Male orangutans exhibit a striking difference: some develop prominent “wide faces,” while others do not. These unique facial structures play a significant role in their biology and social dynamics.
Understanding Cheek Pads
These “wide faces” are technically known as cheek pads or flanges. They consist of fibrous fatty tissue and bone that grow on the sides of a male orangutan’s face, extending from the cheekbones to the temples. These pads give the male a broad, disc-like facial appearance, which can vary in size and shape between individuals and species. For instance, Bornean orangutans tend to have flanges that protrude forward, while Sumatran orangutans often have flatter pads.
Development and Hormonal Influence
Only adult, sexually mature male orangutans develop these distinctive cheek pads. Their development is primarily triggered by hormones, particularly testosterone. This transformation is not immediate upon reaching sexual maturity, which can occur as early as 7 to 10 years of age. Instead, full flange development often begins later, around 15 to 20 years of age, and can take one to two years to complete.
The timing of flange development is also influenced by social and environmental cues. The presence of a dominant flanged male can suppress flange development in other males. Males that develop flanges earlier tend to have sustained higher testosterone levels compared to those whose development is delayed. This suggests an interplay between hormonal predispositions and social conditions in determining when a male undergoes this physical transformation.
Social Role and Mating Advantage
Cheek pads serve multiple functions in an orangutan’s social and reproductive life. They are a visual display of maturity, strength, and reproductive fitness, playing a role in sexual selection by attracting females. Females often prefer flanged males, leading to greater reproductive success for these individuals. The large size of flanged males, including their cheek pads, also serves as an intimidation display towards rival males, helping to establish dominance.
These pads also have an acoustic function, amplifying and directing the “long call,” a loud vocalization unique to flanged males. This booming series of shouts can travel for over a mile through the rainforest, allowing males to advertise their presence and attract mates. The cheek pads, along with a large throat sac, act like a megaphone, projecting the sound forward and enhancing its reach. The long call is crucial for maintaining spacing in their solitary existence and for mediating social relationships.
The Unflanged Male Strategy
Not all adult male orangutans develop cheek pads, even after reaching sexual maturity. These are known as “unflanged males,” and they represent an alternative reproductive strategy. While unflanged males lack the prominent visual display of flanged males, they are still fertile and capable of siring offspring. Their reproductive success often occurs through opportunistic or stealth mating, particularly during periods when dominant flanged males are not present or during times of social instability.
Unflanged males may delay flange development for extended periods, sometimes for decades, or may never develop them at all. The presence of a dominant flanged male can suppress the development of flanges in younger or subordinate males. This allows unflanged males to avoid direct, often dangerous, confrontations with larger, flanged rivals. Their strategy is sometimes described as “go, search, and find,” contrasting with the “sit, call, and wait” approach of flanged males.