Development of foraging skills in two orangutan populations: needing to learn or needing to grow?

A transdisciplinary view on curiosity beyond linguistic humans: animals, infants, and artificial intelligence

Curiosity is a core driver for life-long learning, problem-solving and decision-making. In a broad sense, curiosity is defined as the intrinsically motivated acquisition of novel information. Despite a decades-long history of curiosity research and the earliest human theories arising from studies of laboratory rodents, curiosity has mainly been considered in two camps: 'linguistic human' and 'other'. This is despite psychology being heritable, and there are many continuities in cognitive capacities across the animal kingdom. Boundary-pushing cross-disciplinary debates on curiosity are lacking, and the relative exclusion of pre-linguistic infants and non-human animals has led to a scientific impasse which more broadly impedes the development of artificially intelligent systems modelled on curiosity in natural agents. In this review, we synthesize literature across multiple disciplines that have studied curiosity in non-verbal systems. By highlighting how similar findings have been produced across the separate disciplines of animal behaviour, developmental psychology, neuroscience, and computational cognition, we discuss how this can be used to advance our understanding of curiosity. We propose, for the first time, how features of curiosity could be quantified and therefore studied more operationally across systems: across different species, developmental stages, and natural or artificial agents.

Measuring mantled howler monkey (Alouatta palliata) testes via parallel laser photogrammetry: Expanding the use of noninvasive methods

Parallel laser photogrammetry (PLP), which consists of attaching two or three parallel laser beams at a known inter-beam distance to a camera, can be used to collect morphological measurements of organisms noninvasively. The lasers project onto the photo being taken, and because the inter-beam distance is known, they act as a scale for image analysis programs like ImageJ. Traditionally, this method has been used to measure larger morphological traits (e.g., limb length, crown-rump length) to serve as proxies for overall body size, whereas applications to smaller anatomical features remain limited. To that end, we used PLP to measure the testes of 18 free-living mantled howler monkeys (Alouatta palliata) at La Selva Biological Station, Costa Rica. We tested whether this method could reliably measure this relatively small and globular morphology, and whether it could detect differences among individuals. We tested reliability in three ways: within-photo (coefficient of variation [CV] = 4.7%), between-photo (CV = 5.5%), and interobserver (intraclass correlation = 0.92). We found an average volume of 36.2 cm3 and a range of 16.4-54.4 cm3, indicating variation in testes size between individuals. Furthermore, these sizes are consistent with a previous study that collected measurements by hand, suggesting that PLP is a useful method for making noninvasive measurements of testes.

Heuristic test reveals little effect of learning and maturation on early prey capture experiences in a web-building spider

Behaviors can vary throughout an animal's life and this variation can often be explained by changes associated with learning and/or maturing. Currently, there is little consensus regarding how these processes interact to affect behaviors. Here we proposed a heuristic approach to disentangle the effects of learning and maturation on behavior and applied it to the predatory behaviors of Physocyclus globosus spiderlings. We varied the degree of prey difficulty and familiarity spiderlings received along the first instar and across the molt to the second instar and quantified the time spiderlings spent wrapping prey, as a proxy for prey capture efficiency. We found no overall evidence for learning or maturation. Changes in efficiency were mainly due to the switch from difficult to easy prey, or vice versa. However, there was one treatment where spiderlings improved in efficiency before and after the molt, without a switch in prey type. This provides some indication that difficult prey may offer more opportunity for learning or maturation to impact behavior. Although we found little effect of learning or maturation on prey capture efficiency, we suggest that our heuristic approach is effective and could be useful in investigating these processes in other behaviors and other animals.

Orangutan males make increased use of social learning opportunities, when resource availability is high

Humans' colonization of diverse habitats relied on our ancestors' abilities to innovate and share innovations with others. While ecological impacts on innovations are well studied, their effect on social learning remains poorly understood. We examined how food availability affects social learning in migrant orangutan unflanged males, who may learn from local orangutans through peering (i.e., observational social learning). We analyzed 1,384 dyadic associations, including 360 peering events, among 46 wild Sumatran orangutan and 25 Bornean orangutan males, collected over 18 years. Migrants' peering rates significantly increased with higher food availability and time spent in proximity to others. Furthermore, migrants in the more sociable Sumatran population exhibited significantly higher peering rates compared to the Borneans, suggesting intrinsic and/or developmental effects of food availability on social learning. These findings emphasize the importance of investigating ecological effects on social learning on the immediate, developmental, and intrinsic levels for our understanding of cultural evolution.

