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Orangutans challenge our dichotomous perception of phenotypic sex seeing as they have three sexuallymature adult morphs: females, unflanged males, and flanged males. In males, a significant increase in androgen levels is associated with numerous changes in physical characteristics and behavior that develop during the flanging process. While unflanged males lack these obvious secondary sexual characteristics, they still have higher testosterone levels compared to females. Here, we test whether captive unflanged males and females have similar facial ratios (facial width/bi-orbital distance), since flanges form from a facial compartment that contains androgen receptors and is present prior to flanging. When flanging, males deposit fat to these compartments. In the field, unflanged males can be hard to distinguish from female orangutans, without a clear view of the genitalia. Flanged males (3.9 ± 0.5, range=2.7-5.1, N=20) have significantly wider facial ratios compared to unflanged males (2.4 ± 0.3, range=2.1-3.0, N=9) and females (2.3 ± 0.3, range=1.9-2.7, N=22; F(2,48)= 98.18, p-value >0.0001), who are similar in their facial ratios. Interobserver reliability between measurements (N=74) of the bi-orbital distance (V=1171, p=0.3251), facial width (V=1711, p=0.04779), and facial ratio (V=496.5, p=0.2434) are low. While there are other morphological differences between them, unflanged males and females do not differ in their facial ratios. The lack of significant differences in facial ratios between unflanged males and females, despite the higher testosterone levels in unflanged males, is consistent with them having an alternative 'sneaker' male reproductive strategy that includes mimicking female size and appearance. 

Primates, especially great apes, have slow life histories and long lifespans compared to other mammalian groups. Data pertaining to life history variables can be difficult to collect in the wild considering that species can have longer lifespans than the duration of a field site’s existence. Here, we examine life history variables for captive-born great apes housed in the United States. Using studbook data, we investigate stillbirth rates, age at first birth (AFB), interbirth intervals, number of offspring, twinning rates, mean lifespan and maximum lifespans. Analyses presented here exclude individuals with estimated birthdates. The oldest maximum lifespan was recorded for gorillas (60.07yrs (female); n= 656) followed by chimpanzees (57.40yrs (female); n= 559), orangutans (54.88yrs (female); n= 660), and bonobos (52.15yrs (female); n= 144). Excluding individuals living ≤10 years of age, mean lifespan is similar for all great apes in captivity (F(3,417)= 0.849, p= 0.467; bonobos: 23.5 ± 9.8yrs; chimpanzees: 25.2 ± 10.6yrs; gorillas: 26.2 ± 10.2yrs; orangutans: 24.2 ± 10.0yrs). The stillbirth rate is highest in chimpanzees (0.147; n=559) then gorillas (0.144; n=658), bonobos (0.125; n=144), and orangutans (0.010; n=681). Gorillas (11.4 ± 3.7yrs) have a younger AFB, on average, than chimpanzees (15.9 ± 6.9yrs), bonobos (14.3 ± 6.9yrs), or orangutans (14.4 ± 5.1yrs) (F(3,377)= 13.97, p< 0.0001). We discuss how our findings are influenced by changes in husbandry practices as well as the captive environment. By examining the life histories of captive populations, we highlight the plasticity these species exhibit in relation to the timing of developmental and reproductive events. 

Monitoring health status is a critical aspect of primate conservation, yet can be difficult to noninvasively investigate in the wild. Internal body temperature, a marker of health in endotherms, has been tested in humans and chimpanzees using two different fecal temperature methods: using the peak internal temperature (PIT) or applying a sigmoid curve (SC). We tested both methods on wild and rehabilitant Bornean orangutans to determine if either is a feasible methodology for arboreal mammals. The SC method involves a series of temperatures for each sample that we fitted to a sigmoid curve, whereas the PIT method involved a single peak temperature recording. Estimates from the two methods were not significantly different in either our wild (T(88)= -2.0781, P=0.0406) or rehabilitant (T(29)= -2.8404, P=0.0082) samples. Adult rehabilitant body temperatures (N=9; 34.62 ± 1.32°C) were estimated to be hotter than those in the wild (N=107; 33.59 ± 1.66°C), although not significantly different (T(115)=1.9859; P=0.0493). In our model, testing a number of factors, we found height of fecal drop (P=0.0071), fecal weight (P=0.0198), and time of day (P=0.0029) to significantly affect body temperature estimates. Our field sample (N=107) indicates that wild orangutans have an internal fecal temperature, ranging between 29.5 and 37.3°C, lower than mean temperatures for chimpanzees or humans. This supports the finding that orangutans have lower metabolic rates than do most other eutherian mammals. Lower body temperature may serve as a metabolic adaptation of orangutans to survive extended periods of low food availability when energy needs to be conserved. 

