still limited, but they argue that these self-conscious emotions likely play an early role in shaping social behaviour and reputation, functioning as internal regulators that push children toward repair in one case and retreat in the other.

The third major contribution, from Disa Sauter and her team, dismantles one of the most persistent assumptions in popular psychology: that feeling good means smiling. In a study covering twenty-two different positive emotions, grounded in participants’ own personal experiences, the researchers mapped which facial muscles and movements are activated for each emotion. The results reveal that many positive feelings carry their own distinct facial profiles that involve no smile at all. Interest and determination are prime examples; in the case of determination, the defining feature is a raised chin rather than any movement of the mouth. Other positive emotions, such as amusement and elation, do involve distinct smiles, characterised by upturned corners of the mouth and often accompanied by closed eyes and strongly raised cheeks.

The technical significance of this work lies in its demonstration that positive facial behaviour is a heterogeneous family of expressions rather than a single gradient of smile intensity. Feeling good, the researchers show, manifests in a variety of facial configurations, including spontaneous ones, and the classic smile is only one member of that family. For fields ranging from affective computing to clinical screening, which often rely on smile detection as a proxy for positive affect, the findings suggest that such proxies may systematically miss positive states such as interest or determination, and may misread them when they appear without any mouth movement at all.

A fourth study, authored by postdoc Christopher Riddell, Milica Nikolić and colleagues, extends the comparative dimension of the issue by examining mimicry of the play face, an exaggerated, playful expression resembling a smile that typically appears during rough-and-tumble play. The researchers compared infants, adults and orangutans. Both human infants and adults were more likely to produce a play face themselves after observing one in another person than after seeing a neutral expression, and their own expressions mirrored what they saw even without any direct interaction with the person being mirrored. Orangutans, by contrast, rarely showed this behaviour. For them, play faces arise primarily during direct social play, whereas in humans contagious smiling can break free of the play context altogether and spread through observation alone.

Taken together, the nineteen articles of the special issue, published under the title Mechanisms, development, phylogeny and functions of emotional expressions, make a coherent case that emotional expressions are central to development from the very start, shaping both individual behaviour and the formation and regulation of social relationships, from a baby’s first self-initiated smile to the complex body language of shame and pride. In their editorial, Nikolić, Kret, Nummenmaa and Sauter call on the research community to study emotions more within real-life contexts, attending to how expressions emerge, develop and function for both the expresser and the people around them. If the collection is right, the smile that lights up a living room, the guilty glance after a broken vase and the raised chin of a determined child are not merely decorations on an inner life. They are the mechanisms through which that inner life reaches out and builds the social world.

Subject of Research: The development, evolution and social functions of emotional expressions in humans and other animals

Article Title: From a baby's smile to a monkey's grin: how emotions shape our behaviour

Article References: From a baby's smile to a monkey's grin: how emotions shape our behaviour. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: emotional expressions, infant development, smiling, self-conscious emotions, shame, guilt, facial expressions, play face, orangutans, mimicry, social behaviour, developmental psychology

The research, led by Dr. Revathe Thillaikumar, a Marie Sklodowska-Curie Actions Postdoctoral Fellow at the Max Planck Institute of Animal Behavior, together with senior author Dr. Caroline Schuppli and colleagues at Universitas Nasional in Indonesia, is the first to demonstrate that social and individual forms of learning interact over many years to build broad, survival-relevant ecological skills in a wild great ape. Until now, scientists have debated to what extent animals acquire their survival skills by watching others or by experimenting on their own, but the long developmental timelines of great apes have made it extraordinarily difficult to connect learning events to concrete outcomes. This study closes that gap with an unusually detailed dataset and a statistical approach that treats each animal as its own developmental story.

The scale of the challenge facing a young orangutan is hard to overstate. An adult orangutan’s diet consists of nearly 250 distinct food items, selected from thousands of different options scattered across the rainforest maze. Some of those options are potentially poisonous. Beyond simply knowing what to eat, an independent orangutan must master how each food should be processed, and when and where each item can be found in the forest. As Thillaikumar asks, how do infants master this complex diet curriculum during their first years of life? Humans offer a clue, because throughout our evolutionary history we too had to learn an immense set of survival skills across many years of development, drawing on a complex interplay of social and individual learning. But whether wild animals use the same combination of learning forms to build their own survival skills had remained unknown.

