OBJECTIVES:Male and female primates experience different ecological and reproductive constraints, which often lead to differences in how they allocate energy. Since testosterone promotes muscle growth, male primates generally possess greater relative muscle mass than females. Orangutans exhibit a rare phenomenon called male bimaturism, where there are two adult male morphs. Male orangutans reach adulthood as the unflanged morph and may transition to the flanged morph during their lifetime. Here, we investigated muscle mass differences of wild and captive orangutans among the age-sex classes while accounting for flange status. METHODS:Estimated lean body mass (ELBM) can be obtained by comparing urinary creatinine (CR) residuals using specific gravity (SG). We analyzed 2329 urine samples collected from 51 orangutans under human care and 279 samples from 29 wild Bornean orangutans at Gunung Palung National Park, Borneo, Indonesia. RESULTS:In both datasets, flanged males had the greatest mean ELBM followed by females and unflanged males. Flanged males had significantly greater ELBM than adult females and unflanged males in both captive and wild environments. CONCLUSIONS:Flanged males have greater ELBM than unflanged males, suggesting increased energetic investment in muscle mass. Differences in body composition likely reflect the distinctive reproductive strategies and behavioral niches utilized by the two morphs. Flanging is an energetically costly process to undergo and maintain and is associated with a costly reproductive strategy. Because of these sex/morph differences, we recommend indexing urine samples against SG to avoid potential sex biases when indexing with CR.

OBJECTIVES:As a predominantly arboreal animal in the wild, the terrestrial positional behavior of Pongo pygmaeus is poorly understood, having been studied almost exclusively in captive settings. This study uses camera-trap footage to provide the first assessment of wild orangutan terrestrial locomotor and postural behavior on natural substrates. MATERIALS AND METHODS:Video footage of orangutans from motion-activated cameras in the Gunung Palung National Park, West Kalimantan, Indonesia was collected over a five-year period. The resulting 100 instances of orangutan terrestriality were analyzed to document incidences of positional behavior, including hand and foot postures as well as overstride pattern during locomotion and qualitative assessment of hindlimb joint excursions during orthograde behaviors. RESULTS:When locomoting terrestrially, Pongo pygmaeus primarily engaged in quadrupedal fist-walking with heel-strike. Wrist position and overstride pattern during quadrupedal walking were variable both within and between individuals. For posture, individuals were captured standing upright more often than pronograde, both monopedally and bipedally, and almost always with full extension of the hip and knee. DISCUSSION:These observations of wild orangutan terrestrial positional behavior address prior ambiguities related to hand and foot positioning during locomotion. Maintained full extension of the hindlimb in the absence of substrate compliance indicates the form of bipedalism orangutans use in the trees translates to natural terrestrial substrates. Finally, the high proportion of observations with asymmetrical movement and posture may indicate a lateral decoupling of limbs in addition to the expected fore- and hindlimb independence of apes, suggesting orangutan positional adaptability is intrinsic regardless of substrate.

Maternal loss is a detrimental early life adversity with negative consequences for the survival and development of juveniles. These negative consequences can be mitigated by adoption of the orphan. Here we provide the first detailed report on a case of adoption in wild Bornean orangutans (Pongo pygmaeus wurmbii) in Gunung Palung National Park, West Kalimantan, Indonesia. Among other great ape species, orphans have a greater chance of survival if adopted. In many of these cases, the orphans were adopted by siblings or other related individuals who provided critical alloparental care. We compared the behavior of this adoptive sister pairing with mother-offspring pairs in our study population and considered costs and benefits to each sister. The orphaned younger sister survived and reached developmental milestones at a younger age than the population average. Her activity budget also showed a reduction in the percent of time spent playing. The adoptive older sister did not experience a delay in her reproductive timeline due to the adoption, but the age-sex class profile of her social partners differed from other adolescent females in our study population. The daily average distance between the sisters was greater than the distance between mothers and their offspring, but there was no difference in time spent engaged in feeding tolerance compared to mother-offspring pairs. Maternal loss appears to have accelerated the development of independence, while adoption and alloparental care provided by her older sister likely allowed for her survival. This study also highlights the value of genetic identification analysis along with long-term data collection.

