Animal visual illusions

Animals interact visually all the time. Males try to look big and scary to rivals, or sexy to females. Prey animals try to look inedible—or better, invisible—to predators. Sometimes these animals use visual trickery to assist their cause.

You’ve probably encountered visual illusions before. Here are some classic ones:

a) The vertical line looks longer than the horizontal line, even though they're both the same length. b) The top line looks longer than the bottom line, even though both are the same length. c) The middle circles are both the same size, but the one on the left looks bigger. d) The middle grey rectangle is just solid grey, but against the gradient background, it looks like a gradient. e) Both circles are the same shade of orange, but the one surrounded by black looks brighter.

a) The vertical line looks longer than the horizontal line, even though they’re both the same length. b) The top line looks longer than the bottom line, even though both are the same length. c) The middle circles are both the same size, but the one on the left looks bigger. d) The middle grey rectangle is just solid grey, but against the gradient background, it looks like a gradient. e) Both circles are the same shade of orange, but the one surrounded by black looks brighter.

Animals can use visual illusions a) and b) to appear bigger by changing their posture. Vertical stances make you look bigger than horizontal ones, and making a Y with your limbs looks bigger than letting them fall down. So if you’re a male peacock spider trying to look big and sexy to a female, you can raise a pair of back legs up in a Y to look bigger than you really are.

Peacock spider display. Photo by Jurgen Otto*

Peacock spider display. Photo by Jurgen Otto*

Visual illusion c) shows how objects look bigger when they are surrounded by smaller objects. This illusion actually doesn’t work for all animals: baboons don’t get fooled at all, correctly perceiving that the central circles are the same size, while pigeons get fooled in the opposite direction, mistaking the circle surrounded by bigger circles as being bigger.

Female fiddler crabs get taken in by this illusion just like humans, though. Male fiddler crabs show off their enlarged claw to impress females, and bigger claws are better. Females will approach a male more if the male is displaying his claw next to other males with smaller claws: the large-clawed male’s claw looks even larger in comparison with small-clawed neighbors.

Fiddler crab males showing off their claws. The large-clawed male in the group on the right looks especially large-clawed because his neighbors all have considerably smaller claws.

Fiddler crab males showing off their claws. The large-clawed male in the group on the right looks especially large-clawed because his neighbors both have considerably smaller claws.

It’s not clear if male fiddler crabs know enough to use this to their advantage. Male guppies, on the other hand, are on top of it: they prefer to display next to males with smaller color patches than their own. (The title of the paper that found this is “Do unattractive friends make you look better?”)

Male guppy (some sort of domestic hybrid, but displaying the kind of color patches that wild male guppies have). Photo by Chantal Wagner Kornin*

Male guppy (some sort of domestic hybrid, but displaying the kind of color patches that wild male guppies have). Photo by Chantal Wagner Kornin*

You’ll notice that the male guppy doesn’t just have big patches of color, he also has big patches of black. Why would he do that? He’s taking advantage of visual illusion e), putting black near his color patches to make the colors appear brighter and more striking.

Other animals use striking contrasts with black or white to break up their own image. Predators searching for prey look for a specific prey-shaped outline, so the prey use big patches of a contrasting color to disrupt the contour of their body.

The white marks on this fish break up its distinctive fish-shape, making it harder to see.

The white marks on this fish break up its distinctive fish-shape, making it harder to see.

The black and white rings on a Killdeer's neck break up its outline. Photo by Nicole Mays*

The black and white rings on a Killdeer’s neck break up its outline. It’s hard to see from this perspective, but…
Photo by Nicole Mays*

Killdeer, surprisingly hard to see against the rocks because of their disruptive neck markings. Photo by Sergey Yeliseev*

…in a visually complex environment, like this one with stones, the neck markings blend in with the other contrasty edges in the environment and make it difficult to pick out the shapes of the birds. Photo by Sergey Yeliseev*

Killdeer chicks use white and black markings to disrupt their outlines too. Photo by ArielleView*

Killdeer chicks use white and black markings to disrupt their outlines too.
Photo by ArielleView*

And of course what a prey animal uses to hide from predators, a predator will use to sneak up on prey.

