Does the Color Red Trigger Bulls To Attack?

The idiom “red rag to a bull” means to aggravate someone, make them angry, or incite violence. The phrase “seeing red” implies blind rage. Both phrases may be traced to the Spanish tradition of bullfighting, in which the bullfighter (matador) performs a kind of dance with the bull before killing it with the use of a red flag (muleta) and a sword. It’s a logical enough connection to make: The matador waves the muleta, the bull charges horns-first.

However, many on the internet have raised questions about the apparent correlation: “Is it the red color of the muleta that causes the bull to charge? Or is it simply the movement of the muleta and the present threat of the bullfighter in the arena? And if so, why is the muleta red?”

ELI5:Why do bulls go crazy when they see red?
byu/Subbeh inexplainlikeimfive

We looked into the claim, and learned that bulls are not triggered by the color red.

First, bulls are partially colorblind, though not fully color blind. Like all cattle (and most mammals), they are dichromatic, meaning that they can only see two main color types. Unlike humans, they do not possess red retina receptors and can see only in shades of “yellowish-green” and “bluish-purple” light, according to the book “Improving Animal Welfare: A Practical Approach” by Temple Grandin.

Matadors only use the red muleta in the third and final stage of the traditional bull fight. They use a magenta and yellow cape (capote) during the first stages of the fight.

So, if bulls can’t see red the way humans do, why do they charge at the red muleta?

In 2007, the Discovery Channel series “MythBusters” aired an episode titled “Red Rag to a Bull” in which they ran bulls through five tests to determine what causes bulls to charge.

1. They first put a static flag – one white, one blue, one red – in the arena, one at a time. When they released the bull, it charged at all three separate flags, regardless of the color.
2. They then put all three static flags in the arena at once, and again, the bull charged each of the flags, regardless of the color.
3. Next, using a remote controlled zip-line system, they compared how the bull reacted to a moving flag. While a red flag hung static in the arena, the MythBusters moved the blue flag across the zip-line. The bull chased after the blue flag, completely ignoring the static red flag.
4. They then tested each flag held by a human form. The MythBusters placed three foam dummies wearing red, white, or blue with mechanized arms waving a corresponding red, white, or blue flag in the arena. The bull first charged the white dummy, then the blue dummy, and finally, the red dummy. 
5. Lastly, one MythBuster dressed all in red stood motionless in the arena while two professional bullfighters ran around the arena. When they released the bull, it exclusively charged the professional bullfighters, ignoring the motionless person dressed in red. 

The experiment’s finding confirmed that movement is what primarily triggers bulls, not color. 

Another factor is that bulls have a “flight or fight” response when the matador invades their personal space. According to the International Longhorn Association, bulls will attempt to remove themselves (flight) – or remove the threat (fight) – until the invasion is no longer considered a threat. In the context of understanding how to control cattle, the International Longhorn Association says that:

So, if movement and the matador’s encroachment on its personal space is what triggers bulls, as opposed to the color red, why are muletas traditionally red?

Many claim that muletas are red in order to mask the blood spilled when the matador kills the bull by driving a sword between its shoulder blades, with the intent of piercing the heart or aorta.

TIL that the red cape used during a bullfight (muleta) is red in order to hide the bull’s blood, not to infuriate it. Bulls are colorblind.
byu/haren_ intodayilearned

However, we did not find definitive evidence that muletas are red to mask the bull’s blood. The muleta is traditionally red; the use of both the capote and the muleta are culturally significant to the practice of bullfighting going back centuries. According to Madrid Bullfighting:

We should note that in the first two-thirds of the bullfighting performance, when the matador uses the magenta-and-yellow capote, he, along with the picador on horseback and three banderilleros, do draw the bull’s blood prior to the final third. The picador stabs the bull first with the intention of weakening its neck and shoulder muscles. The banderilleros stab the bull in its shoulders next, using two banderillas, or sharp barbed sticks. Both of these stabbings occur prior to the final third, when the matador (literally meaning “killer”) uses the red muleta and delivers the final sword thrust, which undoubtedly draws more blood than that drawn by the picador and banderilleros. 

We reached out to multiple historians and scholars of Spanish history and bullfighting inquiring why muletas are red, and will update this story if we receive a response. 

In sum, the color red, specifically, does not trigger bulls. Rather, movement and the threat of the matador’s presence sends bulls into a fight-or-flight response, causing them to attack.

Sources