Blowing the lid off! Bottle-directed, extractive foraging strategies in synurbic bonnet macaques Macaca radiata in southern India

Nonhuman individuals and groups, living in anthropogenic landscapes, often adopt adaptive foraging strategies, mediated by their day-to-day interactions with humans and their artefacts. Exploring such novel behavioral manifestations, especially in the Anthropocene, offers us insights into behavioral innovations and their transmission in such rapidly changing ecologies. In this study, employing field experiments, we investigated an example of human-induced, extractive foraging behavior - the extraction of liquid contents from plastic bottles - in a synurbic bonnet macaque Macaca radiata population. The main aims of the study were to examine the distribution, diversity, inter-individual variability and intra-individual flexibility of bottle-directed manipulative behaviors, and to explore the social and environmental factors driving this behavioral practice. We video-recorded the manipulation of partially filled plastic bottles and the extraction of liquid across four groups of bonnet macaques in southern India. Two socio-demographic factors - age class and group membership - and one environmental factor - food provisioning - were identified as major determinants of inter-individual variation in the performance of sophisticated manipulative techniques and in bottle-opening success. Our results also suggest that age-related physical maturation, experiential trial-and-error learning, and possibly social learning contributed to the acquisition of foraging competence in this task. These findings illuminate the mechanisms underlying inter-individual behavioral variability and intra-individual behavioral flexibility amongst free-ranging individuals of a cercopithecine primate species, traditionally known for its ecological adaptability and behavioral plasticity. Finally, this study documents how the presence of humans, their artefacts and their activities facilitate the development of certain behavioral traditions in free-ranging nonhuman populations, thus providing valuable insights into how human-alloprimate relations can be restructured within the increasingly resource-competitive environments of the Anthropocene.

Application of a parallel laser apparatus to measure forearms and flanges of wild Bornean orangutans (Pongo pygmaeus wurmbii)

We constructed a parallel laser photogrammetry apparatus constructed from commercially available parts, and measured forearm lengths and flange widths of 16 wild Bornean orangutans. Our objectives were to validate our method and apparatus, discuss issues encountered, and construct preliminary growth curves. For adult males, we also compared flange width to forearm length as a way to investigate the relationship between body size and flange development. We used a camera cage around a DSLR camera, on top of which we attached two parallel green lasers. We estimated error with repeatability, accuracy, and interobserver reliability measures, and measured forearm lengths in three different ways to see which was most consistent. The longest forearm measure was the most repeatable (CV = 1.64%), and was similar to flange repeatability (3.50%). Accuracy measurements of a known object were high (error = 0.25%), and Interobserver discrepancy low (3.74%). Laser spacing increased with distance to the subject, but we corrected for this using calibration photos after each session. We transparently discuss the issues we encountered with the aim that this accessible method can help expand the use of laser photogrammetry. Preliminary measurements show that male flange widths and forearm length do not reliably increase in tandem, and that female growth plateaus at around the age at first birth (15 years old). We conclude with suggested improvements to the apparatus and method to ensure the lasers remain parallel.

Using optimal foraging theory to infer how groups make collective decisions

Studying animal behavior as collective phenomena is a powerful tool for understanding social processes, including group coordination and decision-making. However, linking individual behavior during group decision-making to the preferences underlying those actions poses a considerable challenge. Optimal foraging theory, and specifically the marginal value theorem (MVT), can provide predictions about individual preferences, against which the behavior of groups can be compared under different models of influence. A major strength of formally linking optimal foraging theory to collective behavior is that it generates predictions that can easily be tested under field conditions. This opens the door to studying group decision-making in a range of species; a necessary step for revealing the ecological drivers and evolutionary consequences of collective decision-making.

