For the first time, biologists have shown that the brilliant iridescence and gloss found in jewel beetles protects them by working as a deceptive warning colouration after being detected by avian predators, according to a recent study in the journal Animal Behaviour.
They found that iridescence may confer a survival benefit by inducing hesitation or even an aversion response in attacking birds, and that the changing colours, a key feature of iridescence, is the characteristic important for this effect.
Iridescence is a form of structural colouration – meaning that the colours come from micro or nanostructures in the material, as opposed to pigments. Another famous form of structural colouration is the vibrant blue of the brilliant South American blue morpho butterflies (Morpho peleides).
In iridescence, the hue and intensity of colours varies depending on the angle at which it is viewed. This striking feature has evolved independently in a wide range of organisms, from birds like the magpie and starling, to many insects including rose chafers, rosemary beetles and in the demoiselle.
A team of researchers from Bristol University’s CamoLab in the UK have investigated why this metallic colouration has evolved so many times in the animal kingdom and what makes it such a successful anti-predator strategy.
Previously, they’d discovered that that iridescence can act as a highly efficient form of camouflage in the jewel beetle (Sternocera aequisignata), but whether it could also protect prey after being detected was unknown.
“One of the challenges when studying the functions of such highly reflective structural colouration has been to separate the effects of the changeability of colours, the hallmark of iridescence, from the effects of simply having multiple colours at the same time, and also to separate the effects of gloss from the effects of iridescence,” says lead author Dr Karin Kjernsmo, from the University of Bristol’s School of Biological Sciences.
Beetle beauty reducing the probability of attack
The team studied this by presenting real and artificial jewel beetle wing cases (the hardened forewings, called elytron, which cover the functional wings) to 32 chicks of the domestic fowl, Gallus gallus domesticus, that had never seen this kind of prey before.
They tested the birds’ willingness to attack glossy and iridescent prey by presenting them with both iridescent and non-iridescent wing cases (with the same overall range of colours) as well as glossy and matte versions of the two.
These wing cases had mealworm placed underneath them, so the birds had to peck and flip over the artificial prey item to eat the food reward. Interestingly, they found that iridescence reduced the probability that chicks would attack.
The birds initially hesitated to attack the iridescent prey, but not non-iridescent prey, suggesting that having multiple colours displayed at the same time is not enough to provoke aversion, but that the colour changeability of iridescence is the feature important for this protective effect.
This study is also the first to isolate the effects of gloss from iridescence, as a benefit of glossiness over matte appeared in the third trial.
“Here we have, for the first time, effectively managed to test for each of these two effects on their own, and shown that both iridescence and gloss can protect prey even post-detection, providing yet another adaptive explanation for the evolution and widespread existence of iridescence,” adds Kjernsmo.
Beetles are insects that form the order Coleoptera, and they’re distinguished from most other insects by their front pair of wings. Hardened into wing-cases called elytra, these wings protect their second functional pair of flying wings from damage.
Beetles are an incredibly large and diverse group of organisms, with 169 families and 340,000 species known to science. The SCINEMA International Science Film Festival entry, the Caretakers, takes us on a journey into the weird, wonderful lives of a variety of beetle species and asks: what does the future hold for these incredibly important animals – and for us if they disappear?
You can watch the film here.