Immature wild orangutans acquire relevant ecological knowledge through sex-specific attentional biases during social learning.

As a part of growing up, immature orangutans must acquire vast repertoires of skills and knowledge, a process that takes several years of observational social learning and subsequent practice. Adult female and male orangutans show behavioral differences including sex-specific foraging patterns and male-biased dispersal. We investigated how these differing life trajectories affect social interest and emerging ecological knowledge in immatures. We analyzed 15 years of detailed observational data on social learning, associations, and diet repertoires of 50 immatures (16 females and 34 males), from 2 orangutan populations. Specific to the feeding context, we found sex differences in the development of social interest: Throughout the dependency period, immature females direct most of their social attention at their mothers, whereas immature males show an increasing attentional preference for individuals other than their mothers. When attending to non-mother individuals, males show a significant bias toward immigrant individuals and a trend for a bias toward adult males. In contrast, females preferentially attend to neighboring residents. Accordingly, by the end of the dependency period, immature females show a larger dietary overlap with their mothers than do immature males. These results suggest that immature orangutans show attentional biases through which they learn from individuals with the most relevant ecological knowledge. Diversifying their skills and knowledge likely helps males when they move to a new area. In sum, our findings underline the importance of fine-grained social inputs for the acquisition of ecological knowledge and skills in orangutans and likely in other apes as well.

Comparative performance of orangutans (Pongo spp.), gorillas (Gorilla gorilla gorilla), and drills (Mandrillus leucophaeus), in an ephemeral foraging task.

A goal of the comparative approach is to test a variety of species on the same task. Here, we examined whether the factors that helped capuchin monkeys improve their performance in a dichotomous choice task would generalize to three other primate species: orangutans, gorillas, and drill monkeys. In this task, subjects have access to two options, each resulting in an identical food, but one (the ephemeral option) is only available if it is chosen first, whereas the other one (the permanent option) is always available. Therefore, the food-maximizing solution is to choose the ephemeral option first, followed by the permanent option for an additional reward. On the original version (plate task), the options were discriminated by the color and pattern of the plates holding the food, while on two subsequent versions we used altered cues that we predicted would improve performance: (1) the color of the foods themselves (color task), which we hypothesized was relevant to primates, who choose foods rather than substrates on which foods are found when foraging, and (2) patterned cups covering the foods (cup task), which we hypothesized would help primates avoid the prepotent response associated with visible food. Like capuchins, all three species initially failed to solve the plate task. However, while orangutans improved their performance from the plate to the color task, they did not for the cup task, and only a few gorillas and no drills succeeded in either task. Unfortunately, our ability to interpret these data was obscured by differences in the subjects' level of experience with cognitive testing and practical constraints that precluded the use of completely identical procedures across species. Nonetheless, we consider what these results can tell us, and discuss the value of conducting studies across multiple sites despite unavoidable differences.

Apes produce tools for future use.

There is now growing evidence that some animal species are able to plan for the future. For example great apes save and exchange tools for future use. Here we raise the question whether chimpanzees, orangutans, and bonobos would produce tools for future use. Subjects only had access to a baited apparatus for a limited duration and therefore should use the time preceding this access to create the appropriate tools in order to get the rewards. The apes were tested in three conditions depending on the need for pre-prepared tools. Either eight tools, one tool or no tools were needed to retrieve the reward. The apes prepared tools in advance for future use and they produced them mainly in conditions when they were really needed. The fact that apes were able to solve this new task indicates that their planning skills are flexible. However, for the condition in which eight tools were needed, apes produced less than two tools per trial in advance. However, they used their chance to produce additional tools in the tool use phase-thus often obtaining most of the reward from the apparatus. Increased pressure to prepare more tools in advance did not have an effect on their performance.

Nest-building orangutans demonstrate engineering know-how to produce safe, comfortable beds.

Nest-building orangutans must daily build safe and comfortable nest structures in the forest canopy and do this quickly and effectively using the branches that surround them. This study aimed to investigate the mechanical design and architecture of orangutan nests and determine the degree of technical sophistication used in their construction. We measured the whole nest compliance and the thickness of the branches used and recorded the ways in which the branches were fractured. Branch samples were also collected from the nests and subjected to three-point bending tests to determine their mechanical properties. We demonstrated that the center of the nest is more compliant than the edges; this may add extra comfort and safety to the structure. During construction orangutans use the fact that branches only break half-way across in "greenstick" fracture to weave the main nest structure. They choose thicker branches with greater rigidity and strength to build the main structure in this way. They then detach thinner branches by following greenstick fracture with a twisting action to make the lining. These results suggest that orangutans exhibit a degree of technical knowledge and choice in the construction of nests.

