Wild Bornean orangutans experience muscle catabolism during episodes of fruit scarcity

Background/Question/Methods

Pronounced temporal and spatial variation in the availability of food resources can produce energetic deficits in organisms. Fruit-dependent Bornean orangutans face extreme variation in fruit availability and experience negative energy and protein balance during episodes of fruit scarcity. We evaluate the possibility that orangutans of different sexes and ages catabolize muscle tissue when the availability of fruit is low. We assess variation in muscle mass by examining urinary creatinine adjusted by specific gravity as a non-invasive measure of estimated lean body mass (ELBM). We examined variation in ELBM using a generalized additive mixed model (GAMM) with FAI and age-sex class as fixed effects and individual ID as a random factor.

Results/Conclusions

Despite orangutans having a suite of adaptations to buffer them from fruit scarcity and associated caloric deficits, ELBM decreased during low fruit periods in all age-sex classes (F(5.094) = 13.6, p<0.001). As predicted, adult male orangutans had higher ELBM than adult females and immatures. Contrary to expectation, flanged and unflanged males did not differ significantly in ELBM. These findings highlight the precarity of orangutan health in the face of rapid environmental change and add to a growing body of evidence that orangutans are characterized by unique metabolic traits shaped by their unpredictable forest environment.