Orangutans possess a distinctive feature: the large, fleshy cheek pads, known as flanges, that adorn the faces of some males. These prominent structures prompt questions about their purpose. Beyond mere adornment, these cheek pads are intertwined with the biology and social dynamics of these apes.
Defining the Flange: What Are Cheek Pads?
Cheek pads, or flanges, are broad, disc-like formations on the faces of certain adult male orangutans. Composed primarily of fibrous fatty and connective tissue, they have a firm yet pliable texture. They are a clear example of sexual dimorphism, found only on sexually mature, dominant males.
Size and shape vary between individuals and between Bornean and Sumatran orangutans. Bornean males often have wider, more protruding flanges, while Sumatran males tend to have narrower, flatter ones. Flanged males also develop a large throat sac, which aids vocalizations.
Evolutionary Purpose: Why They Develop
Development of these cheek pads is a secondary sexual characteristic, signaling sexual selection. Their appearance indicates a male’s maturity, health, and dominance to mates and rivals. Flange growth is hormonally driven, primarily by increased testosterone during sexual maturation. Males with this testosterone increase typically develop these features.
Flanges are an “honest signal” of genetic fitness and good health. Only males in prime physical condition can invest the energy to develop and maintain such large structures. This display suggests a male’s robust physiological state and ability to thrive, making him attractive to females seeking healthy offspring. Flange development timing links to testosterone, with earlier development often indicating higher adult testosterone.
Social Significance and Behavior
Cheek pads function prominently in orangutan social behavior, enhancing a male’s apparent size and making him seem more imposing. This visual augmentation is advantageous during territorial displays, deterring rivals without physical confrontation. Flanges, with the throat sac, also amplify the distinctive “long call” produced by flanged males. These deep, roaring calls travel long distances, sometimes nearly a mile, announcing the male’s presence.
These long calls attract receptive females and warn other males, maintaining spacing between dominant individuals. Females often prefer flanged males for mating, viewing these signals as indicators of dominance and fitness. This preference is a significant factor in the reproductive success of flanged males, who adopt a “sit, call, and wait” strategy.
The Unflanged Male: An Alternative Strategy
Not all sexually mature male orangutans develop prominent cheek pads; some remain “unflanged” or in “arrested development.” These males retain a smaller, more juvenile-like appearance, lacking flanges and the large throat sac. Despite their appearance, unflanged males are sexually mature and capable of fathering offspring.
Unflanged males often employ an alternative reproductive strategy, sometimes called “go, search, and find.” They are less conspicuous and move more freely through territories without directly challenging dominant flanged males. This allows them to mate opportunistically, sometimes resorting to forced copulation, though this behavior is not exclusive. Unflanged males can later develop flanges, highlighting flexibility in orangutan social structures. This transformation can occur if environmental or social conditions change, such as the removal of a dominant flanged male.