Can animals use iridescent colours to communicate?

A new paper from the University of Melbourne reveals how animals use beautiful but unreliable iridescent colours as communication signals. Special adaptations enable animals to control how these shifting colours appear so that they can convey reliable information.

The new work now published in Trends in Evolution and Ecology draws together studies from across the animal kingdom to discover how animals control the appearance of iridescent colors in nature. 

"Iridescence is tricky to study because the hue that you see depends on the position of the viewer and the direction of light," said senior author, Dr Amanda Franklin from the School of BioSciences. 

"That means that iridescent colors change constantly, so it's hard to see how they can convey reliable information. The number one rule for communication is that the information must be reliable - it's the same for both animals and humans!"

But paradoxically, iridescent colours, like dazzling butterfly wings or dramatic peacock feathers, are widespread in the natural world. 

Co-author and PhD student Leslie Ng explains: "By studying how animals detect and process iridescence, we can get a better idea of when iridescence is actually a useful communication signal. 

"Reliable iridescent signals usually come with behavioural or physical adaptations that help animals control the visual effect. For example, male Anna's hummingbirds precisely control their courtship flights so that their iridescent throats appear a constant bright pink colour to watchful females."

Dr Franklin said organisms can do beautiful things with light. 

"Through evolution, they have adapted microstructures to produce specific effects. Some use microstructures to control the precise angle at which the hue of iridescent colours appears to shift. In this way, they control the information they communicate with color." 

Ms Ng said many studies suggest iridescent colours are important for courtship or camouflage but rarely consider how these flashy signals are actually seen by animals. "Because of this, we know very little about how iridescence is processed in the animal's brain." 

The detection of iridescent signals also depends on how organisms display their colour patches, and the physical position of both the signaler and viewer. For example, an iridescent colour can be processed differently if it is flashed quickly, or if the colours are fast-moving. 

Lead author, Professor Devi Stuart-Fox, said the insights shed new light on the colourful world of animal communication and highlight the challenges of studying accurately how iridescent colours work in nature. 

"Nature provides a testing ground for the detection and processing of dynamic and colourful signals," she said. "Understanding how animals reliably use and produce these shifting signals can help the development of bio-inspired iridescent materials designed for human observers."