When guppies get angry, they don’t see red. Instead, their eyes literally flash black.

The small freshwater fish, native to the rivers of Trinidad, live in intense competition for food. It has been previously noticed that the little fish can change their eye color from silver to black and back again, but due to the difficulties of manipulating this no one could prove why.

Now a new study, published in Current Biology, has shown that the fish turn their eyes black when they are acting aggressively and want to defend their food from rivals. What is more, their default eye color is actually black, meaning that they are basically always suppressing rage.

“Eyes are one of the most easily recognised structures in the natural world and many species go to great lengths to conceal and camouflage their eyes to avoid unwanted attention from predators or rivals,” explained Professor Darren Croft, who co-authored the paper. “However, some species have noticeable or prominent eyes and, for the most part, it has remained a mystery as to why this would be.”

Investigating this is surprisingly difficult, as it requires manipulating the eyes of the creatures you’re studying. This led the researchers of this latest study down the route of creating lifelike fish robots in order to test real-life guppies.

They found that a large robot fish with black eyes would successfully deter other fish from coming near the food, but that a large fish with silver eyes or even a small fish with black eyes had no such luck. This suggests that the color of the eyes is what is known as an honest signal, in that a small fish cannot act aggressively and keep the food as other fish know that they can overpower it.

The differences in eye color are not only limited to the guppies, though. Other species of fish, such as salmon and tilapia, can also change the color of their eyes. Again, this is related to aggression but with subordinate fish displaying the darker pigment and the dominant flashing silver.

This has caused the researchers to wonder whether or not many other species could be changing their eye color as a warning signal, too. This would make sense, as both predators and prey tend to look for the eyes of their adversary, making them the perfect instruments for sending a message to their opposite number.

Yet despite many animals having brightly colored eyes, not much research has been done to find out why this might be the case. The team hope that their new work might help change this.