The economics of brain size evolution in vertebrates

Across the animal kingdom, we see remarkable variation in brain size. This variation has even increased over evolutionary time. Traditionally, studies aiming to explain brain size evolution have looked at the fitness benefits of increased brain size in relation to its increased cognitive performance in the social and/or ecological domain. However, brains are among the most energetically expensive tissues in the body and also require an uninterrupted energy supply. If not compensated, these energetic demands inevitably lead to a reduction in energy allocation to other vital functions. In this review, we summarize how an increasing number of studies show that to fully comprehend brain size evolution and the large variation in brain size across lineages, it is important to look at the economics of brains, including the different pathways through which the high energetic costs of brains can be offset. We further show how numerous studies converge on the conclusion that cognitive abilities can only drive brain size evolution in vertebrate lineages where they result in an improved energy balance through favourable ecological preconditions. Cognitive benefits that do not directly improve the organism's energy balance can only be selectively favoured when they produce such large improvements in reproduction or survival that they outweigh the negative energetic effects of the large brain.

Orangutan mothers adjust their behaviour during food solicitations in a way that likely facilitates feeding skill acquisition in their offspring

Immature orangutans acquire their feeding skills over several years, via social and independent learning. So far, it has remained uninvestigated to what extent orangutan mothers are actively involved in this learning process. From a fitness point of view, it may be adaptive for mothers to facilitate their offspring’s skill acquisition to make them reach nutritional independence faster. Food solicitations are potential means to social learning which, because of their interactive nature, allow to investigate the degree of active involvement of the mother. To investigate the role of food solicitation and the role of the mother in immatures’ foraging skill acquisition, we analysed 1390 food solicitation events between 21 immature Sumatran orangutans (Pongo abelii) and their mothers, collected over 13 years at the Suaq Balimbing orangutan population. We found that solicitation rates decreased with increasing age of the immatures and increased with increasing processing complexity of the food item. Mothers were more likely to share complex items and showed the highest likelihoods of sharing around the age at which immatures are learning most of their feeding skills. Our results indicate that immature Sumatran orangutans use food solicitation to acquire feeding skills. Furthermore, mothers flexibly adjust their behaviour in a way that likely facilitates their offspring’s skill acquisition. We conclude that orangutan mothers have a more active role in the skill acquisition of their offspring than previously thought.

The role of great ape behavioral ecology in One Health: Implications for captive welfare and re-habilitation success

Behavior is the interface through which animals interact with their environments, and therefore has potentially cascading impacts on the health of individuals, populations, their habitats, and the humans that share them. Evolution has shaped the interaction between species and their environments. Thus, alterations to the species-typical "wild-type" behavioral repertoire (and the ability of the individual to adapt flexibly which elements of the repertoire it employs) may disrupt the relationship between the organism and its environment, creating cascading One Health effects. A good example is rehabilitant orangutans where, for example, seemingly minor differences from wild conspecifics in the time spent traveling on the ground rather than in the forest canopy can affect an individual's musculoskeletal and nutritional health, as well as social integration. It can also increase two-way transmission of infectious diseases and/or pathogens with local human populations, or potentially with neighboring wild populations if there are no geographical barriers and rehabilitants travel far enough to leave their release area. Primates are well known ecosystem engineers, reshaping plant communities and maintaining biodiversity through seed dispersal, consuming plants, and creating canopy gaps and trails. From the habitat perspective, a rehabilitant orangutan which does not behave like a wild orangutan is unlikely to fulfill these same ecosystem services. Despite the importance of the diversity of an ape's behavioral repertoire, how it compares to that of wild conspecifics and how it alters in response to habitat variation, behavior is an often under-appreciated aspect of One Health. In this review, focusing on orangutans as an example of the kinds of problems faced by all captive great apes, we examine the ways in which understanding and facilitating the expression of wild-type behavior can improve their health, their ability to thrive, and the robustness of local One Health systems.

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.

The development and maintenance of sex differences in dietary breadth and complexity in Bornean orangutans

ABSTRACT

Orangutans show a pronounced sexual dimorphism, with flanged males (i.e., males with fully grown secondary sexual characteristics) reaching twice the size of adult females. Furthermore, adult orangutans show sex-specific dispersal and activity patterns. This study investigates sex differences in adult foraging behavior and sheds light on how these differences develop in immatures. We analyzed 11 years of feeding data on ten adult female, seven flanged male, and 14 immature Bornean orangutans (Pongo pygmaeus wurmbii) at Tuanan in Central Kalimantan, Indonesia. We found that the diets of the adult females were significantly broader and required more processing steps before ingestion than the diets of flanged males. We also found evidence for a similar difference in overall diet repertoire sizes. For the immatures, we found that whereas females reached 100% of their mothers' diet spectrum size by the age of weaning, males reached only around 80%. From the age of 4 years on (i.e., years before being weaned) females had significantly broader daily diets than males. We found no difference in daily or overall diet processing intensity of immature males and females but found preliminary evidence that immature males included fewer items of their mother's diet in their own diets that were processing-intensive. Overall, our results suggest that by eating a broader variety and more complex to process food items, female orangutans go to greater lengths to achieve a balanced diet than males do. These behavioral differences are not just apparent in adult foraging behavior but also reflected in immature development from an early age on.

SIGNIFICANCE STATEMENT

In many species, males and females have different nutritional needs and are thus expected to show sex-specific foraging behavior. Sex differences in several aspects of foraging behavior have been found in various species, but it remains largely unclear when and how those develop during ontogeny, which is especially relevant for long-lived altricial species that learn foraging skills over many years. In our study, we analyzed a cross-sectional and longitudinal data set containing more than 750,000 feeding events of adult and immature Bornean orangutans (Pongo pygmaeus wurmbii). We found that adult females had significantly broader and more complex diets than males. We also found that these differences started to develop during infancy, suggesting that immature orangutans prepare for their sex-specific foraging niches long before those become physiologically relevant while they are still in constant association with their mothers and before being frequently exposed to other role models.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s00265-021-03014-3.

Wild chimpanzee offspring exhibit adult-like foraging patterns around the age of weaning

OBJECTIVES

The prolonged juvenile period exhibited by primates is an evolutionary conundrum. Here we examine wild chimpanzee feeding development in the context of two hypotheses regarding prolonged development in primates: the needing-to-learn hypothesis and the expensive brain hypothesis.

MATERIAL AND METHODS

We studied wild chimpanzee (Pan troglodytes schweinfurthii) offspring at Gombe National Park, Tanzania. We analyzed 41 years of observational behavioral data collected between 1975 and 2016 from 81 offspring. We characterized feeding development in the first 10 years of life via four different measures: (1) proportion of observation time spent feeding; (2) diet composition; (3) diet breadth; and (4) diet maturity as measured by similarity to maternal diet. We used mixed effects models to examine changes with age and by sex, while controlling for season.

RESULTS

Feeding time, diet breadth, and diet maturity exhibited the most substantial increases with age in the first 6 years, with no significant change thereafter. Males and females showed different patterns of change in diet breadth by age, but did not differ by age 10. Diet composition did not change significantly with age and did not differ by sex.

DISCUSSION

We found that chimpanzee offspring attained adult-like feeding behaviors between 4 and 6 years of age, concomitant with the completion of weaning. Thus, our data do not support the needing-to-learn feeding skills hypothesis of a prolonged juvenile period, but additional data are needed to evaluate how and when adolescent chimpanzees are able to make foraging decisions independent of their mothers. Existing data on growth provides support for the expensive brain hypothesis, however, these hypotheses are not necessarily mutually exclusive. As more studies across taxa accumulate sufficient datasets on a range of developmental metrics, we will be able to achieve a more robust understanding of prolonged development in primates.

Fermentation Ability of Gut Microbiota of Wild Japanese Macaques in the Highland and Lowland Yakushima: In Vitro Fermentation Assay and Genetic Analyses

