Tiny guppy fish change their eye colour to warn off rivals, new research shows.

When facing an opponent, Trinidadian guppies rapidly turn their irises from silver to black before attacking their adversary.

This makes their eyes more conspicuous and is an “honest” signal of aggression – larger guppies do it to smaller ones they can beat in a fight, but smaller ones do not return the gesture.

As part of the study, the researchers made visually realistic robotic guppies to see what would happen if smaller fish displayed their aggressive motivation – and larger fish flocked in to compete with the small imposters for food.

Lead author Dr Robert Heathcote, from the University of Exeter, said: “Trinidadian guppies can change their iris colour within a few seconds, and our research shows they do this to honestly communicate their aggressive motivation to other guppies.

“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.”

Professor Darren Croft, also from the University of Exeter, said many species go to great lengths to conceal and camouflage their eyes to avoid unwanted attention from predators or rivals.

“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 study, Dynamic Eye Colour As An Honest Signal Of Aggression, is published in the journal Current Biology.

-AAP