Scientists first described the black eyes in the 1980s, and suggested that they were signals of dominance and aggression, like a gorilla beating its chest or a dog baring its teeth. Heathcote’s team certainly saw that black-eyed guppies were far more likely to perform aggressive behaviors than to receive them. But in those conflicts, eye color could be incidental. To truly see if black eyes are a signal, the team needed to do an experiment. Somehow, they had to manipulate the color of a fish’s eyes to see how other guppies would react.

“We thought about sticking contact lenses on them for probably more time than we’d like to admit,” says Heathcote. When that failed, they tried making animated guppies on an iPad. But tablets are tuned to human eyes, and since guppy eyes see the world very differently, the fish completely ignored their virtual avatars. Heathcote was lamenting this second failure in a pub, when his drinking companion and colleague Jolyon Troscianko, who studies animal camouflage, suggested making a robot.

The team pressed a dead fish into some resin, to make a silicone mold that could churn out model guppies. They then photographed live fish with black and silver eyes, recalibrated the images for guppy vision, and printed them onto sheets of clingfilm. They stretched the colored sheets over the silicone models, which they attached to a fishing line on a motor. And voila: a robo-guppy of bespoke eyes and size, which could be made to angrily thrash over a bit of food.

The team found that guppies were more likely to go for food that was guarded by a silver-eyed robot than an otherwise identical black-eyed one—but only if the robots were bigger than them. If the robots were smaller, the guppies were more likely to try and loot the black-eyed one’s hoard.

This confirms not only that the dark irises are signals of aggression—but that they’re honest signals. They say that a guppy is prepared to fight, but they also tell rivals that there’s something to fight for. If the signaler doesn’t have the buff to back up its bluff, it will pay the price. Or, in other words, “wimpy fish that act strong get beaten up,” says Elizabeth Tibbetts from the University of Michigan, who studies the evolution of social signals.

Many other fish can change the color of their eyes, including distant relatives of guppies like salmon and tilapia. In those cases, it’s the subordinates who have black irises, and the dominant individuals who go silver. Heathcote thinks that these changes might be more common and more important than previously thought. After all, “many animals, predators and prey, are so attuned to looking for eyes,” he says. So if a species wants to send a clear message, why not do it through what is already one of the most conspicuous billboards on their body? Some researchers have even suggested that the whites of human eyes are social signals too, allowing us to more easily work out what our peers are looking at.