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Primate faces provide information about a range of variant and invariant traits, including some that are relevant for mate choice. For example, faces of males may convey information about their health or genetic quality through symmetry or facial masculinity. Because perceiving and processing such information may have bearing on the reproductive success of an individual, cognitive systems are expected to be sensitive to facial cues of mate quality. However, few studies have investigated this topic in non-human primate species. Orang-utans are an interesting species to test mate-relevant cognitive biases, because they are characterised by male bimaturism: some adult males are fully developed and bear conspicuous flanges on the side of their face, while other males look relatively similar to females. Here, we describe two non-invasive computerised experiments with Bornean orang-utans (Pongo pygmaeus), testing (i) immediate attention towards large flanges and symmetrical faces using a dot-probe task (N = 3 individuals; 2F) and (ii) choice bias for pictures of flanged males over unflanged males using a preference test (N = 6 individuals; 4F). In contrast with our expectations, we found no immediate attentional bias towards either large flanges or symmetrical faces. In addition, individuals did not show a choice bias for stimuli of flanged males. We did find exploratory evidence for a colour bias and energy efficiency trade-offs in the preference task. We discuss our null results and exploratory results in the context of the evolutionary history of Bornean orang-utans, and provide suggestions for a more biocentric approach to the study of orang-utan cognition.

Orang-utans are well-known for their vocal flexibility. However, the described vocal repertoire of female orang-utans is relatively limited. This anecdotal report discusses the occurrence of long call-like vocalisations in two zoo-housed Bornean orang-utan females. While Temmy demonstrated these vocalisations in response to playbacks of male long calls, Eloise proactively produced vocalisations during her ovulation before her reproductive senescence. The vocalisations produced by Temmy and Eloise seem to differ in many aspects from known orang-utan calls, although they bear some similarities to male long calls. Thus, the vocal repertoire of female orang-utans may be more extensive than previously thought. We carefully suggest that these calls might indicate proceptivity, while also discussing methodological limitations and theoretical considerations.

Perceiving emotions in others is at the foundation of higher-order social cognition. The importance of emotions is evidenced by the fact that they receive prioritized attention at early stages of processing the environment in humans and some other primates. Nevertheless, we do not fully understand how emotion modulates attention over longer durations in primates, particularly in great apes. Bonobos, one of our closest relatives, stand out in emotion processing and regulation among great apes. This makes them an interesting comparison species and a valuable model for studying the evolution of emotion perception in hominids. We investigated how bonobos and humans spontaneously attend to emotionally valent scenes in a preferential looking task using eye-tracking. With Bayesian mixed modelling, we found that bonobos and humans generally looked longer at emotional scenes, mainly of conspecifics. Moreover, while bonobos did not have a bias toward emotional human scenes, humans sustained their attention toward bonobos playing, grooming and having sex. Furthermore, when exploring an immediate bias for emotions, humans showed a bias toward affiliative human scenes, and bonobos showed a bias away from bonobos-in-distress scenes. These findings suggest that emotions modulate attention at early and later attentional stages in bonobos, similar to humans.

For folivores, socio-ecological models predict scramble competition and egalitarian dominance relationships with female dispersal, but female Asian colobines do not all nicely fit these predictions. Alternative explanations concern an absence of competition, and the Folivore Paradox, where group size is limited by social factors such as infanticide. Relevant data are scarce, but our review shows that colobine foraging costs increase with group size, yet female fitness may increase or decrease. Dominance relationships vary from despotic to egalitarian, and are individualistic. The lack of female nepotism in despotic species (or populations) still requires explanation. Female dispersal is found in egalitarian and some despotic species, yet costs may be low if females migrate into a group with familiar females. Asian colobine social organization seems to follow one of two patterns. First, in seasonal species, food limits group size, while infanticide does not; these groups may experience contest competition. Second, in a-seasonal species groups are uni-male, the infanticide risk is high and infanticide, not food, may limit group size. This proposal requires further testing and may also apply to uni-male frugivores. This overview of the socio-ecological patterns in Asian colobines highlights that ecological and social selection pressures in interaction determine primate social organization.

In contrast to the African great apes, orangutans (Pongo spp.) are semisolitary: Individuals are often on their own, but form aggregations more often than expected by chance. These temporary aggregations provide social benefits such as mating opportunities. When fruit availability is high, costs of aggregating should be lower, because competition is less pronounced. Therefore, average party size is expected to be higher when fruit availability is high. This hypothesis would also explain why orangutans in highly fruit-productive habitats on Sumatra are more gregarious than in the usually less productive habitats of Borneo. Here, we describe the aggregation behavior of orangutans in less productive Sumatran habitats (Sikundur and Batang Toru), and compare results with those of previously surveyed field sites. Orangutans in Sikundur were more likely to form parties when fruit availability was higher, but the size of daily parties was not significantly affected by fruit availability. With regard to between-site comparisons, average party sizes of females and alone time of parous females in Sikundur and Batang Toru were substantially lower than those for two previously surveyed Sumatran sites, and both fall in the range of values for Bornean sites. Our results indicate that the assessment of orangutans on Sumatra as being more social than those on Borneo needs revision. Instead, between-site differences in sociality seem to reflect differences in average fruit availability.

Surveys show that zoo-housed great apes occasionally interact with local wildlife. Bonobos and chimpanzees interact aggressively with and sometimes consume wildlife. Gorillas may also interact with local wildlife, but less often in an aggressive way and consumption was rare. Here we report the case of an adolescent female western lowland gorilla in Apenheul Primate Park (Apeldoorn, The Netherlands) that persistently catches and handles ducklings. Prior to observation we proposed four possible explanations, which are not mutually exclusive: play, meat eating, need for abnormal plucking, and allomothering. We saw the female catch ducklings at eight occasions (9 ducklings in total) and she handled ducklings (at least 19) on ten out of 17 observation days. Ad libitum observations showed that the female spent much time plucking the feathers of the duckling, handling it carefully. In addition, she regularly placed a duckling on her back during locomotion. Eating of a carcass was not observed and playing with a carcass was very rare. Based on these observations, we propose that not meat eating and play, but allomothering and abnormal plucking may explain this idiosyncratic behavior. This female probably invented the behavior herself.