How to be invisible
Moths, sharks, seahorses, stick insects, crab spiders and spider crabs all use different forms of disguise to hide from those who want to eat them—or to better ambush their prey. What can we learn from them?
Camouflage is deceptively simple. A fairy tern’s eggs are speckled cream and tan like the sand it rests on. A cabbage tree moth has dun-brown stripes that mimic the veins on a dried-out tī kōuka/cabbage tree leaf. Somehow, the moth knows just how to orient its wings so that the patterns form continuous lines with the striations on the leaf. A stick insect holds its front legs stretched out in Superman pose to make it look more stick-like, and it moves its body rhythmically—some would say creepily—to mimic a twig swaying in the wind.
But as scientists learn more about how animals see the world, camouflage turns out to be more complicated than it first appears.
Take the North Island lichen moth (Declana atronivea). It looks like a living Rorschach test, as though someone has splattered whorls of black ink onto its snow-white wings, and its caterpillars pretend to be bird poo on a stick. True to its name, the moth lives among the curly silver-black lichen commonly found on tree bark in New Zealand forests.
Cassie Mark, a University of Auckland doctoral candidate, is trying to figure out how the moth’s unique colouration evolved, and exactly how it conceals itself. Is it trying to match the colours and patterns of the lichen it rests on (background matching)? Are its Rorschach patterns designed to bleed into the background, obliterating the boundary of the moth so that predators can’t make out its shape (disruptive colouration)? Or is it actually pretending to be a tiny patch of lichen, allowing it to go unrecognised even on brown bark or leaf litter (masquerade)?
Mark realised that lichen moths are probably multitasking, using all three camouflage strategies at once. And, unlike the other 165,000 butterfly and moth species in the world, the lichen moth has an extra string to its bow: asymmetry.
Each lichen moth has a unique pattern of black splotches—so far, so normal. But one of its wings is not like the other: each individual’s left side is subtly different from its right, making it one of only two known insects with asymmetric wing patterns.
Some moths are more asymmetric than others. Mark carried out experiments where she made tiny life-sized lichen moth models out of paper and pastry, stuck them to trees, and measured how often birds bit them. The models with more asymmetry had fewer bite marks in the pastry, suggesting that the wonkiness improves the moth’s camouflage. But if asymmetry works so well, why don’t more insects try it?
The answer may have something to do with flight, says Mark. To fly well, you need both your wings to be the exact same shape, and the fact that so few insects have managed to evolve asymmetric wings suggests the genes for wing shape and pattern must be close together. How the lichen moth managed to pull it off remains a mystery Mark hopes to solve.
In the animal world, camouflage is widely used and wildly successful. It’s the reason for stripes and spots and skin that changes colour and some forms of bioluminescence and seaweed-shaped fin flaps and see-through sea creatures.
Death-by-predator is one of the strongest drivers of evolution, and many predators hunt by sight, says Jolyon Troscianko, a behavioural and sensory ecologist at the University of Exeter in the United Kingdom.
“Some animals do reach a ripe old age and die of natural causes, but that’s insanely rare. The main source of death in the wild is being eaten.”
All that death makes predation a powerful evolutionary lever, Troscianko says. Predators eat the prey they can find. The ones they don’t find survive to breed. And one of the best ways to hide from visual predators is camouflage.
This explains why so many New Zealand animals are dull greens and browns: before humans arrived, the top predators here were the giant raptors, Haast’s eagle and Eyles’ harrier. Looking boring, freezing when threatened, and being nocturnal are great strategies for staying alive when your main predators are sharp-sighted diurnal birds. (When they’re rats, cats, dogs and stoats—well, not so much.)
How does a creature start looking like a stick or a leaf or a piece of lichen in the first place? Camouflage itself is often used to explain Charles Darwin’s theory of evolution by natural selection. The classic example, featured in classes and textbooks for decades, involves another moth.
