Did you see that toad in the picture? Okay that was probably an easy one. How about the owl? A bit harder, no? Lastly, that white blob on the snow? Is that a bird? Alright, you get the point. We have long been taught about these amazing camouflages put on by animals to help them escape the eyes of hungry predators. However, animals do not just stand for poses like in these photos. As soon as the animal starts to move, their perfect camouflages lose effect. So, a question remains: is there a pattern that can conceal the prey even when they are moving?
An idea is that high-contrast patterns with repeated elements, like a zebra, can become blurry and featureless when the animal moves fast enough. This phenomenon is also called the “flicker fusion effect”. You remember that one time when the merry-go-around at the park went a-wire and the horses became a blurry patch of solid color when everyone got thrown into the air? That is what is at play here. But this idea was never tested, and by how much is “fast enough”? If a rabbit is running at the speed of light, it would not even matter if that will make it look less conspicuous – because it would just be impossible for the wolf to catch it. And that is what the scientists are trying to figure out – what is the best combination of speed and pattern interaction that can conceal the prey on the go?
In their latest publication “Pattern and Speed Interact to Hide Moving Prey”, Umeton et al. set up a laboratory system where mantids act as predators and moving pattern patches on the computer screen as “preys”. The patterns are designed to look different and move at different speeds (ecologically relevant speeds at which small insects move). They then recorded the reaction of the mantids to these images to obtain a probability of response for each pattern + speed combination.
There are four image patterns flying across the screen: wide-striped (A), narrow-striped (B), background-matching camouflage (C), and pure gray (D). For each pattern, they can move at three speed levels: slow, medium, and fast. The speeds are carefully designed so that the wide-striped pattern would remain visible at all speeds, while the narrow-striped pattern more and more blurred as the speed increases. The prediction is that, the mantids would react less to the narrow-striped pattern at high speeds, and that the probability of response to narrow-striped pattern would be similar to that to the pure gray pattern, because the two should look similar at higher speeds.
Now hold on to your horses before we jump to the results. We still need to make sure that the mantids are, say, more likely to attack fast-moving objects just in general. Umeton et al. thus included a pure black pattern during testing and found the probability of response to be high at all three speed levels. Now that it is clear that motion blur is the only thing that makes the difference, let us see what they found.
Fig 1. Patterns used in the experiment and the probability of responses they elicit at different speeds.
As it turned out, responses to narrow-striped pattern were reduced with increasing speed. (Fig1. B) Compared with the wide-striped pattern, narrow-striped pattern elicited significantly less reactions from the mantids at medium and fast speeds. (Fig1. A, B) And at these speeds, the mantids responded similarly to the narrow-striped and the pure gray patterns. (Fig1. B, D) These results line up perfectly with the prediction made before.
How about the background-matching camouflage patterns? While the narrow-striped pattern is more conspicuous than the camouflage at slow speed, surprisingly, when the camouflage pattern moved fast enough, they became less advantageous. (Fig1. B, C)
In their study, Umeton et al. effectively showed that a moving animal can be patterned in a way that helps reduce detection by predators. Even high-contrasting patterns, which were thought to make preys more visible to predators, can become a good camouflage if the prey moves at sufficient speed. Specifically, striped patterns can escape the eyes of predators by taking advantage of the limitations of their visual perception. Outside the laboratory, bumblebees are a good example of the ecological application of this finding, as they carry the narrow-striped pattern used in this experiment. (The yellow and black stripes might look quite wide in the picture, but remember bumblebees are small creatures!) Furthermore, Umeton et al. hypothesized that animals can utilize high-contrasting patterns for signaling when they are static, and for camouflage when on the run.
There is still one question left unanswered: why are background-matching prey less advantageous when moving? To answer this question, we need to understand how eyes work. The classical motion detectors rely on two factors to track movement: (a) the shifts in intensity of light at the moving edges of an object, and (b) the internal contrast of the object. The gray patterned used in the experiment was the least possible pattern to elicit responses at slow speeds for this reason – it contrasts little with the background on the edges and appears uniform on the internal. The background-matching pattern, on the other hand, has internal dark and light features that easily trigger motion detectors when moving. In natural scenes, the “background” texture has more variation on broader scales, and animals wishing to blend in need to adopt camouflages that involve “wide” patterns, much like the wide-striped pattern in this experiment. These wide patterns require much faster speeds to appear uniform than finer ones, and thus are less advantageous.
The findings of Umeton et al. might bring us one step closer to deciphering how natural selection acts on how animals are patterned. (Maybe it can bring about new explanations for the black and white stripes on zebras!) In the ecology context, this experiment will help us further understand the ways in which animals adapt to the mechanisms of the sensory systems of their predators. There could be a secret arm race happening right under our eyes!