Monitoring health status is a critical aspect of primate conservation, yet can be difficult to noninvasively investigate in the wild. Because mammals are endothermic, body temperature can be used as a health marker for primates. Using a method previously tested on chimpanzees and humans, we estimated body temperature of wild Bornean orangutans by measuring the internal temperature of fecal samples. Upon quickly collecting a fecal sample after defecation, we recorded internal temperature of the sample at 20-sec intervals for six minutes. Data included a series of temperatures for each sample that we fitted to a sigmoid curve, which was used to estimate body temperature. Estimated body temperature was not affected by sex (F(2,92)= 0.431, P= 0.651), weather (F(2,92)= 1.175, P= 0.313), or collection time (r= -0.074, N= 95, P= 0.468). Estimated body temperature was higher for fecal samples that fell from lower estimated heights (r= -0.23, N= 95, P= 0.0004) and were heavier (r= 0.23, N= 75, P= 0.0475). We compare these results from the field to captive fecal samples, taking place on the ground, to determine the accuracy of this field method. From our field samples (N=95), orangutans appear to have a lower internal body temperature (33.44 ± 1.74 °C) on average than either chimpanzees or humans. Previous studies have demonstrated that orangutans have a lower metabolic rate than other great apes. Lower body temperature may serve as a metabolic adaptation of orangutans to survive extended periods of low food availability when energy needs to be conserved. 

Primate health status affects individual fitness and survival, yet is difficult to noninvasively investigate in the wild. Using a method tested on chimpanzees and humans, we estimated temperature of fecal samples of Bornean orangutans as a proxy for body temperature. Upon defecation, we recorded peak internal temperature of the samples. Estimated body temperature was influenced by height of defecation (r= -0.23, N= 95, P= 0.0004) and sample weight (r= 0.23, N= 75, P= 0.0475). These estimates were not affected by sex (F(2,92)= 0.431, P= 0.651) or weather (F(2,92)= 1.175, P= 0.313). Our method allowed for fast, consistent sampling, such that time from defecation to collection did not affect the results (r= -0.074, N= 95, P= 0.468), confirming reliable fecal temperatures can be collected from orangutans. We compare our results from the field to captive fecal samples, finding higher body temperatures in captivity. From our samples (N=95), orangutans appear to have a lower internal body temperature (33.44 ± 1.74 °C) on average than either chimpanzees or humans. Previous studies have demonstrated that orangutans have a lower metabolic rate than other great apes. Lower body temperature may serve as a metabolic adaptation of orangutans to survive extended periods of low food availability when energy must be conserved. 

The Gunung Palung Orangutan Project has conducted research on critically endangered wild Bornean orangutans (Pongo pygmaeus wurmbii) since 1994 in Gunung Palung National Park, West Kalimantan, Indonesia. A major goal of our broad-ranging research on orangutan behavior and ecology is to understand how the unique rainforest environment of Southeast Asia, characterized by dramatic changes in fruit productivity due to unpredictable mast fruiting, impacts orangutan behavior, physiology, and health. Much of our research has been devoted to the development of non-invasive techniques and an integrated biology approach – using hormonal assays, fecal processing, nutritional analysis, genetics, and behavioral ecology – and has led to an increased understanding of the ecological and evolutionary pressures shaping orangutan adaptations. Our results show that the extended life history and very slow reproductive rate of orangutans are adaptations to their environment. Orangutans in the Gunung Palung landscape, as elsewhere across Borneo and Sumatra, also face a series of conservation challenges, including extensive habitat loss and the illegal pet trade. We highlight how our investigations of orangutan health status, ecosystem requirements, and the assessment of orangutan density using ground and drone nest surveys have been applied to conservation efforts. We describe our project’s direct conservation interventions of public education and awareness campaigns, sustainable livelihood development, establishment of village-run customary forests, investigation of the illegal pet trade, and active engagement with Indonesian government organizations. These efforts, in concert with the development of local scientific and conservation capacity, provide a strong foundation for further conservation as orangutans face a challenging future.