Male orangutans (Pongo spp.) exhibit bimaturism, an alternative reproductive tactic, with flanged and unflanged males displaying two distinct morphological and behavioral phenotypes. Flanged males are larger than unflanged males and display secondary sexual characteristics which unflanged males lack. The evolutionary explanation for alternative reproductive tactics in orangutans remains unclear because orangutan paternity studies to date have been from sites with ex-captive orangutans, provisioning via feeding stations and veterinary care, or that lack data on the identity of mothers. Here we demonstrate, using the first long-term paternity data from a site free of these limitations, that alternative reproductive tactics in orangutans are condition-dependent, not frequency-dependent. We found higher reproductive success by flanged males than by unflanged males, a pattern consistent with other Bornean orangutan (Pongo pygmaeus) paternity studies. Previous paternity studies disagree on the degree of male reproductive skew, but we found low reproductive skew among flanged males. We compare our findings and previous paternity studies from both Bornean and Sumatran orangutans (Pongo abelii) to understand why these differences exist, examining the possible roles of species differences, ecology, and human intervention. Additionally, we use long-term behavioral data to demonstrate that while flanged males can displace unflanged males in association with females, flanged males are unable to keep other males from associating with a female, and thus they are unable to completely mate guard females. Our results demonstrate that alternative reproductive tactics in Bornean orangutans are condition-dependent, supporting the understanding that the flanged male morph is indicative of good condition. Despite intense male-male competition and direct sexual coercion by males, female mate choice is effective in determining reproductive outcomes in this population of wild orangutans.

Gunung Palung National Park (GPNP) and the surrounding region in West Kalimantan, Indonesia, host a significant population of Critically Endangered Bornean orangutans (Pongo pygmaeus wurmbii) amidst various conservation challenges, including habitat destruction, the illegal pet trade, and human-wildlife conflict. The Gunung Palung Orangutan Conservation Program (GPOCP) employs diverse strategies, including environmental education, to address these challenges. Environmental education helps to connect local students with conservation efforts, educate them about endangered endemic species, and highlight the relationship between human behavior and ecosystem health. Environmental education is critical in fostering sustainable behaviors and raising awareness about conservation issues, particularly in areas where human activities threaten primate biodiversity. Such programs can promote long-term conservation outcomes by engaging communities and empowering individuals to take action. Here, we review the effectiveness of GPOCP's environmental education initiatives, focusing on school visits conducted between 2016 and 2023 that concentrated on orangutan conservation. We analyze data from standardized pre- and post-activity surveys administered to students participating in puppet shows and lectures. Results indicate a substantial increase in knowledge and positive shifts in attitudes toward orangutan conservation among students. However, challenges remain in evaluating long-term behavioral changes and the relative contribution of different conservation interventions. Future efforts will explore tailored educational approaches and impact assessments to enhance conservation outcomes and foster community engagement in safeguarding Bornean orangutans and their habitat.

Abstract Conservation funding is currently limited; cost‐effective conservation solutions are essential. We suggest that the thousands of field stations worldwide can play key roles at the frontline of biodiversity conservation and have high intrinsic value. We assessed field stations’ conservation return on investment and explored the impact of COVID‐19. We surveyed leaders of field stations across tropical regions that host primate research; 157 field stations in 56 countries responded. Respondents reported improved habitat quality and reduced hunting rates at over 80% of field stations and lower operational costs per km2 than protected areas, yet half of those surveyed have less funding now than in 2019. Spatial analyses support field station presence as reducing deforestation. These “earth observatories” provide a high return on investment; we advocate for increased support of field station programs and for governments to support their vital conservation efforts by investing accordingly.