To answer the question, the team drew on long-term data from the Suaq Balimbing research station in Gunung Leuser National Park, Indonesia, where an international team of researchers and field assistants follows wild Sumatran orangutans across their entire lives. For this study, the researchers analyzed data from 21 immature orangutans recorded from birth until independence at around 8.5 years of age. Across 4,700 hours of observation, they tracked three key behaviors: when the youngsters closely observed their mothers and other nearby orangutans through a behavior known as peering, which serves as a behavioral indicator of social learning; when they explored different items in their habitat, indicating individual learning; and which food items they successfully ate without help from their mothers, a direct measure of ecological competence.

The analytical challenge was formidable. Because orangutans develop so slowly, there is a multi-year lag between a learning event and any measurable change in behavior, as Schuppli explains. To bridge that gap, Thillaikumar worked with theoretical statistician Professor Paul-Christian Bürkner of TU Dortmund University to analyze just under seven thousand records of social and individual learning, keeping each animal’s history separate. This individual-level modeling was, in her words, a linchpin of the study, because it finally allowed the researchers to link the two parts that had previously been unbridgeable: learning and skill development. Rather than averaging across the population, the team could trace how each animal’s own learning tendencies shaped its own eventual diet.

The first major finding is that orangutan infants have genuinely individual learning styles. Over their long development, the immatures differed substantially from one another in how frequently they engaged in social versus individual learning. Thillaikumar admits the team was astonished by how different the infants were from one another in their tendencies to learn. Yet despite this variation, a clear pattern emerged across the group: all immatures engaged in social learning more frequently than individual learning, on average as much as five times more often. The explanation, Schuppli suggests, lies in the economics of risk. Learning purely through individual exploration can be dangerous for a naive orangutan, especially for immatures who are still growing and cannot afford to miss out on feeding opportunities. The safer option may simply be to watch and learn from knowledgeable role models, such as their mothers.

Social learning, it turns out, does far more than provide a safe alternative to exploration. When an infant engaged in social learning, it triggered subsequent exploration for as long as two and a half hours afterward. Each time an infant peered at its mother or another orangutan, it produced a prolonged ripple effect on its own subsequent explorations, priming the youngster to investigate items it had just seen handled by someone else. To disentangle the contributions of the two processes, the authors focused on explorations that were independently initiated, allowing them to test how social and individual learning worked together rather than merely replacing one another.

The answer is that the two forms of learning operate in tandem across the entire developmental period. Infants that engaged in high rates of both social and individual learning ended up with the broadest diet profiles by the time they became independent. Conversely, when both forms of learning were low, individuals ended up with the narrowest diets, producing a striking three-fold difference in diet breadth between the highest and lowest learners. Perhaps most intriguingly, increased social learning could partially compensate for low individual learning, improving diet breadth even when independent exploration was infrequent. As Thillaikumar puts it, social learning stepped in to do the heavy lifting, a result that underlines just how significant it is in the lives of great apes.

For Schuppli, the findings demand a more nuanced appreciation of the mechanisms underlying great ape cultures. It is about more than just social or individual learning, she says; it is about the interplay of complex learning mechanisms that has driven the development of a broad, highly survival-relevant cultural competence in one of our closest living relatives in the wild. That interplay, she argues, suggests that the sophisticated cultural learning seen in humans may have deep evolutionary roots, predating the split between our lineages by many millions of years. In other words, the recipe for human culture, a blend of attentive observation and hands-on experimentation sustained over a long childhood, may already have been simmering in the forests where our ape relatives live today.

Questions remain. The study shows that heightened learning produces broader diets, but exactly how this advantage arises is still open. Are high-learning orangutans simply accumulating knowledge about more items, or are they actually getting better at learning itself? The team now plans to look beyond diet breadth to determine whether greater learning also improves energy intake, and ultimately whether it translates into measurable differences in survival and reproduction. As co-author Dr. Sri Suci Utami-Atmoko of Universitas Nasional notes, questions like these can only be answered by watching animals in their natural environment from birth to death, because long-term field studies provide the time and space to follow long-lived animals across their whole histories. For orangutans, she says, these studies offer a window into how culture and survival have become intertwined over evolutionary time. For the rest of us, they offer a humbling reminder that the path to competence, in the rainforest as in our own species, is neither purely taught nor purely discovered, but built patiently from both.

Subject of Research: How social and individual learning styles shape ecological skill development in wild orangutans

Article Title: Learning style predicts survival skills in young orangutans

Article References: Learning style predicts survival skills in young orangutans. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: orangutans, social learning, individual learning, animal culture, Sumatra, diet breadth, great apes, childhood development, ecological competence, field study, Science journal, Max Planck Institute