- Jewel beetles (Sternocera aequisignata) have iridescent wing cases that change color depending on which angle the light hits them, like a peacock’s feather or an opal.
- A new study published in Current Biology finds that this iridescence may help the beetles hide from predators in the wild like birds.
- The study authors ran experiments that showed birds were least likely to spot the iridescent wing cases compared to static mono-colored wing cases, and even humans had the most difficulty locating them.
- This finding is important because it provides evidence for the first time that iridescence, rather than being a disadvantage, can aid a species’ survival by helping conceal it.
A new study in Current Biology shines a light on how iridescence in jewel beetles (Sternocera aequisignata) helps them to hide from predators. Iridescence produces the mesmerizing hues of a male peacock’s feathers, the varied brilliance in opal, and the soupy rainbow that sometimes graces soap bubbles. Asian jewel beetles sport a metallic iridescent wing case that changes color depending on the angle of light that strikes it.
“I have always been fascinated by animal coloration, ever since I was a young girl investigating the beautiful butterflies and beetles that inhabit our gardens,” said Karin Kjernsmo, an evolutionary and behavioral ecologist at the University of Bristol, U.K., and first author of the paper. “In particular, I am mostly interested in how animals or prey can use their colors and patterns to avoid getting eaten by their enemies or predator.”
Bright shiny things attract attention: this is true even in the natural world. Which begs the question: whose attention? For the male peacock, its stunning plumage attracts peahens, but it also makes them conspicuous targets for predators like jungle cats. Given that bright colors make species stick out, it is baffling to scientists that iridescence is seen in everything from beetles to caterpillars to birds. The new study shows that this may not be such a bad survival strategy — as a way of befuddling predators.
“Our findings provide the first evidence to support Thayer’s more than century-old idea that biological iridescence can provide a survival benefit for prey through concealment,” Kjernsmo said. “For sexually selected traits, the costs of iridescence may be lower than generally assumed.” U.S. naturalist Abbott Thayer, considered the “father of camouflage,” propounded seminal but controversial ideas about the use of coloration as camouflage. He argued that all coloration was in the service of crypsis, a species’ ability to blend into its surroundings — a view that was widely discredited because it ignored the role that coloration plays in sexual selection.
Usually, what determines the color of an object or organism is the presence of pigments that absorb a slice of the visible light spectrum, reflecting the rest. For example, plant leaves contain the pigment chlorophyll, which absorbs all but green light. Optical nanostructures arranged in recurring patterns, on the other hand, impart color by dispersing light, not absorbing it, so the angle at which light hits them determines which colors are projected. The 17th-century British naturalist Robert Hooke proved this by performing a simple experiment where he dunked peacock feathers in water and noted that the “fantastical” coloration disappeared. The coloration was not innate, but dependent on how light fell upon it.
Kjernsmo and her team carried out a more elaborate experiment. It involved placing beetle wing cases of different hues — ranging from green, black, purple, blue, a static rainbow, to “jewel beetle” iridescent — amid natural foliage. The cases were fortified with mealworm beetle (Tenebrio molitor) larvae, a common prey for birds. The researchers wanted to see how birds would reacted to the different hues of the wing cases.
Their observations showed that iridescent wing cases were less likely to be spotted and thus attacked by birds. Next, the researchers put humans to the task. They also found it difficult to locate the iridescent wing cases, even though they were actively looking for them. This was considered significant because in the first experiment, the birds could have been choosing to ignore the iridescent wing cases even if they spotted them. This second experiment appeared to confirm that the birds really didn’t spot the wing cases.
“This study seems very thoughtfully designed and I feel like it’s a very useful and important step forward in determining the evolutionary origins of these ‘living jewels,'” said Ainsley Seago, an expert on the Coleoptera order of beetles to which jewel beetles belong. Seago, who works for the New South Wales state government in Australia and was not involved in this study, said that apart from showing that iridescence could help conceal the beetles, the research also demonstrated that the colors blue and purple are relatively easily spotted by birds. This could hold a clue, she said, to unlocking one of the enduring mysteries of biology: why the color blue is so rare in nature.
The researchers haven’t quite figured out why iridescence threw the birds off. In earlier research, Kjernsmo and her colleagues found that bumblebees were confused by the shifting array of colors. For birds, who search for food by sight, the changing colors may make it challenging to identify the shapes of their prey. Seago, however, cautioned that the study design could be a factor in the birds’ poor showing. After all, jewel beetles are from southern Asia and not native to the U.K., where the experiment was performed. The birds, meanwhile, were in their natural habitat, where they were unlikely to have ever encountered these particular beetles before. The authors say the fact that the birds “readily attacked” the purple and blue cases, despite also not having encountered anything like them in their habitats, would suggest that they didn’t spot the iridescent ones.
What remains a mystery is what exactly the birds were seeing. Seago suggested it would be useful to find out which kinds of birds attacked the wing cases. “This seems important as not every bird species perceives the same colors, and bird color vision is quite different from that of humans,” she said.
The study authors believe that iridescence might play a similar role in other species as well. Kjernsmo said they were working on a more technology-heavy experiment to test their ideas about camouflage. They plan to use AI to determine, based on the natural environments of different species, what its optimal camouflage should be, and then compare that to its actual coloration.
Citation:
Kjernsmo, K., Whitney, H. M., Scott-Samuel, N. E., Hall, J. R., Knowles, H., Talas, L., & Cuthill, I. C. (2020). Iridescence as camouflage. Current Biology, 30, 1-5. doi:10.1016/j.cub.2019.12.013
Kjernsmo, K., Hall, J. R., Doyle, C., Khuzayim, N., Cuthill, I. C., Scott-Samuel, N. E., & Whitney, H. M. (2018). Iridescence impairs object recognition in bumblebees. Scientific Reports, 8(8095). doi:10.1038/s41598-018-26571-6
(Banner Image: A jewel beetle (Sternocera aequsignata). Image courtesy: Bristol Museums, Galleries & Archives)
Malavika Vyawahare is a staff writer for Mongabay. Find her on Twitter: @MalavikaVy.
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