Generally when we think of camouflage, subtle colours and soft shapes that resemble the background come to mind, not obvious patterns with bold colours and crisp outlines.
However, as optical illusions show, the visual system can be tricked. This goes for the visual systems of pretty much any animal, not just humans and in the evolutionary arms race between predator and prey, several animals have evolved to take advantage of the eye’s fallibility.
Rather than blend in, they hide by standing out. No conversation about bold colour patterns in animals could possibly begin by discussing anything other than the zebra. So let us start there; how did the zebra get its stripes? Well, more specifically, why did the zebra get its stripes?
There are, of course, several theories about the evolution of the bold, black and white stripes, ranging from them having a cooling effect to their being an effective form of defence against predators. Since this is an article about vision, we will set aside the thermoregulation theory and focus on stripes as defence.
Looking at figure 1, it is hard to believe black and white stripes could help an animal living in the dusty, shrubby brown landscape of the savannah. Surely, it would make more sense to be the same colour as the background in order to stand out less? The problem with background-matching camouflage (that is the kind where, as you may have guessed from the name, the animal’s colours and patterns match their background environment, making it difficult for predators to spot them) is that its effectiveness is limited to environments that match.
As soon as the animal moves from that environment, their background-matching no longer works. In practice, an animal such as a zebra will appear against many different backgrounds as they move about. They will also appear against different backdrops depending on where they are being observed from; a lion crouching in the grass may see them silhouetted against the blue sky, while a lion perched on top of a rock or ridge for an ambush will see them against the ground.
Environments change too. For instance, the Serengeti undergoes a dramatic seasonal shift from dry and arid to wetland. So, it is difficult for an animal to background-match its environment when that environment or background is constantly changing.
There are exceptions of course, which I will discuss further on. The other issue with background-matching camouflage is that it only works well when the animal is stationary. As soon as the animal moves, it becomes very obvious.
This is due to the way visual systems detect movement. Visual systems are capable of detecting (at least) two types of motion; first-order and second-order. First-order motion is based on changes in luminance (brightness) and contrast over time, while second-order motion is based on changes in texture or contrast.
For example, a dark horse moving across a green field would be an example of first-order motion, while the movement of a sand-coloured and patterned flatfish over the sandy ocean floor would be an example of second order motion.
When still, the flatfish matches the sand in both colour and pattern, making it very hard to spot. However, as soon as it moves, it becomes immediately obvious to eyes like ours. Given the constraints of background-matching camouflage, how else can an animal protect itself from a predator’s sight?
Enter dazzle camouflage. Dazzle camouflage is less about not being seen and more about not being seen correctly. Spotting potential prey is only the first step in capturing it. A predator needs to correctly judge both its distance and its speed. This is where the optical illusion of dazzle camouflage comes in. Seen among a herd, it is very difficult to tell which black and white stripes belong to which zebra. The high-contrast pattern hides the outline of the zebra, making it hard for a predator to single one out, which is problematic when it comes to the catch. Not only that, but the stripes may also create an optical illusion that makes it difficult for predators to judge the zebra’s speed and direction of movement.
In experiments with human volunteers, scientists have shown that high-contrast patterns like stripes can cause a marked change in perceived speed, making an target seem faster or slower than it really was depending on the type of pattern. In other words, bold high-contrast patterns like stripes can effectively mask an animal’s true position and movement, hampering a predator’s attempts to catch it.
It is interesting to note that such effects have not been overlooked by the military. It is not just nature’s background-matching camouflage that they have copied for their uniforms, but this bold dazzle camouflage too. Back in World Wars I and II, ships were painted bold, high-contrast patterns of stripes and geometric shapes. It was thought that these patterns would make it difficult for the sailors on enemy ships to correctly calculate their speed and distance, leading them to miss with any attack.
Of course, as the technology of weapons targeting changed from human to machine vision and beyond, such patterns would no longer be effective, which is why we do not see such crazily-painted warships these days (sadly). We have been talking about predators up until now in the sense of a big animal, such as a lion, a leopard or a human, but small animals, such as insects, can be every bit as dangerous to a zebra.
Not only can they lose a substantial amount of blood to biting flies, but species such as the tsetse fly can also be a spreader of serious disease. It is therefore imperative zebras minimise the amount they are bitten. It is now thought that their bold stripes play a role in this too.So how exactly can stripes affect the visual system of a fly? Just as for a lion, they might make it difficult for the fly to detect exactly where the zebra is and prevent the fly from landing successfully.
Flies use a visual cue called optic flow to control their flight when landing. Optic flow is the pattern that the perceived motion of the visual surroundings makes on the retina. As a fly flies towards an object, they depend on a reliable optic flow cue to control how fast they approach as they land. Stripes may interfere with this pattern, causing the fly to misjudge the flight dynamics of its approach and either crash straight into its target (ie the zebra) or miss it entirely. That is one theory.
Another theory is that the stripes startle the fly, disrupting their ability to land. This relies on the fact that flies, which have compound-type rather than our own camera-type eyes, have relatively poor resolution, making the world appear much more blurred.
As a result, even the relatively thick, chunky stripes on zebras will blend together at relatively short distances, making the zebra appear grey. This works on the same principle as the dots of coloured ink used in comic book printing or even the pixels on modern-day devices. If an object (ie a dot or a stripe) is below the resolution of an eye, its features, such as colour, will blend with its surroundings.
So, the black and the white stripes will average to a uniform, solid grey for a typical fly flying around 2-3m from the zebra. However, as they get closer, the thick stripes will become resolvable (as they appear larger from the fly’s point of view) and then the zebra that looked grey is now suddenly black and white. This sudden change in appearance may startle the fly, or cause an upset to its visually-guided flight dynamics. Either way, it means a less likely landing for the fly on the zebra.
These theories are currently being tested both in the laboratory and in the field. But zebras are not the only animals with stripes. Cuttlefish are well known for their colour-changing abilities, but it is less well known that they can also dress themselves in stripes. And what is more, these stripes are not static like the zebra’s, but move rapidly across the cuttlefish’s body. They have specialised cells in their skin that allow them to change colour and pattern dynamically at will. They can use their colour-shifting abilities as a means of communication and for camouflage.
Since they can adapt their colours to suit almost any natural environment, they can overcome one of the main limitations of static background-matching camouflage, so they are probably not using stripes for that. Nor are they using stripes as protection against biting flies, obviously. So, what are they for?
In a reversal of what we see on land, cuttlefish actually use the stripes when they are hunting. Using their bodies like a display screen, they ‘play’ a pattern of horizontal black and white stripes that move downwards across their heads towards the tips of their tentacles. This is displayed directly in front of their prey, such as a crab.
There are two theories for how the stripes aid the cuttlefish in hunting. Either they somehow mesmerise their prey (insert the obvious ‘look into my eyes’ joke here) or the movement of the stripes across their body masks their actual movement towards their prey, allowing them to get closer than they otherwise would have been able to had the prey been able to accurately judge their movement. Either way, it makes for an impressive but deadly display.
