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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. 

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.