Evidence for a midlife crisis in great apes consistent with the U-shape in human well-being.

Recently, economists and behavioral scientists have studied the pattern of human well-being over the lifespan. In dozens of countries, and for a large range of well-being measures, including happiness and mental health, well-being is high in youth, falls to a nadir in midlife, and rises again in old age. The reasons for this U-shape are still unclear. Present theories emphasize sociological and economic forces. In this study we show that a similar U-shape exists in 508 great apes (two samples of chimpanzees and one sample of orangutans) whose well-being was assessed by raters familiar with the individual apes. This U-shaped pattern or "midlife crisis" emerges with or without use of parametric methods. Our results imply that human well-being's curved shape is not uniquely human and that, although it may be partly explained by aspects of human life and society, its origins may lie partly in the biology we share with great apes. These findings have implications across scientific and social-scientific disciplines, and may help to identify ways of enhancing human and ape well-being.

Children conform to the behavior of peers; other great apes stick with what they know.

All primates learn things from conspecifics socially, but it is not clear whether they conform to the behavior of these conspecifics--if conformity is defined as overriding individually acquired behavioral tendencies in order to copy peers' behavior. In the current study, chimpanzees, orangutans, and 2-year-old human children individually acquired a problem-solving strategy. They then watched several conspecific peers demonstrate an alternative strategy. The children switched to this new, socially demonstrated strategy in roughly half of all instances, whereas the other two great-ape species almost never adjusted their behavior to the majority's. In a follow-up study, children switched much more when the peer demonstrators were still present than when they were absent, which suggests that their conformity arose at least in part from social motivations. These results demonstrate an important difference between the social learning of humans and great apes, a difference that might help to account for differences in human and nonhuman cultures.

The ecology of primate material culture.

Tool use in extant primates may inform our understanding of the conditions that favoured the expansion of hominin technology and material culture. The 'method of exclusion' has, arguably, confirmed the presence of culture in wild animal populations by excluding ecological and genetic explanations for geographical variation in behaviour. However, this method neglects ecological influences on culture, which, ironically, may be critical for understanding technology and thus material culture. We review all the current evidence for the role of ecology in shaping material culture in three habitual tool-using non-human primates: chimpanzees, orangutans and capuchin monkeys. We show that environmental opportunity, rather than necessity, is the main driver. We argue that a better understanding of primate technology requires explicit investigation of the role of ecological conditions. We propose a model in which three sets of factors, namely environment, sociality and cognition, influence invention, transmission and retention of material culture.

Orangutan (Pongo spp.) whistling and implications for the emergence of an open-ended call repertoire: a replication and extension.

One of the most apparent discontinuities between non-human primate (primate) call communication and human speech concerns repertoire size. The former is essentially fixed to a limited number of innate calls, while the latter essentially consists of numerous learned components. Consequently, primates are thought to lack laryngeal control required to produce learned voiced calls. However, whether they may produce learned voiceless calls awaits investigation. Here, a case of voiceless call learning in primates is investigated--orangutan (Pongo spp.) whistling. In this study, all known whistling orangutans are inventoried, whistling-matching tests (previously conducted with one individual) are replicated with another individual using original test paradigms, and articulatory and acoustic whistle characteristics are compared between three orangutans. Results show that whistling has been reported for ten captive orangutans. The test orangutan correctly matched human whistles with significantly high levels of performance. Whistle variation between individuals indicated voluntary control over the upper lip, lower lip, and respiratory musculature, allowing individuals to produce learned voiceless calls. Results are consistent with inter- and intra-specific social transmission in whistling orangutans. Voiceless call learning in orangutans implies that some important components of human speech learning and control were in place before the homininae-ponginae evolutionary split.

The majority of genetic variation in orangutan personality and subjective well-being is nonadditive.

The heritability of human personality is well-established. Recent research indicates that nonadditive genetic effects, such as dominance and epistasis, play a large role in personality variation. One possible explanation for the latter finding is that there has been recent selection on human personality. To test this possibility, we estimated additive and nonadditive genetic variance in personality and subjective well-being of zoo-housed orangutans. More than half of the genetic variance in these traits could be attributed to nonadditive genetic effects, modeled as dominance. Subjective well-being had genetic overlap with personality, though less so than has been found in humans or chimpanzees. Since a large portion of nonadditive genetic variance in personality is not unique to humans, the nonadditivity of human personality is not sufficient evidence for recent selection of personality in humans. Nonadditive genetic variance may be a general feature of the genetic structure of personality in primates and other animals.

