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When orangutans were shown two photos side by side, one of a dominant flanged male (having large pads on the sides of their faces) and one of an unflanged male, they spent more time looking at the flanged male.

Researchers from Utrecht University, Apenheul, and Leiden University, in the Annals of the New York Academy of Sciences. Previous studies suggest that humans also pay more attention to members of their own species with more masculine faces.

"This shows that our attention system has deep evolutionary roots," says Utrecht University behavioral biologist Tom Roth.

There are two types of adult male orangutans. Some look much like females, while others are larger and heavier, with distinctive flanges, pads along the sides of their heads. These bigger males also have a throat pouch for making calls and sport longer hair.

"Developing into a large male with flanges takes a lot of energy and can take several years," Roth says.

When it comes to mate choice, however, these larger, flanged males have a clear advantage, as female orangutans prefer them over smaller males.

"But what exactly makes these males more appealing?" Roth wondered. "What do females base their choice on? Do they, for instance, spend more time looking at males with flanges?"

Eye tracking

By exploring these questions, Roth hopes to gain deeper insight into how zoos can better pair animals for breeding programs. "Right now, the main focus is on genetic compatibility," he says.

"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."

To investigate the question, Roth and his colleagues used eye-tracking technology. With a special camera, they were able to precisely monitor where each orangutan was looking. The team used the technique while showing the animals two photos at the same time, one of the head of a flanged male and one of the head of an unflanged male, for four seconds.

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

Diluted lemonade

For the test, the apes needed to keep their heads still in just the right position. How did the researchers manage that? "The animals are always free to choose whether they want to take part," Roth explains. "If they do, they receive a small amount of diluted lemonade through a drinking nipple during each test. They keep their lips on the nipple, which positions their heads perfectly for us to take the measurements."

This way, the researchers tested four orangutans, three females and one male. Interestingly, the male also spent more time looking at the flanged head.

"This study shows that flanges provide relevant information for conspecifics. However, we cannot yet say with certainty what exactly motivates the animals to focus on flanged males. Is it really about mate choice, or is something else at play? For example, unflanged males tolerate each other, but flanged males are dominant over other males. So the flanged male can also be a threat."

Broader trend

Yet, this study fits into a broader trend. Growing evidence shows that the attention of humans and other animals is often automatically drawn to evolutionarily relevant cues about potential partners, competitors, or threats.

"Many of these processes happen without much conscious thought," Roth says. "The best-known example is our attention to predators. People still have a strong attention bias towards photos of snakes and spiders. That is a good thing, because if you have to think about it before you pay attention, it might already be too late."

Roth has also previously studied attention in humans. "We asked people to rate photos of other people for attractiveness. We then combined those photos in an eye-tracking experiment.

"The results showed that people focus much more on faces they find attractive. Other researchers' experiments also suggest that both men and women pay more attention to men with masculine faces, with smaller eyes, thicker eyebrows, a wider nose, and a strong jawline. I think most people don't even realize they are doing this."