Last updated May 30, 2018 at 1:12 pm
Shimmering colour conceals more than it reveals.
Bumblebees find it difficult to identify shapes in iridescent colours, researchers have established, confirming a theory first raised more than a century ago and potentially shedding new light on predator-prey camouflage strategies.
A team of scientists led by Karin Kjernsmo of the University of Bristol in the UK trained a number of bumblebees (Bombus terrestris) to recognise artificial flowers, rewarding them with sugar water each time they landed on ones with particular shapes.
They then substituted the targets with flowers of the same shape but cut from iridescent material, and found that the insects became confused and were less able to identify the ones containing rewards.
In a paper published in the journal Scientific Reports, Kjernsmo and colleagues suggest that the results might explain why iridescence is common across the natural world. Examples include not just flowers, but also the carapaces of certain beetle species, and the plumage of some birds.
Iridescence, they posit, is an effective camouflage strategy, reducing the chances of predation.
Although bumblebees are not themselves predators, their visual systems have been well studied and are used frequently as models for those of other insects, especially predatory wasps and hornets.
To human eyes, an iridescent beetle stands out from its surroundings and is instantly visible. However, to wasps and hornets, for which it represents prey, the iridescence may serve to disrupt its shape and outline, making it much harder to detect.
“Thus, if you are a visual predator searching for the specific shape of a beetle (or other prey animal), iridescence makes it difficult for predators to identify them as something edible,” explains Kjernsmo.
“We are currently studying this effect using other visual predators, such as birds as well. This because birds are likely to be the most important predators of iridescent insects.”
Iridescence arises because regular repeating nanostructures on a surface reflect light at a multitude of different angles, producing the effect of ever-changing colours.
Although the current study provides the first concrete evidence that the effect confuses potential predators, the suggestion that it did so was first made in 1909 by American artist and naturalist Abbot Thayer.
In a book, Thayer concluded that “brilliantly changeable or metallic colours are among the strongest factors in animals’ concealment”.
“Even without motion, the animal’s surface, which would show all in its true place and plane if it were plainly coloured, is by its iridescence made to appear ‘dissolved’ into many depths and distances,” he wrote.
Thayer became known as the “father of camouflage” and his work had a powerful influence on American military planners in World War I.
His theories led to a radical new camouflage system for naval ships. Dubbed “razzle dazzle”, it involved vessels being painted with thick bands of intersecting and fragmented lines – a little like a zebra – based on the assumption that the form would disrupt the visual perception of enemy look-outs.
Kjernsmo and colleagues happily acknowledge Thayer’s work, and credit him with formulating the question that lies at the base of their own investigation: “How can something that is both brilliant and changeable contribute to concealment?”
The study validates Thayer’s contention that blending into the background is not the only way to hide from predators. Sometimes, being big and bold is equally effective.
After World War I Thayer’s ideas gradually faded from focus. In their place, biologists favoured other theories – notably, mate attraction – to explain iridescence.
All that now seems set to change once more.
“This study has wider implications for how we understand animal vision and camouflage – now when we see these shiny beetles we can know that their amazing colours have many more functions than previously thought,” says Kjernsmo.