Visual discrimination in an orangutan (Pongo pygmaeus): measuring visual preference.

Although previous studies have confirmed that trained orangutans visually discriminate between mammals and artificial objects, whether orangutans without operant conditioning can discriminate remains unknown. The visual discrimination ability in an orangutan (Pongo pygmaeus) with no experience in operant learning was examined using measures of visual preference. Sixteen color photographs of inanimate objects and of mammals with four legs were randomly presented to an orangutan. The results showed that the mean looking time at photographs of mammals with four legs was longer than that for inanimate objects, suggesting that the orangutan discriminated mammals with four legs from inanimate objects. The results implied that orangutans who have not experienced operant conditioning may possess the ability to discriminate visually.

Experimental investigation of orangutans' lithic percussive and sharp stone tool behaviours.

Early stone tools, and in particular sharp stone tools, arguably represent one of the most important technological milestones in human evolution. The production and use of sharp stone tools significantly widened the ecological niche of our ancestors, allowing them to exploit novel food resources. However, despite their importance, it is still unclear how these early lithic technologies emerged and which behaviours served as stepping-stones for the development of systematic lithic production in our lineage. One approach to answer this question is to collect comparative data on the stone tool making and using abilities of our closest living relatives, the great apes, to reconstruct the potential stone-related behaviours of early hominins. To this end, we tested both the individual and the social learning abilities of five orangutans to make and use stone tools. Although the orangutans did not make sharp stone tools initially, three individuals spontaneously engaged in lithic percussion, and sharp stone pieces were produced under later experimental conditions. Furthermore, when provided with a human-made sharp stone, one orangutan spontaneously used it as a cutting tool. Contrary to previous experiments, social demonstrations did not considerably improve the stone tool making and using abilities of orangutans. Our study is the first to systematically investigate the stone tool making and using abilities of untrained, unenculturated orangutans showing that two proposed pre-requisites for the emergence of early lithic technologies-lithic percussion and the recognition of sharp-edged stones as cutting tools-are present in this species. We discuss the implications that ours and previous great ape stone tool experiments have for understanding the initial stages of lithic technologies in our lineage.

Orangutan pantomime: elaborating the message.

We present an exploratory study of forest-living orangutan pantomiming, i.e. gesturing in which they act out their meaning, focusing on its occurrence, communicative functions, and complexities. Studies show that captive great apes may elaborate messages if communication fails, and isolated reports suggest that great apes occasionally pantomime. We predicted forest-living orangutans would pantomime spontaneously to communicate, especially to elaborate after communication failures. Mining existing databases on free-ranging rehabilitant orangutans' behaviour identified 18 salient pantomimes. These pantomimes most often functioned as elaborations of failed requests, but also as deceptions and declaratives. Complexities identified include multimodality, re-enactments of past events and several features of language (productivity, compositionality, systematicity). These findings confirm that free-ranging rehabilitant orangutans pantomime and use pantomime to elaborate on their messages. Further, they use pantomime for multiple functions and create complex pantomimes that can express propositionally structured content. Thus, orangutan pantomime serves as a medium for communication, not a particular function. Mining cases of complex great ape communication originally reported in functional terms may then yield more evidence of pantomime.

Happy orang-utans live longer lives.

Nonhuman primate ageing resembles its human counterpart. Moreover, ratings of subjective well-being traits in chimpanzees, orang-utans and rhesus macaques are similar to those of humans: they are intercorrelated, heritable, and phenotypically and genetically related to personality. We examined whether, as in humans, orang-utan subjective well-being was related to longer life. The sample included 184 zoo-housed orang-utans followed up for approximately 7 years. Age, sex, species and number of transfers were available for all subjects and 172 subjects were rated on at least one item of a subjective well-being scale. Of the 31 orang-utans that died, 25 died a mean of 3.4 years after being rated. Even in a model that included, and therefore, statistically adjusted for, sex, age, species and transfers, orang-utans rated as being "happier" lived longer. The risk differential between orang-utans that were one standard deviation above and one standard deviation below baseline in subjective well-being was comparable with approximately 11 years in age. This finding suggests that impressions of the subjective well-being of captive great apes are valid indicators of their welfare and longevity.

Semantics of primate gestures: intentional meanings of orangutan gestures.