Wild Japanese macaques (Macaca fuscata Blyth) living in the highland and lowland areas of Yakushima are known to have different diets, with highland individuals consuming more leaves. We aim to clarify whether and how these differences in diet are also reflected by gut microbial composition and fermentation ability. Therefore, we conduct an in vitro fermentation assay using fresh feces from macaques as inoculum and dry leaf powder of Eurya japonica Thunb. as a substrate. Fermentation activity was higher for feces collected in the highland, as evidenced by higher gas and butyric acid production and lower pH. Genetic analysis indicated separation of highland and lowland in terms of both community structure and function of the gut microbiota. Comparison of feces and suspension after fermentation indicated that the community structure changed during fermentation, and the change was larger for lowland samples. Analysis of the 16S rRNA V3-V4 barcoding region of the gut microbiota showed that community structure was clearly clustered between the two areas. Furthermore, metagenomic analysis indicated separation by gene and pathway abundance patterns. Two pathways (glycogen biosynthesis I and D-galacturonate degradation I) were enriched in lowland samples, possibly related to the fruit-eating lifestyle in the lowland. Overall, we demonstrated that the more leaf-eating highland Japanese macaques harbor gut microbiota with higher leaf fermentation ability compared with the more fruit-eating lowland ones. Broad, non-specific taxonomic and functional gut microbiome differences suggest that this pattern may be driven by a complex interplay between many taxa and pathways rather than single functional traits.

When ontogeny recapitulates phylogeny: Fixed neurodevelopmental sequence of manipulative skills among primates

Neural development is highly conserved across distantly related species of different brain sizes. Here, we show that the development of manipulative complexity is equally cumulative across 36 primate species and also that its ontogeny recapitulates phylogeny. Furthermore, larger-brained species reach their adult skill levels later than smaller-brained ones, largely because they start later with the simplest techniques. These findings demonstrate that these motor behaviors are not modular and that their slow development may constrain their evolution. Complex foraging techniques therefore critically require a slow life history with low mortality, which explains the limited taxonomic distribution of flexible tool use and the unique elaboration of human technology.

The emergence of emotionally modern humans: implications for language and learning

According to the Cooperative Breeding Hypothesis, apes with the life-history attributes of those in the line leading to the genus Homo could not have evolved unless male and female allomothers had begun to help mothers care for and provision offspring. As proposed elsewhere, the unusual way hominins reared their young generated novel phenotypes subsequently subjected to Darwinian social selection favouring those young apes best at monitoring the intentions, mental states and preferences of others and most motivated to attract and appeal to caretakers. Not only were youngsters acquiring information in social contexts different from those of other apes, but they would also have been emotionally and neurophysiologically different from them in ways that are relevant to how humans learn. Contingently delivered rewards to dependents who attracted and ingratiated themselves with allomothers shaped their behaviours and vocalizations and transformed the way developing youngsters learned from others and internalized their preferences. This article is part of the theme issue 'Life history and learning: how childhood, caregiving and old age shape cognition and culture in humans and other animals'.

Bringing up baby: Maternal responsiveness, secondary attachments, and the development of infant social competence in wild olive baboons (Papio anubis)

Caregiver responsiveness and presence of secondary attachments play a crucial role in children's socio-cognitive and emotional development, but little is known of their effect on the development of non-human primates. Here we present the results of a 16-month behavioral study conducted on 22 wild infant olive baboons (Papio anubis) at the Uaso Ngiro Baboon Project, Kenya. This is the first study to examine the effects of maternal responsiveness and secondary attachments on the development of infant social behavior in a wild primate species that does not breed cooperatively. The data track maternal responsiveness and the rates of two behavioral indicators of infant social competence-orienting toward interactions and social play-over the course of the first year of life. Maternal responsiveness decreased as infants grew older, while infant orientation toward interactions and play behavior increased. Infants with poorly responsive mothers were more likely to have secondary attachments, and infants with secondary attachments to siblings oriented more frequently to social interactions than those with secondary attachments to adult/subadult males or with no secondary attachments. These findings indicate that variation in maternal responsiveness and presence of secondary attachments can influence the development of social competence in olive baboon infants.

Information transfer through food from parents to offspring in wild Javan gibbons

The adaptive functions of food transfer from parents to their offspring have been explained mainly by two mutually non-exclusive hypotheses: the nutritional and informational hypotheses. In this study, we examined the functions of food transfer in wild Javan gibbons (Hylobates moloch) by testing these hypotheses from both infants’ and mothers’ perspectives. We observed 83 cases of food solicitations that resulted in 54 occasions of food transfers in three groups over a 19-month period in Gunung Halimun-Salak National Park, Indonesia. Infants initiated all solicitations directed at their mothers with one solicitation towards a father. Food solicitation rate decreased as infant age increased and ceased before weaning. As predicted by the informational hypothesis, infants solicited more food items difficult to obtain and preferred by their parents. On the contrary to the nutritional hypothesis, infants solicited low-quality items more often than high-quality items. Mothers did not change probability of food transfer according to the food characteristics or infant age. Hence, our results suggest that the primary function of food transfer from mother to infant Javan gibbons seems to be information transfer rather than nutritional aids, similarly to great apes.

