Wednesday, March 3

Sparkly exoskeletons may help camouflage beetles from predators

Iridescence sparkles across many branches of the
tree of life, from dazzling ruby-throated hummingbirds to bright, metallic
beetles. While ostentatious coloration can woo mates, scientists had assumed it
also attracted predators. But new evidence suggests an unexpected benefit of
iridescence — camouflage.

Asian jewel beetles (Sternocera aequisignata)
boast brilliantly iridescent exoskeletons, and the fact that both males and
females share this trait suggests its importance outside of mating. To see if
iridescence affected whether beetles were detected by hungry birds, behavior
ecologist Karin Kjernsmo at the University of Bristol in England and colleagues
pinned mealworm-stuffed iridescent beetle wing cases to forest leaves along
with non-iridescent ones artificially colored blue, green, purple, rainbow or black.
All 886 targets — iridescent and matte — represented the spectrum of colors in
the iridescent shell, allowing researchers to disentangle the effects of
individual colors from the ever-changing sparkle of iridescence.  

beetle wing cases
Wing cases of the Asian jewel beetle tilted different ways show the diversity of colors iridescence can produce. Most animal colors are produced by pigments, but iridescence is structural. Microscopic layers interfere with how a surface reflects light, and can generate different colors depending on the angle of view.K. Kjernsmo

After two days, the iridescent
“beetles” were less likely to have been attacked by birds than all the other colors, except black, researchers report
January 23 in Current Biology. Birds “killed” 85 percent of purple and
blue targets, but less than 60 percent of iridescent targets, Kjernsmo says.
“It may not sound like much, but just imagine what a difference this would make
over evolutionary time.”

It’s unclear if birds had trouble seeing iridescence
or were avoiding it, for example if they associated it with poisonous prey. But
Kjernsmo suggests the rapidly changing colors could disrupt normal
image-forming processes.

Humans proved worse than birds in detecting
iridescent beetles. In a second experiment, 36 people walked a forest path
while trying to spot both iridescent and dull beetle cases affixed to leaves in
plain sight. Humans on average identified nearly 80 percent of matte blue and
purple cases, but only 17 percent of iridescent cases — suggesting to the
researchers that iridescence can function as camouflage.