Normally, its eyes are silver. But when the tiny Trinidadian guppy (Poecilia reticulata) gets all worked up, the eyes turn pitch black as a warning for other guppies to rack off, scientists have discovered. And they had a fun time finding this out.

Freshwater guppies - small, defenseless and popular as aquarium pets - are towards the bottom of the food chain, and when they experience a high level of predation, they don't display a lot of aggression.

However, in safer environments, such as above waterfalls, they tend to get aggressive with each other, fighting over resources such as food - and, as previous research has observed, black eyes in some guppy species correlates with aggressive behaviour and dominance.

They can change their eye colour in just a few seconds. But whether the guppies were using this deliberately was unknown - so animal behaviour experts designed an experiment to find out.

"Experimentally showing that animals use their eye colouration to communicate with each other can be very difficult, so we made realistic-looking robotic fish with differing eye colours and observed the reaction of real fish," explained Robert Heathcote of the University of Exeter.

They created a mould of a dead guppy, and created silicone replicas. Then they gave the replicas either black or silver eyes, based on photographs of living guppies, and put them on a fishing line attached to a motor.

These robotic guppies were then placed over food, and made to appear that they were moving about. The team found that the other guppies were more likely to move in on the food if the robot had silver eyes than if it had black eyes.

But if the black-eyed robot was smaller than the real guppies, it got disproportionately targeted by the real fish.

This is because the guppies, the researchers concluded, were using the black eyes as a signifier of what they call "honest" aggression.

Not only do black eyes indicate an absolute willingness to enter an altercation, they also indicate that the black-eyed guppy has something worth defending.

Hence the larger guppies targeted the small black-eyed robot - they knew that they would have the upper hand in an altercation, and that the smaller robot had something they would want.

In the real world, smaller guppies don't tend to display black eyes to larger ones - it's only used by guppies against those they know they can dominate.

The researchers think that the behaviour they observed by the large guppies is why this dynamic exists: act tough when you're not, and you will get a thrashin'.

Scientists still don't know how the guppies change the colour of their eyes, but the research is yet another demonstration of the importance of eyes in the animal kingdom.

"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," said Darren Croft from University of Exeter.

"However, some species have noticeable or prominent eyes and, for the most part, it has remained a mystery as to why this would be. This research gives a new insight into the reasons behind why some animals have such 'conspicuous' eyes."

The research has been published in the journal Current Biology.