Animals often communicate through signals that go beyond sound or movement. In orangutans, the flange – large cheek pads that develop in dominant males – are more than just striking facial features.

Recent research shows that flange not only help males stand out but also capture the attention of other orangutans. The study highlights how such traits may influence both attraction and vigilance in the social world of great apes.

A research team led by Leiden University found that orangutans pay more attention to images of flanged males compared to unflanged ones. The bias mirrors patterns in humans, where masculine features often draw the eye.

“This shows that our attention system has deep evolutionary roots,” said Tom Roth, a behavioral biologist at Utrecht University. Such shared mechanisms suggest that sexual selection has shaped cognition across primates.

Why cheek pads matter

Male orangutans display extreme sexual dimorphism. Some remain unflanged, while others develop into heavier males with large cheek pads, throat sacs, and long hair.

These flanges require years of energy investment and are linked to higher testosterone levels. Females typically prefer flanged males, possibly because they signal genetic quality.

At the same time, flanged males may pose risks. They dominate rivals and might represent threats in social interactions.

For females, even a low possibility of infanticide could encourage caution. This dual role – attractive yet potentially dangerous – may explain why flanges command such attention.

Eyes on dominance

To test this, the team used eye-tracking technology with zoo-housed orangutans. When presented with paired images, the apes were more likely to fixate first on the flanged male.

They also spent longer gazing at him. The bias held in both controlled portraits and more naturalistic images.

“The orangutans looked at the flanged male about 55 to 60 percent of the time and at the unflanged male 40 to 45 percent of the time,” said Roth. “That is a substantial difference for this kind of study.”

Attention bias towards flanged males

Interestingly, the experiments also revealed a perceptual quirk: orangutans tended to fixate first on the left side of the screen, regardless of which male was shown.

This mirrors findings in humans and other animals, suggesting a general bias in how attention is directed.

Beyond group patterns, individual orangutans showed unique preferences. Some females consistently favored specific flanged males, hinting at personal variation in attraction or interest.

However, the small sample size leaves open whether these differences reflect genuine preference or random variation.

Signals beyond attraction

Attention toward flanged males may not be purely about mate choice. For males, it could signal competition. For females, it could represent vigilance against dominance or potential risks.

“This study shows that flanges provide relevant information for conspecifics,” said Roth. “However, we cannot yet say with certainty what exactly motivates the animals to focus on flanged males.”

Such complexity echoes human patterns. People often focus on masculine faces, which may signal both attractiveness and dominance.

Just as in humans, attention in orangutans might be shaped by overlapping factors: desire, competition, and caution.

Broader evolutionary picture

These findings fit into a larger framework of how visual attention supports survival and reproduction.

In both humans and other primates, attention is biased toward evolutionarily important stimuli. These include predators, rivals, potential mates, or subtle signals of social dominance that can shape interactions.

Detecting these cues quickly and automatically provides a survival advantage, since hesitation in recognizing a threat or opportunity could reduce reproductive success.

Orangutans join macaques and humans in showing clear attentional biases toward sexually selected traits. This suggests that visual systems across primates have evolved to prioritize cues linked not only to fitness and competition but also to long-term strategies of mate choice, alliance building, and risk avoidance.

The consistent focus on flanged males demonstrates how strongly cognition has been shaped by pressures of natural and sexual selection, guiding attention toward traits that influence both attraction and social hierarchy.

Future research on flanged males

Future research could explore whether female orangutans show stronger biases during fertile periods, linking attention more directly to mating motivation.

Studies could also combine visual and vocal cues, since flanged males are known for their long calls, which may reinforce their visual presence.

“But just like with humans, there has to be more than a genetic match. The animals also need to be interested in each other as partners,” Roth said.

Understanding attention may help improve breeding programs in zoos and deepen our grasp of how cognition and sexual selection intertwine.

The study is published in the journal Annals of the New York Academy of Sciences.

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