Early sociability fosters later exploratory tendency in wild immature orangutans

Exploration is essential for skill acquisition and strongly facilitates cognitive performance. In humans, it is widely known that exploration and later cognitive performance are highly dependent on early social inputs. Here, we aim to shed light on the evolutionary roots of this process by studying the effects of variation in opportunities for social learning on the exploratory tendency of immature orangutans (Pongo spp.) in nature. We based our analyses on mixed cross-sectional, longitudinal data of exploration by immatures and their mothers. Current exploration rates were correlated with levels of past experienced sociability, but not with current food abundance or with maternal condition, and only partly with genetic similarity. We conclude that the dependence of cognitive development on socially triggered exploration, which underlies the construction of cognitive skills such as intelligence, existed before the emergence of the human lineage.

Animal cultures: how we've only seen the tip of the iceberg

For humans we implicitly assume that the way we do things is the product of social learning and thus cultural. For animals, this conclusion requires proof. Here, we first review the most commonly used procedure for documenting animal culture: the method of exclusion, which charts geographic behavioral variation between populations as evidence for culture. Using published data, we show that, whereas it is an adequate proof of principle, the method of exclusion has major deficiencies when capturing cultural diversity and complexity. Therefore, we propose a new method, namely the direct counting of socially learned skills, which we apply to previously collected data on wild orangutans. This method reveals a far greater cultural repertoire among orangutans, and a different distribution of cultural elements among behavioral domains than found by the method of exclusion, as well as clear ecological correlates for most cultural elements. The widespread occurrence of social learning ability throughout the animal kingdom suggests that these conclusions also apply to many other species. Culture is most likely more widespread and pervasive than commonly thought and an important avenue to local adaptation. The complex and normative dimensions of culture seem unique to our species, but were most likely built upon a very broad, pre-existing cultural capacity that we inherited from our ancestors.

Refining the ecological brain: Strong relation between the ventromedial prefrontal cortex and feeding ecology in five primate species

To survive in complex and seasonal environments, primates are thought to rely upon cognitive capacities such as decision-making and episodic memory, which enable them to plan their daily foraging path. According to the Ecological Brain hypothesis, feeding ecology has driven the expansion of the brain to support the corresponding development of cognitive skills. Recent works in cognitive neurosciences indicate that cognitive operations such as decision-making or subjective evaluation (which are contextual and dependent upon episodic memory), relied critically upon a small part of the frontal lobe, often referred to as the ventromedial prefrontal cortex (VMPFC). Several authors suggested that this area might be important for foraging, but this has never been tested. In the present study, we quantified the relation between the size of the VMPFC (along with other cerebral measures: the whole brain, the gyrus rectus and the somatosensory cortex) and key socio-ecological variables in five primate species (Macaca mulatta, Macaca fuscata, Gorilla gorilla, Pan troglodytes and Homo sapiens). We hypothesized that the size of the VMPFC would be greater in primates with a large dietary spectrum and complex foraging strategies. We also hypothesized that the impact of feeding ecology would be stronger on this specific region than on other regions (somatosensory cortex) or on more global cerebral measures (e.g., whole brain). In line with these hypotheses, we found that all cerebral measures were more strongly related to feeding ecology than group size, a proxy for social complexity. As expected, the VMPFC volume is more precisely related to feeding ecology than the whole brain, and appears to be critically related to dietary quality. Thus, combining a comparative approach with predictions coming both from behavioral ecology and cognitive neurosciences, our study provides evidence that feeding ecology played a key role in the development of specific cognitive skills, which rely upon the expansion of a specific cortical area.