Sexually-selected infanticide by males is widespread across primates. Maternal protection is one of many infanticide avoidance strategies employed by female primates. Bornean orangutan (Pongo pygmaeus wurmbii) mothers with younger offspring are less social with males than mothers with older offspring. Additionally, the distance between a mother and offspring decreases in the presence of male conspecifics, but not female conspecifics. We hypothesized that mothers are responsible for the change in mother-offspring proximity when males are present. Using a year of behavioral data from orangutans in Gunung Palung National Park, we tested whether the Hinde Index, a ratio of the number of approaches and leaves between two individuals, was indicative of mother or offspring proximity maintenance across different social groupings. The semi-solitary social organization of orangutans allows us to observe different social groupings. We found that the mother-offspring Hinde Index was typically indicative of offspring maintenance of proximity. However, the presence of male conspecifics was associated with an increase in the Hinde Index which indicates that mothers are responsible for the decrease in mother-offspring distance when males are present. The decrease in mother-offspring distances and increase in Hinde Index when males are present indicates that mothers react to the presence of males in a protective manner. We suggest this may be an infanticide avoidance behavior by mother orangutans.

Research in primate nutritional ecology uses concepts of quality, preference, and energy content to explain food choice: a primate consumes high-quality, preferred, and/or high-energy foods when available, and moves to other foods when they are not. Many frugivorous primates are thought to maximize energy, suggesting that high-energy fruit is a quality/preferred resource. However, recent research suggests a more complicated picture, including nutrient balancing and state-dependent foraging decisions. This brings into question whether a food is inherently high- or low-quality or whether this depends on previous feeding choices of the consumer. We tested whether a frugivorous primate, Pongo pygmaeus wurmbii , selects nonfruit foods when fruit is available. To test selectivity, we looked for three behaviors: (1) orangutans leaving available fruit for nonfruit foods, (2) evidence of orangutans navigating to nonfruit foods, and (3) orangutans selecting nonfruit when fruit was available and nearby. We use data from 51 focal follows (611 feeding bouts, 15 animals) in Gunung Palung National Park, Indonesia. We found when orangutans leave available fruit, it often is for nonfruit foods. We found evidence of orangutans navigating for nonfruit foods. GPS data show that orangutans leave or pass by fruit to consume other foods. When orangutans consumed nonfruit resources, 26% of the time, there was at least one available fruit resource within a 50-m radius. These data suggest that orangutans select nonfruit foods, strengthening the argument that feeding choice is consumer state-dependent, and leading us to argue for a revision of the use of “quality” as a descriptor of primate foods.

In humans, individuals’ social setting determines which and how language is acquired. Social seclusion experiments show that sociality also guides vocal development in songbirds and marmoset monkeys, but absence of similar great ape data has been interpreted as support to saltational notions for language origin, even if such laboratorial protocols are unethical with great apes. Here we characterize the repertoire entropy of orangutan individuals and show that in the wild, different degrees of sociality across populations are associated with different ‘vocal personalities’ in the form of distinct regimes of alarm call variants. In high-density populations, individuals are vocally more original and acoustically unpredictable but new call variants are short lived, whereas individuals in low-density populations are more conformative and acoustically consistent but also exhibit more complex call repertoires. Findings provide non-invasive evidence that sociality predicts vocal phenotype in a wild great ape. They prove false hypotheses that discredit great apes as having hardwired vocal development programmes and non-plastic vocal behaviour. Social settings mould vocal output in hominids besides humans.

Conservation strategies are rarely systematically evaluated, which reduces transparency, hinders the cost-effective deployment of resources, and hides what works best in different contexts. Using data on the iconic and critically endangered orangutan (Pongo spp.), we developed a novel spatiotemporal framework for evaluating conservation investments. We show that around USD 1 billion was invested between 2000 and 2019 into orangutan conservation by governments, nongovernmental organizations, companies, and communities. Broken down by allocation to different conservation strategies, we find that habitat protection, patrolling, and public outreach had the greatest return on investment for maintaining orangutan populations. Given the variability in threats, land-use opportunity costs, and baseline remunerations in different regions, there were differential benefits per dollar invested across conservation activities and regions. We show that although challenging from a data and analysis perspective, it is possible to fully understand the relationships between conservation investments and outcomes and the external factors that influence these outcomes. Such analyses can provide improved guidance toward a more effective biodiversity conservation. Insights into the spatiotemporal interplays between the costs and benefits driving effectiveness can inform decisions about the most suitable orangutan conservation strategies for halting population declines. Although our study focuses on the three extant orangutan species of Sumatra and Borneo, our findings have broad application for evidence-based conservation science and practice worldwide.

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.