Humans admire peacocks for their stunning tail feathers, but until recently no one could say whether the display actually drew the eyes of peahens. Now scientists, strapping cameras onto the heads of the female birds, have obtained evidence of what it is about peacock tails that attracts the opposite sex.

The peacock’s tail, or train, was a riddle that vexed Charles Darwin as he sought to devise his theory of evolution: The principle of natural selection suggested that a species would develop traits that enhanced its chances of survival, but the extravagantly outsize train seemed more like a burden than an aid. “The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!” the puzzled scientist wrote in 1860.

His eventual solution was the concept that some traits might be useful for sex rather than survival. In other words, peacock tails are aids to courtship that run the risk of fatally impairing the males in exchange for giving them a better shot at a mate.

However, it has remained uncertain how exactly peacock tails enticed peahens: Was it the iridescent eyespots? The length of the feathers?

The technology known as eye-tracking is beginning to provide answers.

This femeale peacock, a peahen, is equipped with a video camera on her head in order to study how sight plays a role in choosing a mate and other interactions. (Courtesy of Jessica Yorzinski)

Eye-tracking research has been conducted in the laboratory for more than a century. Once the necessary cameras became light enough for humans to wear, scientists conducted studies outdoors. For instance, wearable eye-trackers developed by New York-based Positive Science with funding from the U.S. Naval Research Laboratory could be used to learn how combatants detect camouflaged targets.

Now eye-trackers are lightweight enough for animals to wear, giving scientists a tool to learn more about how other creatures view the world. This year, for example, Japanese researchers reported equipping chimpanzees with such gear to learn how they look at humans as well as at the rewards they are offered during experiments.

For the peacock experiment, reported in the August edition of the Journal of Experimental Biology, Positive Science collaborated with evolutionary biologist Jessica Yorzinski, then a graduate student at the University of California at Davis, to develop eye-trackers small enough for peahens.

Scientists from UC Davis and Duke University then rigged 12 birds with headgear equipped with two cameras — one aimed at one of the bird’s eyes, the other at the scene in front of the animal. A wireless transmitter mounted on the bird’s back sent video from both cameras to a recorder nearby, and when those feeds were combined, a yellow dot would mark exactly where the bird’s gaze was directed.

Gradual daily training accustomed the peahens to wearing the apparatus. First, researchers fit each peahen with an empty backpack, then slowly added filler until it approximated the weight of the battery and transmitter. Then, each bird was fitted with a soft plastic helmet. Finally, the cameras were added to the helmet, and the transmitting apparatus was put into the backpack. “I got many scratches from their sharp claws as they tried to escape,” Yorzinski, who is now a postdoctoral researcher at Purdue University, recalled in an interview.

At first, Yorzinski was afraid the weight of the device — about 13 ounces — would inhibit the birds’ normal activities. However, by the end of the familiarization process, which sometimes took months, she said the peahens behaved normally while wearing the eye-tracker — walking, eating and even mating with it on.

“This was a very ingenious way of finding out what this species finds important in the world,” said Eric Knudsen at Stanford University, who studies the nervous systems of birds.

In experiments, each peahen was placed alongside two peacocks in an outdoor enclosure. The males vied for attention with elaborate shows, shaking their wings and rattling their train of plumes, hooting and dashing toward peahens if the females appeared ready to mate.

The scientists weren’t surprised when they saw that the peahens looked at each peacock when it fanned its tail feathers upward. But, intriguingly, the females focused nearly completely on the bottom part of the train, close to the ground. They mostly ignored the conspicuous upper fan.

However, the peahens did pay attention to the upper feathers when the lower train was obscured — as it might be if the males were relatively far away and partially hidden by the dense vegetation of the birds’ natural habitat in India. This suggests that the flashy upper train is mostly a long-distance attraction signal, but the lower feathers are more important to close-up courtship.

“It was exciting to literally see through the eyes of the females to learn how they perceive potential mates,” Yorzinski said.

Now Yorzinski and her colleagues are placing eye-trackers on peacocks to see how males judge their rivals. They also plan to alter the appearance of the males’ lower trains to try to figure out what the females find attractive.

Eye-trackers may reveal many other details about how animals behave. “You can imagine investigating what monkeys look at as they swing from branch to branch, or what a cat looks at while it’s stalking a bird — how does it divide its attention between the bird and also the path in front of it?” said neuroethologist Michael Land at the University of Sussex in England. “When red deer males are banging antlers together, what are females looking at to judge them?

“An especially fascinating animal to examine might be the chameleon, because each of their eyes can move independently — chameleons point their eyes in different directions to look for prey, and then bring both eyes to the front to target prey, which they capture by sticking out their tongues,” Land added. “How do they control both eyes and get them to work separately and together?”

Choi is a freelance writer based in New York.