Great ape gesture has become a research topic of intense interest, because its intentionality and flexibility suggest strong parallels to human communication. Yet the fundamental question of whether an animal species' gestures carry specific meanings has hardly been addressed. We set out a systematic approach to studying intentional meaning in the gestural communication of non-humans and apply it to a sample of orangutan gestures. We propose that analysis of meaning should be limited to gestures for which (1) there is strong evidence for intentional production and (2) the recipient's final reaction matches the presumed goal of the signaller, as determined independently. This produces a set of "successful" instances of gesture use, which we describe as having goal-outcome matches. In this study, 28 orangutans in three European zoos were observed for 9 months. We distinguished 64 gestures on structural grounds, 40 of which had frequent goal-outcome matches and could therefore be analysed for meaning. These 40 gestures were used predictably to achieve one of 6 social goals: to initiate an affiliative interaction (contact, grooming, or play), request objects, share objects, instigate co-locomotion, cause the partner to move back, or stop an action. Twenty-nine of these gestures were used consistently with a single meaning. We tested our analysis of gesture meaning by examining what gesturers did when the response to their gesture did not match the gesture's meaning. Subsequent actions of the gesturer were consistent with our assignments of meaning to gestures. We suggest that, despite their contextual flexibility, orangutan gestures are made with the expectation of specific behavioural responses and thus have intentional meanings as well as functional consequences.

Do apes know that they could be wrong?

When confronted with uncertain or incomplete information in decision-making situations, monkeys and apes opt for either escaping the situation or seeking additional information. These responses have been interpreted as evidence of metacognitive abilities. However, this interpretation has been challenged. On the one hand, studies using the information-seeking paradigm have been criticized because subjects may simply engage in a search for information routine (e.g., search until spot the reward) without any metacognitive involvement. On the other hand, studies using the escape response paradigm have been criticized because subjects may not recognize their own state of uncertainty but have learned to use the escape response in the presence of certain stimuli configurations that create uncertainty. The current study attempted to address these two criticisms by presenting great apes (seven gorillas, eight chimpanzees, four bonobos, seven orangutans) with a seeking information task whose basic procedure consisted of presenting two hollow tubes, baiting one of them and letting subjects choose. Conditions varied depending on whether subjects had visual access to the baiting, the cost associated with seeking information, the time interval between baiting and choosing, the food quality and the additional information offered regarding the food's location. Although subjects showed a high retrieval accuracy when they had witnessed the baiting, they were more likely to check inside the tube before choosing when high stakes were involved (Experiment 3) or after a longer period of time had elapsed between the baiting and the retrieval of the reward (Experiment 2). In contrast, providing subjects with indirect auditory information about the food's location or increasing the cost of checking reduced checking before choosing (Experiment 1). Taken together, these findings suggest that subjects knew that they could be wrong when choosing.

Bonobos, chimpanzees, gorillas, and orang utans use feature and spatial cues in two spatial memory tasks.

Animals commonly use feature and spatial strategies when remembering places of interest such as food sources or hiding places. We conducted three experiments with great apes to investigate strategy preferences and factors that may shape them. In the first experiment, we trained 17 apes to remember 12 different food locations on the floor of their sleeping room. The 12 food locations were associated with one feature cue, so that feature and spatial cues were confounded. In a single test session, we brought the cues into conflict and found that apes, irrespective of species, showed a preference for a feature strategy. In the second experiment, we used a similar procedure and trained 25 apes to remember one food location on a platform in front of them. On average, apes preferred to use a feature strategy but some individuals relied on a spatial strategy. In the final experiment, we investigated whether training might influence strategy preferences. We tested 21 apes in the platform set-up and found that apes used both, feature and spatial strategies irrespective of training. We conclude that apes can use feature and spatial strategies to remember the location of hidden food items, but that task demands (e.g. different numbers of search locations) can influence strategy preferences. We found no evidence, however, for the role of training in shaping these preferences.

Specialization in the vicarious learning of novel arbitrary sequences in humans but not orangutans.

