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.

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.

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.

Haemadipsid leeches are ubiquitous inhabitants of tropical and sub-tropical forests in the Indo-Pacific region. They are increasingly used as indicator taxa for biomonitoring, yet very little is known about their basic ecology. For example, to date no study has assessed the occurrence and distribution of haemadipsid leeches across naturally occurring gradients within intact habitats. We analyzed a long-term data set (2012-2020) on the closely related tiger (Haemadipsa picta) and brown (Haemadipsa spp.) leech species to investigate if and how abiotic and biotic factors influence their occurrence across a gradient of forest types at an undisturbed tropical rainforest site in Indonesian Borneo. We compared a series of negative binomial mixed models and found that, of the abiotic factors, soil moisture had the largest positive effect on encounter rates of both leech species. Among biotic factors, forest type had differential effects on counts of the two species: while tiger leech counts were greater in low elevation forest types, brown leech counts were greater in high elevation forest types. Additionally, we found that the presence of one species had a positive effect on the presence of the other species. Finally, our results show that the tiger leech has a narrower distribution, being restricted to lower elevation forest types with higher water retention, suggesting that the tiger leech could be more sensitive to lower soil moisture levels. Abstract in Indonesian is available with online material.