The peppered moth is found across the temperate Northern Hemisphere. Before the Industrial Revolution, the vast majority of these moths had pale wings speckled with black—patterns which helped to camouflage the moths on light-barked, lichen-covered trees. But a genetic mutation gave a few of the moths much darker wings.
When England’s coal-burning factories caused widespread pollution, blackening trees and buildings with soot and killing lichen, the pale moths were easily spotted by birds and eaten. The dark ones blended in to their new background and survived. By 1895, 98 per cent of England’s peppered moths were black. When pollution-control measures were introduced in the 20th century, the lighter peppered moths predominated again. (The case of the peppered moth was controversial for a while, but was vindicated by new research in 2012.)
But nature is always more complex than the textbook example. Sometimes, the best way of going unnoticed is to be the family weirdo.
It’s thought that predators who hunt by sight carry around in their heads a picture of their favourite prey—a search image. “Imagine you’re a bird hopping around in the woods, and you eat snails,” says Troscianko. “You find an orange snail, and now you have in your head a search image of this orange-coloured snail that was really tasty.” You find a couple more, and eat those, too. And as you hop around looking for orange snails, you overlook the white snail or the pink snail nearby.
But as most of the orange snails disappear, you start noticing stripy snails. You give up on orange snails and start looking for stripy ones instead—and your bird brain struggles to keep both patterns front of mind. Over time, the proportions of differently coloured snails fluctuate as birds update their search images.
For an individual snail, then, standing out from the crowd can actually be a good thing. “That explains a lot of the diversity that we see in nature,” says Troscianko.
To appreciate nature’s vast diversity, just look underwater. The oceans are home to a mind-blowing variety of camouflage strategies, which have given rise to some of the weirder forms of body organisation seen on this planet.
Many spider crabs—including Notomithrax ursus, New Zealand’s decorator crab—create intricate floral arrangements on their backs from pieces of seaweed. Some fish and seahorses masquerade as algae, their colour and ruffles perfectly concealing them among the weeds.
Other species are so cryptic—difficult to spot—that we’ve only just realised they exist, says Auckland Museum marine biologist Tom Trnski. He’s thinking of the candy-coloured pygmy pipehorse (Cylix tupareomanaia), which was hiding in plain sight among the kelp and coralline algae of the Northland coast until 2017, when Trnski and others described and named it.
Even more impressive is the tropical Pacific’s mimic octopus—itself discovered by science only in 1998—which can alter its colour, patterns, and form in order to resemble as many as 15 other species, including sea snakes, jellyfish, lionfish and flounder.
Camouflage is crucial in the open ocean, where there is nowhere to hide. For this reason, many baby sea creatures are completely transparent.
If you tip a crayfish larva into your hand with some water, says Trnski, “the only reason you’ll know it’s there is because it’s moving”. Other larvae pretend to be siphonophores—colonies of jellyfish-like stinging organisms—that predatory fish avoid.
It’s harder to make muscles, eyes, guts, and gonads transparent, so adults need different strategies.
Some fish, such as trevally, use mirror camouflage. Reflective plates on their silvery skin and scales bounce light in such a way the fish blend into the background from wherever you’re looking at them—except for underneath, which is why trevally are as thin as they can possibly be, while still having somewhere to put their internal organs.
Recent studies suggest fish are better mirrors than actual mirrors, camouflaging even more completely by manipulating the way polarised light is reflected through special structures under their skin.
This research was funded by the United States Navy, in the long tradition of the military attempting to copy nature’s camouflage. One of the more bizarre efforts was the British Navy’s “dazzle camouflage” during World War I. Thousands of battleships were painted in wonky black and white stripes, with the aim of breaking up the vessel’s outline and disguising where it was heading. Journalists at the time described the look as resembling “floating Cubist paintings”, “a Russian toyshop gone mad”, and “an intoxicated snake”.
In the deep, things get even stranger. As you descend, red light is lost first, which means that bright red skin—like that on New Zealand’s orange roughy—looks black at depth, and can pretty much turn you invisible if you live 100 metres or more below the surface.
