Facts So Romantic

1-Trick Chameleon: Predators Learn to See Through Camouflage

Probably the worst thing to happen to you, if you’re an animal playing the game of life, is to be eaten by some bigger beast. If you’ve already managed to successfully reproduce by then, as far as evolution is concerned, maybe it’s OK for you to shuffle off that mortal coil. Still, I imagine it’s a terrible way to go. Luckily, evolution has endowed certain creatures with various tools to avoid being gobbled up as a mid-day snack, and one of the most widespread anti-predator defense systems is camouflage.

The drive to find food is nature’s grand game of hide and seek. As a prey species improves its ability to hide, predators improve their ability to seek. If camouflage is a clever way to keep a secret, then evolution has endowed some of its toothier beasts with some impressive skills for revealing those secrets.

If an animal wishes to remain unseen, then it needs to achieve two things. First, it needs to avoid detection—to avoid the predator’s powers of visual perception. The most obvious way to do that is with background matching, where an animal’s coloration and patterning matches that of the environment in which it spends its time, like the leaf-littered floor of a forest, or the light-colored bark of a tree.

This lesson is taught to high school biology students the world over in the story of England’s peppered moths. In the mid-20th century, when coal smoke and soot darkened English trees, the rare dark variety of moth blended in better, and birds ate more of the lighter variety, driving up the proportion of the dark moth. Several decades later, England cleaned up its air and the trees became lighter, giving better cover to the lighter moth, which again became predominant.

The problem with background matching is that the body outline remains intact. The other major mode of camouflage is disruption, which presents different advantages. Disruptive camouflage breaks up the body’s outline by introducing additional edges, like a zebra’s stripes, making it hard for a predator to separate a target from the visual landscape in which it occurs. 

Can you spot the prey animals in the photos below? Can you identify the type of camouflage? Article continued below image.

Top row: cicada in Japan from far and close (low-contrast disruptive). Second row: horned ghost crab in Borneo from far and close (background matching). Third row: mantis (background matching). Fourth row: nightjar (low-contrast disruptive)Cicada and crab: Martin Stevens. Mantis and nightjar: Jolyon Troscianko.

But predators aren’t simple machines that get one chance to detect, or not detect, their prey. Hunting is an active process, and predators, like all animals, can learn. So the second thing an animal needs to do to remain uneaten is to inhibit the ability of their predators to learn to detect them.

For the predator, this is a problem not of perception, but of cognition. And it’s something that Dr. Jolyon Troscianko, a postdoc at the University of Exeter in England, is investigating. In a paper published last month in the journal PLoS ONE, Toschianko points out, “while some types of camouflage may be powerful in preventing initial detection, they may be learnt more readily than other prey [that is, camouflage] types. Therefore, the overall benefit of a camouflage strategy may reflect the combined outcome of preventing both initial detection and predator learning.”

To experimentally assess which types of visual camouflage better interfered with learning, Toschianko had human participants play a simple computer game. A computer-generated photo was presented on a touch screen, and the participant had to touch the spot on the photo where they thought they saw a triangle that looked like a hidden “moth.” There were no tricks; every photo contained one.

First the researchers found that it took their participants longest to spot the moths when they were using disruptive coloration rather than the other strategies, and that was especially true when the pattern was of high contrast. However, as the moths’ contrast levels increased above the level of the background, they became easier to spot. In the middle was an ideal contrast level.

The results got more interesting when it came to the participants’ ability to learn to detect their digital prey over time. Disruptive patterns, when of high contrast, were learned more quickly than low-contrast disruptive camouflage. The researchers say “there may be a trade-off in having disruptive coloration, with high-contrast patterns reducing initial detection, but facilitating predator learning if encounters occur frequently or increase.” In other words, a strategy involving high-contrast disruptive camouflage seems to help predators learn, but only when learning opportunities would occur frequently or increase in frequency over time.

The sweet spot for that trade-off depends on the broader ecology in which the predator and prey species find themselves. According to Toschianko, high-contrast disruptive markings are best when predators only occasionally encounter the same prey type. For example, a relatively rare prey species with short-lived predators might benefit from this strategy, or a prey species that is subject to predation from multiple other species, or in environments where a single predator hunts multiple prey species. For example, some insects are eaten by a variety of birds, and birds often hunt a variety of insect species. On the other hand, he says, “high contrast markings would be costly when species face more specialist long-lived predators, or are highly abundant.” Monkeys, for example, are relatively long-lived and thus have ample opportunities to improve their hunting skills. This is perhaps why their prey are more likely to use low-contrast disruptive coloration.

Now that Toschianko has begun to understand the animals that use camouflage to hide, he is turning his attention to the seekers. In some ecological environments, it may make sense for predators to search for areas of high contrast within the visual field. In other scenarios, it may be better to take a more general approach, like, “search for a hidden triangle shape,” which could lead the predator to find things that are generally moth-like in appearance. To begin to uncover the strategies that predators use in spotting their prey, Toschianko, together with project leaders Martin Stevens, also of Exeter, and Claire Spottiswoode of the University of Cambridge, have launched an online game called Find the Nightjar (background on the game here). In the game, players look at photos of birds called nightjars camouflaged against the leaf litter of the African bush, and then click the photo in the spot where they think they’ve spotted the bird. The specific nightjar species in the game is found in South Africa and Zambia, where their ground nesting habits make them particularly vulnerable to predation by animals like monkeys and mongooses. To test how the bird’s camouflage works in that particular setting, the photos are modified to look as they would to monkeys and mongooses. Fooling humans, after all, is not the nightjar’s goal.

It’s a frustrating game to play. Nightjars, it turns out, are really good at hiding. After 20 minutes at it, I realized I’d make a terrible monkey but that I could eat reasonably well as a mongoose, as long as I had my species’ fondness for eating raw nightjars.


Jason G. Goldman received his Ph.D. in developmental psychology at the University of Southern California in Los Angeles and writes a blog called The Thoughtful Animal, hosted at Scientific American. His doctoral research focused on the evolution and architecture of the mind, and how different early experiences might affect innate knowledge systems.

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