Glimmering dragonflies, butterflies, hummingbirds, all dressed up in the finery of iridescence give us some of the most beautiful sights of cottage country. A new study offers insight into the reasons behind their showy looks.
In the natural world, colouration can serve several functions. For some animals, such as red-winged blackbirds and cardinals, vibrant colours signal to potential mates their health and good genes. Other species of brightly coloured animals (such as monarch butterflies) use their markings as a warning, avoiding predation by broadcasting that they’re a toxic choice for a meal, or that messing with them can result in a sting, as with bees. Still others ride on the colourful coattails of these animals’ reputation by mimicking their markings, and scaring off would-be predators. And lastly, for some animals, such as deer fawns and bitterns, colours and patterns help animals hide in their natural habitat.
Iridescence is a special kind of colouration, created by nano-structures in insect wings and bird feathers, rather than pigments. “When most people think of iridescence in animals we tend to think of peacocks’ tails and hummingbirds‘ throat-patches: colours to get you noticed by potential mates, not to hide you from predators. Up until now, any research related to the function of iridescence have mainly been focused on iridescence as a signal in mate choice, or as a form of aposematism, which is warning coloration,” says Karin Kjernsmo, the lead researcher on a new study published in January in Current Biology.
The new research, out of the University of Bristol, builds on an idea written more than a century ago, in 1909, by American naturalist Abbott Thayer (known as the ‘father of modern camouflage theory’), that “Brilliantly changeable or metallic colors are among the strongest factors in animals’ concealment.”
“Thayer’s idea sounds completely counterintuitive because how can colours that are both brilliant and changeable aid an animals’ concealment?” says Kjernsmo. “Interestingly to us, this longstanding idea about the biological function of iridescence seemed to have been forgotten or neglected over the years, and our study is the first to show that these ideas have traction.”
The research team tested the counterintuitive theory by hiding pretend insect exoskeletons with different colouration in a forest setting and observing which “beetles” were discovered by birds. Since they couldn’t be sure that the birds wouldn’t avoid some colours of insects (thinking them dangerous perhaps), the second part of the study sent humans out to look for the decoys.
Their results were surprising: “Both birds and humans really do have difficulty spotting iridescent objects in a natural, complex, forest environment,” Kjernsmo says. And that that’s especially the case in environments with glossy backgrounds. “I am sure that many will initially be surprised to hear that iridescence can act to conceal—we were positively surprised by this finding too,” she says. This newly understood benefit of iridescence may help to explain why it is found so frequently in nature.
And beetles aren’t the only ones using bling as camo. “We don’t for a minute imagine that the effect is something unique to jewel beetles. Indeed, we’d be a bit disappointed if it was. The reason we chose this species is that both sexes are iridescent so that makes sexual signalling somewhat less likely as a function of the colour. Also, these beetles really are very vividly coloured (the wing cases are used in jewellery) so, if we found these beetles could be concealed by their colours, it increases the chances that many iridescent species could be using their colours this way.”
So when we are out in the woods and we see an animal exhibiting a dramatic colouration, how can we try to guess what they get out of their big, shiny display? Is it giving them camouflage from predators, or attracting mates? Or is it warning off predators with bright colours? Kjernsmo offers some clues.
In iridescent animals where males and females look very different from each other (called sexual dimorphism), as in many species of birds of paradise or the peacock where it’s only the males that have these elaborate iridescent ornaments and females look dull in comparison, she says “it is clear that iridescence has evolved via sexual selection and serves as an important signal in mate choice.”
And how can you tell when it’s not a factor in mate selection? Kjernsmo explains, “the most convincing cases that iridescence is used to protect prey from predators, rather than mate attraction, is when we find iridescence in non-reproductive life stages of animals such as beetle grubs, caterpillars and butterfly chrysalises, as well as in cases where both males and females of the same species look the same, as in cases where they both are equally iridescent.”
Can it ever serve more than one function? Kjernsmo thinks it can, hiding the insect from an distance, and then looking“very conspicuous up close against a dull background, in which case it could work as a warning signal, even as an honest warning signal if the colour is paired with a secondary defence such as toxins or sharp spines.”
Whether black or brightly coloured, glossy or matte, the colours of animals provide clues as to where they live and the challenges they face. “I hope that the next time someone spots an iridescent animal, either in their own garden or when our for a walk in the woods,” Kjernsmo says, “they would be curious about why animals look the way they do and simply allow themselves to be marvelled by the beauty of our natural world.”
Watch the video below to see the study come to life: