TY - JOUR TI - Understanding bimaturism: the influence of social conditions, energy intake, and endocrinological status on flange development in Bornean orangutans (Pongo pygmaeus wurmbii) DO - https://doi.org/doi:10.7282/t3-69nv-7813 PY - 2019 AB - Bimaturism in orangutans is defined as two different secondary sexual characteristics of a mature male, unflanged and flanged. Bimaturism has been hypothesized to be condition-dependent, which includes how social interactions, energetic status, and physiological status regulate flange development. Yet we still have little understanding of what triggers the development of secondary sexual characteristics in adult males and why there is such of variation in the timing of flanging. This study examined relationships between social behavior, energy intake, and endocrinological hormones to flange development in Bornean orangutans (Pongo pygmaeus wurmbii). By quantifying the orangutans’ flange size from three different male’s developmental stages (unflanged, developing flange, and flanged), I observed that 1) Bimaturism in orangutans is provoked by social interactions. Social interactions were related to the size of the orangutans’ flange such that males with smaller flanges tended to have greater interactions. Moreover, the amount of social interactions was related to the cortisol production in males with greater flange size. 2) Bimaturism in orangutans is not nutrient-dependent. Total energy intake did not differ among developmental stages and by accounting body mass per Kg, flanged males tended to have the lowest energy intake compared to other stages. 3) Bimaturism in orangutans seems to be influenced by endocrinological status. High testosterone levels were correlated to the flange development, and elevated cortisol levels were highest in unflanged males and may have inhibited flange development. KW - Ecology and Evolution KW - Bornean orangutan -- Development -- Endocrine aspects LA - English ER -