When a peacock fans his plumage and struts his stuff, it’s an impressive sight. Or so it appears to us humans. What really matters, of course, is what the female he’s trying to impress makes of it. In a new study, scientists mounted tiny eye-tracking cameras on the heads of peahens to try to get inside their minds as they watched males’ courtship displays.

The findings suggest that what a female pays attention to when she sizes up a potential mate isn’t what some researchers had thought.

All those dramatic eyespots? Meh. But the width of his feather train? She’s definitely checking that out. And when he turns around and shakes his tail feathers? That’s totally hot.

‘We thought it would be a novel way to actually ask what she’s interested in.’

The peacock’s tail gave Darwin fits. At first, it seemed to fly in the face of his theory of natural selection. How could evolution possibly favor such cumbersome and conspicuous accoutrement? The very sight of those feathers, Darwin famously wrote to a colleague, made him sick. He soon realized, however, that the feathers might serve another purpose: enhancing the male’s reproductive success even as they made him more visible and vulnerable to predators. The concept of sexual selection was born, and the peacock’s tail remains a textbook example of it to this day.

But exactly what it is about the male’s display that females find attractive is far less clear.

Studies with feral peafowl at a British wildlife park in the 1990s suggested that it’s the ornamentation. Behavioral ecologist Marion Petrie of Newcastle University and her colleagues found that males with more eyespots mate more often. When the researchers used scissors to snip off 20 eyespots from several males, females showed less interest in them. Petrie’s work suggested that in the mind of a peahen, eyespots are pretty sexy.

If that’s true, she should spend a lot of time looking at them when the male does his display, says Michael Platt, a neuroscientist at Duke University and co-author of the new paper, published today in the Journal of Experimental Biology. Platt has previously used eye-tracking equipment to study primate behavior, including the social interactions of freely-moving lemurs, and in the new study he and colleagues developed an even smaller system that could fit on the head of a peahen.

“We thought it would be a novel way to actually ask what she’s interested in instead of going on our intuitions about what traits might be important,” says Jessica Yorzinski who collaborated with Platt on the study as a grad student and is now a postdoc at Purdue University.

The eyetracker consists of two cameras attached to a helmet that loops over the bird’s beak. It weighs just 25 grams, about as much as four quarters. One camera is trained on the pupil of one eye and tracks its position (for these experiments, the other eye was covered); the other captures the scene in front of the bird. A battery-powered transmitter strapped to the bird’s body wirelessly beams the data to a nearby computer.

The team used the eyetracker to study courting peafowl at an outdoor enclosure.

In the video above, the small box in the top right corner is the view of the peahen’s eye, and the yellow dot indicates where she’s directing her gaze. One thing that stands out is how little time she spends looking at the male as he shows off his elaborate train of feathers. She spends most of the time looking at the ground and glances his way only once in a while. She watches a squirrel run by. Then she seems to take an interest in the male, looking back and forth across the bottom of his train as if she’s sizing it up. Then it’s back to the ground for a bit. And then — what’s this? He turns around and shakes his backside. This maneuver clearly commands her attention, if only briefly.

After analyzing dozens of hours of such video from 16 peahens, the researchers conclude that during a male’s display, females devote considerable attention to the lower part of his train, virtually ignoring the top part of the train, his head, and crest. Females spent relatively little time looking at his eyespots. “It’s pretty suggestive that whatever the females are interested in, it doesn’t seem to be the eyespots themselves,” Platt said.

Well, what is it then?

“It’s reasonable to speculate here that what matters is something like size rather than ornamentation,” he said. “Size is related to a males’ age, and a males’ age is a pretty good predictor of breeding quality in these birds, so that wouldn’t be a bad strategy in these birds.”

The researchers found that females did look at a male’s eyespots more when he was farther away or when the lower part of his body was obscured. “They might serve the function of helping to call the female over, but once she gets there they don’t really seem to matter anymore,” Platt said.

“It’s technically very difficult to do this,” said Mary Hayhoe, a perceptual psychologist at the University of Texas, Austin, who was not involved with the study. “It gives you an interesting perspective on animal cognition from animal’s perspective.”

Eyetracking studies can reveal aspects of cognition that are hard to catch from studying behavior alone, Hayhoe says. The strangely compelling video below provides an example of this from her work with people: as a woman makes a peanut butter and jelly sandwich, her eye movements reveal that she mentally moves on to the next step before her hands finish the preceding one. She looks for the jelly before she’s done with the peanut butter, for example.

“People think of sensory systems as passive, but really you have to go out and select the information from the world that’s relevant to you,” Hayhoe said.

As for what’s relevant to peahens, Petrie doesn’t think the new study rules out the male’s eyespots. “It shows that the train is important, but the eyespots could still be important too.”

“To me, the nicest result is the aspect of attention grabbing,” Petrie said. “When they wobble their wings, the females look more.”

Platt, meanwhile, is already thinking of other potential applications of eyetracking devices to study attention in animals in their natural environments, or something similar. Very few studies like that have been done. “It’s wide open,” he said. “I could imagine using eyetracking to study how animals navigate a forest, how they search to find food, or how they coordinate behavior when they want to move as a group.”

Video credits: Jessica Yorzinski (top), Mary Hayhoe (bottom)