Sex Differences in Estimated Lean Body Mass of Captive and Wild Orangutans

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.

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.

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.

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.