Disruptive coloring on an African Wild Dog

Disruptive coloring on an African Wild Dog.

Prey have another visual trick up their sleeves, though: motion dazzle. Motion dazzle refers to the fact that when a striped animal moves in the same direction as its stripes, it is harder for an observer (or predator) to resolve how fast and in what direction it is moving, and therefore harder to catch it. This may be one reason why a lot of snakes and lizards are striped lengthwise: when they run or slither forward, they are moving in the direction of their stripes, and can confuse any pursuers with motion dazzle.

Garter snake. Photo by the Oregon Dept. of Fish & Wildlife*

Garter snake. Photo by the Oregon Dept. of Fish & Wildlife*

Skink (probably copper-tailed skink?). Photo by Malcolm Tattersall*

Skink (probably copper-tailed skink?). Photo by Malcolm Tattersall*

One of the more elaborate (and stranger) visual illusions in the animal world is created by male Great Bowerbirds.

Great Bowerbird working on his bower. Photo by sunphlo*

Great Bowerbird working on his bower. Photo by sunphlo*

Male bowerbirds create bowers of sticks, with collections of items in the “court” in front of the bower. The female then stands in the bower to admire the collection of items, watch the male display in the court, and decide whether to mate with him.

Photo by sunphlo*

Photo by sunphlo*

Great Bowerbirds collect white and light grey items (shells, stones, bones, bits of glass) for their court. Then—this is the strange part—they arrange them by size, with smaller items close to the bower and larger items farther from the bower. If you mix the items up randomly, the bowerbird will put them back in proper size order.

Photo by Natalie Tapson*

Photo by Natalie Tapson*

When the female sits in the bower and looks out over the court, the ordered-by-size objects counteract normal perspective: usually, closer objects look bigger and farther objects look smaller, but the size gradient counteracts that, since the farther objects are also objectively larger. The result is that all the objects appear to be the same size to the female, as if perspective did not exist.

visual_illusions_bowers

Left: normal perspective, if all the objects were actually the same size, would make the farther objects appear smaller. Right: since the objects are arranged by size to counteract the effect of perspective, they all appear to be the same size to the female.

We don’t really know why this visual illusion, called “forced perspective,” is considered attractive in male bowerbirds. One idea is that the uniformity of the view presented to the female—all those white objects all looking like the same size—appeals to her visually. There isn’t a lot of uniformity in nature, after all, so it may be a striking contrast to the rest of the world.

Since the criteria other bowerbird species’ females use to evaluate bowers is usually along the lines of “Are all the objects blue?”, I suppose “Are the objects in apparent defiance of perspective?” isn’t objectively any stranger.

Are all the objects blue? Photo by Melanie Cook*

Are all the objects blue?
Photo by Melanie Cook*

Are all the objects snails? Photo by Kasi Metcalfe

Are all the objects snails?
Photo by Kasi Metcalfe*

*Photos obtained from Flickr and used via Creative Commons. Many thanks to these photographers for using Creative Commons!

References:

Endler J, Endler L, Doerr N. 2010. Great bowerbirds create theaters with forced perspective when seen by their audience. Current Biology 20:1679-1684.

Gasparini C, Serena G, Pilastro A. 2013. Do unattractive friends make you look better? Context-dependent male mating preferences in the guppy. Proceedings of the Royal Society B: Biological Sciences 280:20123072.

Kelley LA, Kelley JL. 2013. Animal visual illusion and confusion: the importance of a perceptual perspective. Behavioral Ecology advance access:1-14.

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5 thoughts on “Animal visual illusions

    • Isn’t that an odd one? The first time I read that study, I assumed that the forced perspective must go the opposite way, because if it did, it would make the displaying male appear to be farther away – and therefore appear to be larger than he really is. That would make so much sense! But instead they do it the other way, and we don’t know why.
      And yes, I’m glad we haven’t figured everything out, because then I wouldn’t have a job :-)

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