The slow ape: High infant survival and long interbirth intervals in wild orangutans

Orangutans (Pongo spp.) are reported to have extremely slow life histories, including the longest average interbirth intervals of all mammals. Such slow life history can be viable only when unavoidable mortality is kept low. Thus, orangutans' survivorship under natural conditions is expected to be extremely high. Previous estimates of orangutan life history were based on captive individuals living under very different circumstances or on small samples from wild populations. Here, we combine birth data from seven field sites, each with demographic data collection for at least 10 years (range 12-43 years) on wild orangutans to better document their life history. Using strict criteria for data inclusion, we calculated infant survival, interbirth intervals and female age at first reproduction, across species, subspecies and islands. We found an average closed interbirth interval of 7.6 years, as well as consistently very high pre-weaning survival for males and females. Female survival of 94% until age at first birth (at around age 15 years) was higher than reported for any other mammal species under natural conditions. Similarly, annual survival among parous females is very high, but longevity remains to be estimated. Current data suggest no major life history differences between Sumatran and Bornean orangutans. The high offspring survival is remarkable, noting that modern human populations seem to have reached the same level of survival only in the 20th century. The orangutans' slow life history illustrates what can be achieved if a hominoid bauplan is exposed to low unavoidable mortality. Their high survival is likely due to their arboreal and non-gregarious lifestyle, and has allowed them to maintain viable populations, despite living in low-productivity habitats. However, their slow life history also implies that orangutans are highly vulnerable to a catastrophic population crash in the face of drastic habitat change.

The development of feeding behavior in wild chimpanzees (Pan troglodytes schweinfurthii)

OBJECTIVES

Primates have an extended period of juvenility before adulthood. Although dietary complexity plays a prominent role in hypotheses regarding the evolution of extended juvenility, the development of feeding behavior is still poorly understood. Indeed, few studies have investigated the timing and nature of feeding transitions in apes, including chimpanzees. We describe general patterns of feeding development in wild chimpanzees and evaluate predictions of the needing-to-learn hypothesis.

MATERIALS AND METHODS

We analyzed 4 years of behavioral data (2010-2013) from 26 immature chimpanzees and 31 adult chimpanzees of the Kanyawara community in Kibale National Park, Uganda. Specifically, we examined milestones of nutritional independence (first consumption of solid food and cessation of suckling) as well as developmental changes in feeding time, diet composition, diet breadth, and ingestion rates.

RESULTS

Chimpanzees first fed on solid food at 5.1 months and, on average, suckled until 4.8 years. Daily feeding time of immature individuals reached adult levels between 4 and 6 years, while diet composition showed minor changes with age. By juvenility (5-10 years), individuals had a complete adult diet breadth. Ingestion rates for five ripe fruit species remained below adult levels until juvenility but continued to show absolute increases into adolescence.

DISCUSSION

Chimpanzees acquired adult-like patterns on all feeding measures by infancy or juvenility. These data are inconsistent with the needing-to-learn hypothesis; moreover, where delays exist, alternatives hypotheses make similar predictions but implicate physical constraints rather than learning as causal factors. We outline predictions for how future studies might distinguish between hypotheses for the evolution of extended juvenility.

The effects of sociability on exploratory tendency and innovation repertoires in wild Sumatran and Bornean orangutans

It has been hypothesized that opportunities for social learning affect the size and complexity of the adult skill set of birds and mammals, their learning ability, and thus ultimately also their innovation frequency. To test these predictions we compared rates of social learning, rates of independent exploration (independent learning) and innovation repertoires between individuals of a highly sociable population of Pongo abelii at Suaq Balimbing and a less sociable population of Pongo pygmaeus wurmbii at Tuanan. Suaq immatures showed significantly higher rates of peering, even after controlling for differences in association time and diet complexity, implying that they make disproportionally greater use of their increased opportunities for social learning. As predicted, we found that immatures and adults at Suaq also showed significantly higher rates of exploratory behaviour. The difference between the individuals of the two popuations remained when controlling for association time, suggesting persistent developmental effects, intrinsic differences, or both. Accordingly, Suaq animals had a larger set of learned skills and a higher mean dietary complexity. Our findings show that population level sociability, individual rates of exploration and population-wide repertoires of innovations are positively linked, as predicted.