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A viper's zig-zag colors help blur their predators' vision

Scientists previously thought that animals' color patterns were either warning signs or camouflage — on these snakes, they are both

Sara Zlotnik

Ecology & Evolutionary Biology

University of Florida

Evolutionary biologists are constantly trying to make sense of patterns in nature — from the global-scale patterns of biological diversity to the individual color patterns on an animal’s fur. Prey animals are especially interesting puzzles because of their unique adaptations that help them avoid being eaten. Some evolve extreme camouflage, like stick insects and bottom-dwelling fish. Others consume toxic food which they use as chemical defenses against predators. Toxic or venomous prey often advertise their defenses using bright warning coloration (think wasps, butterflies, and poison frogs). Now, scientists are seeing that venomous snakes can have it both ways: patterns as both advertisement and camouflage.

Snakes may not seem like typical prey animals, but evolutionary biologists actually have a long history of studying snake coloration from an anti-predator perspective. Some venomous snakes, like coral snakes and sea snakes, have the classic black-and-yellow or black-and-red striped pattern that effectively communicates danger to potential predators. In contrast, rattlesnakes use the sound of their rattle as a warning signal instead of relying on their color pattern, which may explain why their coloration is better for camouflage than that of a coral snake.

Recently, biologists have been investigating whether a single color pattern could have evolved multiple defensive functions to protect prey from being seen, attacked, and caught by predators. There's been a lot of focus on European vipers, which have a dark brown or black zig-zag stripe down their back over a light gray or tan background. Although these snakes do not have bright colors, their zig-zag stripe still functions as a warning signal to predators. Studies over the past few decades have confirmed this by measuring the frequency of predator attacks on model vipers made out of clay. These studies have shown that birds attack clay snakes less often if they have a zig-zag stripe than if they are plain-colored or if they have other striped patterns.

While many scientists would leave the story there and accept that the zig-zag stripe evolved solely as a warning signal, evolutionary biologists Janne K. Valkonen and Johanna Mappes went further. In collaboration with researchers at the University of Jyväskylä and the University of Oulu in Finland, Valkonen and Mappes did another clay model experiment, but swapped bird predators for human ones. They hid the model snakes along a hiking trail and asked college students from biology field courses to identify as many as possible while they walked down the trail. College students were worse at finding the snakes with zig-zag stripes than solid-colored ones, indicating that the zig-zag actually provides camouflage in addition to serving as a warning signal.

a large stick bug camouflaged on a stick in the woods

Many species of stick insect are highly camouflaged due to their body shapes and coloration

Pikist

But what if a particularly persistent predator managed to spot a striped viper and, despite the warning of danger, attacked anyway? Could the viper’s zig-zag stripe help it to evade capture as well?

The researchers answered this question by examining a particular visual illusion known as ‘flicker fusion’. Imagine sitting in a speeding car and looking out the window at trees in the distance. If the trees are packed very closely together, they may appear to blur into a single brownish streak on the horizon rather than being distinguishable as individual objects. The same effect could occur with the zig-zags of the vipers. If the snakes move fast enough, the back-and-forth of the zig-zag would blur into a diffuse longitudinal stripe. This pattern-changing effect is thought to make fleeing prey harder to catch by confusing predators and reducing their ability to accurately judge the prey’s speed, shape, and location.

Some predators have better vision than others, which could affect their perception of visual illusions. To figure out which predators would experience the flicker fusion effect when hunting vipers, the researchers first had to measure how fast the snakes could move.

They collected live vipers in Finland and filmed them fleeing from a simulated attack (a quick tail tap was enough to scare them). The researchers then used these videos along with close-up photos of the zig-zags to calculate how many of the snakes' light and dark color patches could pass by a single spot within one second. By comparing these calculations to measurements of various predators' flicker fusion thresholds — the maximum rate at which black and white stripes can speed by before they blur together — they determined that European vipers may use this visual illusion to help them elude capture by mammals, although it is unlikely to be effective against birds.

Valkonen, Mappes, and their colleagues recently published these results in Animal Behaviour, and concluded that the zig-zag stripe found on many European vipers can potentially protect them from being seen, attacked, and caught by predators. And even though some predators may overcome a single defense, they are unlikely to hurdle all three. So, while a casual observer might think these vipers’ color patterns only provide aesthetic benefits, this simple zig-zag has proven to be much more powerful than anyone would have predicted.

