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Unlike any other primate species, orangutans exhibit extraordinary sexual differences in their facial morphology. Two different strategies are available for males that reach sexual maturity: either become fully “flanged” and develop secondary sexual characteristics, which is knownto be only developed by the dominant male, or remain “unflanged” and not develop secondary sexual characteristics. The mechanism for how this bimaturism evolved and how occurs is poorly understood, but both flanged and unflanged males are reproductively successful.

This project explores the physiological mechanism behind bimaturismin orangutan males; namely, are there genetic differences between flanged and unflanged males such that the strategy is inherited, is bimaturism a result of hormonal differences, and how do the two strategies influence reproductive success in male orangutans? Previous studies have shown that males that develop these “flanges” or cheek pads are more likely to have a high reproductive success than the ones who do not develop them. Results suggest that males with flanges emit a loud vocalization calls, also known as long calls, that give them higher opportunity for mates.

Publication Date



orangutans, bimaturism, flanges, unflange, morphology


Animal Studies | Behavior and Ethology

Current Academic Year


Faculty Advisor/Mentor

Amy Rector Verrelli


© The Author(s)

Meta-analysis of the origin of bimaturism in orangutan males