Orangutans, native to the rainforests of Borneo and Sumatra, are known for their distinctive appearance. As the largest arboreal mammals, they spend most of their lives navigating the forest canopy with their exceptionally long arms. Their physical characteristics, particularly their striking facial features, are a subject of interest.
Unique Facial Features
Adult male orangutans often possess specific facial features that set them apart, showcasing sexual dimorphism. The most prominent of these are the large, fleshy cheek pads, also known as flanges, which develop on the sides of their faces, extending from the cheekbones. Along with these flanges, mature males typically develop a large, pendulous throat sac. These features are primarily observed in mature males, while females and younger males generally do not display such pronounced characteristics. The size and shape of these flanges can vary among different orangutan species and individuals.
The Purpose of Cheek Pads
The large cheek pads of male orangutans serve as a visual display, playing a significant role in sexual selection and social dominance. Larger, more developed flanges signal maturity, health, and higher status to both rival males and potential mates. They are used to intimidate other males. Flanges develop as a result of a surge in testosterone. Their presence is linked to greater reproductive success, as females often prefer mating with flanged males.
The Role of the Throat Sac
Complementing the visual display of the cheek pads, the throat sac in flanged male orangutans plays a role in acoustic communication. This inflatable sac functions as a resonance chamber, enabling males to produce loud, far-reaching vocalizations known as “long calls.” These calls travel through the dense rainforest, serving to assert territory, advertise the male’s presence, and warn off rival males. The cheek pads help direct and amplify these sounds forward. This acoustic signaling is important given the semi-solitary nature of orangutans and their forest environment.
Why Some Males Look Different
Not all adult male orangutans develop these prominent facial features, leading to two distinct male morphs: “flanged” and “unflanged” males. Unflanged males lack the large cheek pads and throat sac, and typically appear similar in size to adult females. The development of flanges is influenced by environmental factors and social hierarchy, with the presence of a dominant flanged male often inhibiting these features in other sexually mature males. Unflanged males are still reproductively capable and employ different mating strategies, seeking out females more actively rather than relying on calls and displays. While flanged males have higher reproductive success, unflanged males can also father offspring.