Other deep-sea fish go for “ultra-black” instead of red. This is a layer of particles in the skin which absorbs almost all light and scatters the rest, meaning these animals are tough to see even for predators that hunt with their own bioluminescence. (Ultra-black is so useful that it seems to have evolved multiple times in many distantly related species.)
Speaking of bioluminescence, that’s another camouflage strategy. If any light filters down from above, a fish looking up at another fish sees its outline silhouetted against the sky. Scientists are discovering more and more species with light-producing organs called photophores arranged along their bellies, giving out just enough blue light to disguise the fish against the surface. It’s thought that at least 10 per cent of the world’s sharks, for example, have this ability. “It’s an amazing adaptation to living in the twilight zone,” says Trnski.
Closer to the surface, many pelagic animals, such as sharks and rays, use shades of colour rather than bioluminescence to avoid being seen from both above and below—dark back, pale belly. This countershading doesn’t totally prevent them from being seen from underneath, but is thought to help them blend in with the water when glimpsed from the side.
In the past few years, we’ve learned a lot about animal vision. Advances in spectrometry have enabled us to see—more or less, anyway—what predators see. That’s revealed some of the ways our own vision has blinded us to what’s really going on in nature.
Some of New Zealand’s best camouflagers are our 20 or so species of stick insect. Morgane Merien, a University of Auckland doctoral student, is researching how these masters of disguise conceal themselves. In other words, does a stick insect actually look like a stick?
You may be thinking, well, duh—but Merien isn’t interested in whether humans think so, because we’re not out there snacking on them. To us, a stick insect’s green or brown colouration seems to perfectly blend in with the kānuka twigs it’s perched on. But is it also hidden from a bird’s-eye view? When Merien ran images of stick insects through a modelling program that approximates bird vision, she realised that for birds, the colour doesn’t match at all.
“That tells me that if the bird is not spotting the stick insect, it’s not really about the colour. It could be using a different type of camouflage technique, like masquerade—looking like a stick and behaving like a stick.”
Merien’s research is still under way—it involves making lots of model stick insects and putting them in the bush to see which ones get chewed—but her initial thought is that the stick insect is a camouflage overachiever: “What I’m concluding is that a stick insect has evolved the ultimate appearance to be able to deal with multiple animals’ vision.”
Crab spiders (not to be confused with spider crabs) are found in many countries—tiny jewel-like arachnids that lurk among flower petals, waiting to pounce on unsuspecting bees.
Their white, yellow, or pink colour seems to perfectly match the bloom they’re sitting on. Many species can change their colour when they move from one flower to the next.
Studies of the European crab spider (Thomisus onustus) showed that the spiders were hidden from both their predators (birds) and their prey (insects). Bees never even see the spiders coming, says Kate Umbers, an evolutionary ecologist at the University of Western Sydney who studies camouflage and warning techniques in grasshoppers and frogs.
But Australian crab spiders (Thomisus spectabilis) want to be seen. To human eyes, they look just like the European ones. White spiders blend in nicely on white flowers. And yet under ultraviolet light—a part of the spectrum visible to insects and birds, but not us—a white spider positively glows. “They are really, really conspicuous,” says Umbers.
Instead of blending into the background, the Aussie spiders are using their colour as a lure. For some reason, bees strongly prefer flowers with contrasting colour patterns, and up close, the spider contrasts violently with virtually any bloom it perches on. “It’s not that [bees] can’t see it,” says Umbers. “It’s just that they can’t resist it.”
For Umbers, the crab spider saga is a reminder not to take anything in nature at face value. That’s useful for a scientist, but sometimes it makes her question everything in her life—from why her baby is crying, to whether she should have a coffee. “I can’t stop myself from generating hypotheses about everything,” she says. “Just because it’s most likely doesn’t mean that it’s true.”
Studying evolution can do that to a person. “Everything is almost certainly more complicated than it appears, and you can’t assume that you know anything. It just totally destabilises your view of the world—but in a good way.”