The Scientist - USA (2022 - Spring)

AMANDA FRANKLIN

SPRING 2022 | THE SCIENTIST 25

cles that trap and redirect some wavelengths.

These can take the form of recurring struc-

tures that diffract light, or so-called photonic

crystals, for example. Such features can pro-

duce extraordinarily vivid colors—such as

the famous blue of the morpho butterflies—

or, as in the case of these beetles, give objects

glossy surfaces.

The purpose of these visual effects is

less clear. While a substantial volume of

research suggests that iridescence can

help camouflage an animal—generally by

breaking up its outline, making it harder

to pinpoint—glossiness’s function has

largely remained a mystery.

There are a few different hypotheses

as to why these animals have a mirror-

like sheen, Franklin notes. One is that it

serves as a form of aposematism—the use

of signals, typically visual, such as warn-

ing coloration, to keep predators away. It

might also be a simple byproduct of cuticle

structure: a stronger or lighter carapace,

or one that can cool the animal in hot con-

ditions or repel water, might also happen

to be very shiny. But the leading—though

largely untested—hypothesis, Franklin

says, is that glossiness, like iridescence, can

act as camouflage from predators such as

birds that rely on sight to find food.

To test the idea, Franklin and colleagues

set up a study with two main experiments.

In the first experiment, researchers walked

eight human volunteers equipped with eye-

tracking headgear to four different areas of

forest, and gave them five minutes to find

12 replica beetles in a 2-cubic-meter area.

Half of the beetles were glossy, like the

real thing, and the rest were matte; the

researchers tracked how many of each the

volunteers found.

In the second experiment, a similar set

of replica beetles was placed in field and

forest habitats, with the researchers check-

ing daily for evidence of predation and tak-

ing note of replicas that had been damaged

by birds. Missing replicas were attributed

to bird attacks as well, as birds would be

the only animals in the closed-off areas that

were large enough to fully remove the fake

bugs (Funct Ecol, 36:239–48, 2021).

The results were not exactly as expected,

Franklin explains. “Especially with the

humans, we expected a result one way or

the other: either that [the glossy beetles]

would blend in or stand out more.” But

shiny or matte, it made no difference to

the humans or the birds. Apparently, being

glossy was no help in finding the fake bee-

tles, but no hindrance either.

Karin Kjernsmo, a research associ-

ate at the University of Bristol who wasn’t

involved in the work, isn’t so sure. She’s

done work on iridescence as a camou-

flage mechanism, and is currently study-

ing glossiness’s role as well. She notes that

distance might play a role in how well

glossiness works as camouflage: in some

of her experiments, she found that a glossy

surface made iridescent beetles harder to

spot by humans, and that people had to be

closer to spot them than they did for matte

beetles. The human volunteers in Frank-

lin’s experiment were searching an area six

feet across in all directions, so it’s possible,

Kjernsmo says, that they were too close for

the mirror-like effect to act as camouflage.

She also notes that the Melbourne

team’s findings regarding bird predation

could be interpreted differently. Pre-

dation rates in this experiment ranged

from 3.4 percent to 17.6 percent across

the study sites—in other words, the fake

insects were left alone, or “survived,” 82.4

percent to 96.6 percent of the time. “That

is incredibly high survival in these types

of experiments,” Kjernsmo says. So per-

haps the takeaway for the matte versus

glossy hypothesis is “that both strategies

seem to be good.”

Coleopterist Ainsley Seago, the cura-

tor of invertebrate zoology at the Carnegie

Museum of Natural History who was not

involved in the work, says that the research

is a significant step forward. “Almost nobody

has studied [specularity], so this paper is

incredibly valuable,” she says.

Both Kjernsmo and Seago praise

the authors for publishing their findings

despite not finding the result they expected.

“The fact that they were straight up about

what the hypothesis was and that the data

don’t support it, that alone is commend-

able,” Seago says. There’s a strong bias

Glossiness’s function has

largely remained a mystery.

BUG DOUBLE: The team used replica beetles

with shiny and matte surfaces to study the

effects of glossiness on predation—something

they could measure via peck marks from

attacking birds (right).