Comment Peer Commentary

We ask other scientists from our Consortium to respond to articles with commentary from their expert perspective.

Ashley Marranzino

Marine Biology

University of Rhode Island

Sara, this is a great summary of some fascinating research! I like how you highlight the range of pressures faced by organisms. While it might seem like the answer to a question like “why does a snake have this coloration” would be easy to answer, in reality there are so many different factors that can influence the evolution of traits. As this research shows, there is often more than one reason why a trait evolved a certain way.

I am curious, since these patterns appear to serve as a warning signal to birds, is that warning genuine signal or are the vipers mimicking a pattern from another species?

Sara Zlotnik responds:

Hi Ashley, thanks for your comments! I agree, we often expect that the evolution of a trait is due to a single adaptive function, so it’s exciting to learn about research projects that really delve into the multiple mechanisms through which a trait could have evolved.

Your question about mimicry is also really interesting. I believe that all Vipera species are venomous, so any of these vipers that have a zig-zag stripe are displaying an honest warning signal to potential predators. Their defensive capabilities makes it seem unlikely that the vipers’ color patterns have evolved to mimic another animal. However, it is possible that the zig-zag pattern is under evolutionary pressure due to Mullerian mimicry, which is when multiple venomous or toxic animals mimic each other’s patterns to increase the chance that predators will recognize and avoid that pattern.

Another interesting point is that there are non-venomous snakes that live in the same areas as the vipers and some of them also have zig-zag stripes, which is likely due to Batesian mimicry (when a harmless species mimics a dangerous one to avoid predation). So rather than the vipers mimicking other animals, it is more likely that other animals are mimicking them.

Jaime Chambers

Anthropology

Washington State University

Such an interesting finding and a fascinating write-up! I’ve read that European vipers’ color varies by sex – which you allude to, mentioning their stripes and basal colors range from brown and tan to black and gray. I’m curious how these color variations might affect the “flicker fusion” illusion, and if the researchers accounted for them? Do black-on-gray males with more “contrasty” zig-zag patterns create a more effective illusion than brown-on-tan females?

Sara Zlotnik responds:

Hi Jaime, that’s a really great point that females and males typically differ in their base coloration (brown vs gray) in Vipera berus, which is the species of European viper that was used in the flicker-fusion study. Other European vipers (in the genus Vipera) also have zig-zag stripes, but the various species differ widely in their coloration, and thus, in how much the zig-zag contrasts with the background color, so this question is relevant across species as well.

For the camouflage experiment, the researchers actually tested out clay models of different colors (brown, gray, and black), as well as testing out differences between solid-colored and zig-zag striped snakes. The background color of the snakes did not seem to affect detectability, while the zig-zag reduced the chance of detection for both brown and gray snake models. So overall, the zig-zag should help both female and male Vipera berus to camouflage.

It is less clear how the base color of the snakes affect the flicker-fusion effect. The researchers found that there were no sex differences in fleeing speed, but their calculations were not able to account for differences in the flicker fusion effect due to color contrast between the zig-zag and the background. I would imagine that a zig-zag might appear to ‘fuse’ more easily when placed on low-contrast background, but in that case, it might appear more like a diffuse blob than a longitudinal stripe. As the exact mechanism by which the flicker fusion effect influences predation risk is still out for debate, it is unclear what specific characteristics would maximize the prey’s chance of survival.

Fernanda Ruiz Fadel

Animal Behavior and Behavioral Genetics

University of Tübingen

Very interesting read! I actually learned a lot of new things here, the “flicker fusion” being my favorite new fact. I believe with everything in Biology we are slowly learning that there isn’t really just simple cause and effect, but rather interactions from various sources of pressure and though looking at one thing at a time is easier, we need to consider all aspects we can find. So, as an evolutionary biologist, I am not surprised the color patterns of vipers have so many functions. As a curious person, I am amazed at how one feature can function in so many angles at once - specially that standing still and moving trigger different effects.
I also really like the description of experiments, they sound so cool! I never get tired of how creative researchers get to design experiments and make sure all bias is controlled for.