Britannica Sources used in this answer

Why do older male orangutans develop broad cheek pads?

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An illustration of a female Bornean orangutan (Pongo pygmaeus) with long reddish-brown fur, hanging from tree branches with one arm extended upward and one leg hooked over a branch. A scale bar at the bottom shows lengths of 30 centimeters and 12 inches for size reference. — orangutan Female orangutans (*Pongo*) have the longest breeding interval of any mammal, giving birth on average once every eight years.

Older male orangutans develop broad cheek pads as a secondary sexual characteristic associated with reaching full physical maturity. These pads enhance the size of the head and are linked to increased levels of testosterone.12

The development of these features is closely tied to social conditions. A subadult male may remain in a state of arrested development for 10 to 20 years if he is in the proximity of a dominant, full-grown male, as the presence of the dominant male suppresses the subadult's development. When the dominant male moves away or is removed, the subadult experiences a surge in testosterone, leading to rapid maturation that includes the development of cheek flanges and a significant increase in body weight.12

These physical changes, along with the development of a throat pouch used for the "long call," serve to signal maturity, space out other males, and attract sexually receptive females.2