Some male orangutans have giant, padded cheeks that frame their faces, while others don’t. Now, researchers conducting ape paternity tests reveal that males with cheek pads (called flanges) are far more successful at fathering offspring. The findings were published in Behavioral Ecology and Sociobiology this week. 

Unlike most mammals, orangutans exhibit what’s called bimaturism, by which adult males take on one of two distinct face shapes. Socially subordinate, unflanged males appear similar to females in size and face shape. Dominant, flanged males, on the other hand, exhibit extreme sexual dimorphism: they’re much bigger than females and, in addition to cheek pads, they have a large, pendulous throat sac that helps them bellow their long, deep calls. What’s especially interesting is that males without cheek pads are quite capable of fathering offspring as well. 

"Dominant males have to find and consume more calories. Their movement is restricted as a result of their size, and fights with neighboring dominant males have been known to result in death," Graham Banes of the Max Planck Institute for Evolutionary Anthropology (MPI-EVA) said in a statement. "So, why would a male develop cheek pads if he can father offspring without?" 

To see if flanges make male orangutans more attractive to females, Banes and colleagues studied the reproductive success of Kusasi, the former long-term dominant male at Camp Leakey in Indonesia's Tanjung Puting National Park. (There’s usually only one flanged male in any given area.) They then compared his success with that of subordinate males without cheek pads from the same home range. The team performed paternity tests by extracting DNA from fecal samples they collected in the 50-square-kilometer (19-square-mile) study area. 

It identified 39 known individuals, 12 of which were males. During his decade as top orangutan, Kusasi sired more offspring than any other male. "Paternity could be assigned to 14 candidate offspring, conceived across multiple decades, 10 of which were fathered by Kusasi," said study coauthor Linda Vigilant, also of MPI-EVA, in a statement. The team concluded that cheek pad development is consistent with an evolutionarily stable reproductive strategy, with reproduction highly skewed in a flanged male’s favor.  

That unflanged males also successfully fathered offspring wasn’t a surprise. "The timing, however, was interesting," Banes added. "These other males were typically reproductively successful at the beginning and end of Kusasi's dominant period, when the hierarchy was potentially unclear." That is, they waited for times of rank instability.