Bulls are enraged by the color red.

The bullfight's defining image is one of color, or what appears to be color: the matador's red muleta cape, dramatically extended as the bull charges. For centuries, audiences and commentators assumed the crimson cloth was the engine of the animal's rage. It seemed obvious. Red excited the bull; red was danger; red drove the charge.

The obvious answer was wrong.

In bullfighting, two capes are used. In the opening stages of the fight, the matador and assistants use a large magenta-and-gold capote. Only in the final third, when the matador faces the bull alone in preparation for the kill, does the red muleta appear. The cape's color serves two purposes: tradition and concealment. The deep red obscures blood, both the bull's and, on occasion, the matador's, from spectators in the stands. What it does not do is provoke the bull.

Cattle cannot distinguish red from other colors in that portion of the spectrum. By the 1980s and 1990s, researchers had begun systematically measuring the photopigments of cattle eyes and testing their capacity for color discrimination. A 1989 study published in Applied Animal Behaviour Science examined fighting cattle using discrimination conditioning methods. Bulls were trained to respond to colored stimuli matched against grey samples of equivalent brightness and then tested across the full visible spectrum. The animals could distinguish medium- and long-wavelength colors, what humans perceive as greens through yellows, but they failed consistently to differentiate red from comparable neutral stimuli. The long-wavelength end of the spectrum, where red sits, was essentially invisible to them as a distinct color category.

The physiological explanation was confirmed in 1998, when Gerald Jacobs and colleagues at the University of California, Santa Barbara published measurements of photopigments in cows, goats, and sheep. Using electroretinogram flicker photometry, they identified exactly two classes of cone photoreceptors in cattle eyes, with peak sensitivities near 445 nanometres and 555 nanometres. The photopigment required to see red, a third, long-wavelength cone, was absent. Cattle are dichromats, operating with two color channels where most humans have three.

Which is to say, bulls do not see red any differently than they see green, yellow, or grey.

What drives the charge is the movement. Matadors exploit this directly: the muleta is swung and extended, drawing the bull's attention through motion rather than color. Skilled fighters control the animal by manipulating the speed and direction of the cape's arc. When researchers have tested this, presenting bulls with stationary red cloths while separately presenting moving cloths of other colors, the animals consistently charge the moving object regardless of color. A flapping blue cloth draws an attack just as readily as a red one.

The myth's persistence has something to do with the legibility of the theater. A crowd watching a bullfight sees the brilliant red cape and the furious charge in immediate sequence. The inference that one causes the other is natural, and, like so many natural inferences, incorrect. What the crowd is actually watching is an animal's response to rapid movement combined with accumulated physiological stress: the bull has already been weakened and agitated by the earlier stages of the fight before it faces the muleta.

The red muleta was introduced into bullfighting in the eighteenth century, supposedly by the Ronda matador Francisco Romero, though accounts vary. For nearly three hundred years afterward, the color was credited with powers of agitation. It made for good theater and a satisfying explanation. The explanation happened to be built on a property the animal entirely lacked. Bulls do not see red. They see movement.