Sequence learning underlies many uniquely human behaviours, from complex tool use to language and ritual. To understand whether this fundamental cognitive feature is uniquely derived in humans requires a comparative approach. We propose that the vicarious (but not individual) learning of novel arbitrary sequences represents a human cognitive specialization. To test this hypothesis, we compared the abilities of human children aged 3-5 years and orangutans to learn different types of arbitrary sequences (item-based and spatial-based). Sequences could be learned individually (by trial and error) or vicariously from a human (social) demonstrator or a computer (ghost control). We found that both children and orangutans recalled both types of sequence following trial-and-error learning; older children also learned both types of sequence following social and ghost demonstrations. Orangutans' success individually learning arbitrary sequences shows that their failure to do so in some vicarious learning conditions is not owing to general representational problems. These results provide new insights into some of the most persistent discontinuities observed between humans and other great apes in terms of complex tool use, language and ritual, all of which involve the cultural learning of novel arbitrary sequences. This article is part of the theme issue 'Ritual renaissance: new insights into the most human of behaviours'.

Spontaneous (minimal) ritual in non-human great apes?

The potential for rituals in non-human great apes (apes) is an understudied topic. We derive a minimal definition of ritual and then examine the currently available evidence for it in untrained and non-enculturated apes. First, we examine whether such apes show evidence for the two main components of our minimal definition of ritual: symbolism and copying. Second, we examine if there are actual cases already identifiable today that may fit all aspects of our minimal definition of ritual-or whether there are at least cases that fit some aspects (proto-ritual). We find that apes are not likely to spontaneously practise minimal ritual, but we claim that the highest expected likelihood of occurrence is in the results-copying domain. Yet, we did not find actual cases of minimal ritual in apes-including those involving environmental results. We did, however, find some cases that may match at least part of our minimal ritual definition-which we termed proto-ritual. At least two out of three potential cases of such proto-rituals that we identified (rain dance, object-in-ear and surplus nest-making procedures) do revolve around results. Overall, apes do not show much, or very clear, evidence for even minimal ritual, but may sometimes show proto-ritual. However, dedicated ape ritual studies are currently lacking, and future work may identify ape ritual (or clearer cases of proto-ritual). We discuss the implications of our preliminary finding for inferences of ritual in the last common ancestor of humans and apes. This article is part of the theme issue 'Ritual renaissance: new insights into the most human of behaviours'.

An orangutan in Paris: pondering Proximity at the Muséum d'histoire naturelle in 1836.

When the Muséum d'histoire naturelle in Paris learned in 1836 that it had the chance to buy a live, young orangutan, it was excited by the prospect. Specimens were the focus of the Museum's activities, and this particular specimen seemed especially promising, not only because the Museum had very few orangutan specimens in its collection, but also because of what was perceived to be the orangutan's unique place in the natural order of things, namely, at the very boundary between the animal kingdom and humans. Frédéric Cuvier, the superintendent of the Museum's menagerie, urged that studying the orangutan's mental faculties would help resolve fundamental questions regarding the similarities and differences between animals and humans. Archival and printed sources allow one to reconstruct the orangutan's capture, acquisition, and subsequent career at the menagerie in greater detail than has generally been possible for animals of nineteenth-century zoos. Scientists, artists, the public, the press, and even musicians (Franz Liszt included) sought to engage with the orangutan, seeing in it not just another ape or monkey but a special creature unto itself at the animal/human boundary. Key to their fascination with the orangutan was the question of proximity-just how close was the orangutan to humans? The orangutan's story illuminates not only how the animal-human boundary was conceived at the time but also the problematic status of the zoo as a site for scientific research and the roles of scientific and non-scientific actors alike in constructing how the orangutan was understood.

Social inhibition and behavioural flexibility when the context changes: a comparison across six primate species.

The ability to inhibit previously employed strategies and flexibly adjust behavioural responses to external conditions may be critical for individual survival. However, it is unclear which factors predict their distribution across species. Here, we investigated social inhibition and behavioural flexibility in six primate species (chimpanzees, bonobos, orangutans, gorillas, capuchin monkeys and spider monkeys) differing in terms of phylogenetic relatedness, foraging ecology and social organization. Depending on the social context, individuals could maximize their food intake by inhibiting the selection of a larger food reward in one condition (i.e. inhibition), but not in others, which required them to flexibly switching strategies across conditions (i.e. behavioural flexibility). Overall, our study revealed inter-specific differences in social inhibition and behavioural flexibility, which partially reflected differences in fission-fusion dynamics. In particular, orangutans and chimpanzees showed the highest level of inhibitory skills, while gorillas and capuchin monkeys showed the lowest one. In terms of behavioural flexibility, orangutans and spider monkeys were the best performers, while bonobos and capuchin monkeys were the worst ones. These results contribute to our understanding that inhibition and behavioural flexibility may be linked in more complex ways than usually thought, although both abilities play a crucial role in efficient problem solving.