Fading Into the Background

Mostly limited to ocean animals, transparency is thought to help escape predators by blending the animal in with its environment, but is this what actually happens? (Image Credit: Wikicommons, CC BY 3.0).

Transparency reduces predator detection in mimetic clearwing butterflies (2019) Arias et al., Functional Ecology, https://dx.doi.org/10.1111/1365-2435.13315

The Crux

Predators are one of the strongest forces of selection in the natural world, and as a result it can be quite costly to stand out and be more easily noticed. This means that in order to survive, animals must adapt to avoid predators. Besides running away from what is trying to eat you, your best bet is to evolve body coloration that helps you avoid being seen by a predator.

Animals that rely on blending in will match the color or even the texture of their backgrounds, but when prey species live in areas where they cannot easily blend in (like plankton in the water column) they often evolve to be transparent. Unlike their marine counterparts, transparency is normally rare in terrestrial animals. The clearwing butterfly is one notable exception to this rule, and the authors of today’s paper wanted to test whether or not these clear wings actually reduce predation.

What They Did

To test whether or not transparent wings reduced predator detection, the authors used four different species of clearwing butterfly, each of which varied in degree of wing transparency from mostly opaque to mostly transparent. The authors predicted that the most opaque species would be most easily detected, but their design allowed them to also test if it was the degree of transparency that affected detection, and not just the transparent versus opaque trait.

To simulate predators, the authors used both birds (Great tits, Parus major) and humans. Great tits stood in for the natural predators of these butterfly species, as they have similar vision but have not learned to avoid the bitter-tasting butterflies. Humans were used as a second predator in order to disentangle the effects of prey detection and motivation to attack, as the bird predators may be able to see the butterflies but might not want to attack, leaving the authors unable to tell the difference. The human ‘predators’ simply walked through a corridor and researchers noted which of the butterflies they could spot*.

Did You Know: Predator Search Image

When predators search for prey, they aren’t just looking around and hoping to see what they are looking for, they actually have a fixed idea in their minds of what their prey looks like, which means that the prey will stand out. This is called a “search image”, and it allows for some interesting predator-prey dynamics.

When prey species come in more than one color, for example, the most common color will be the one that is most often eaten by the predator. Because they are the most common, they are what the predator is most likely to come across, thus the search image is based off of this most common form. When the least common form is at some disadvantage (i.e. maybe it can’t lay as many eggs as the most common form) this reduced predation can allow them to persist in populations where they would otherwise be out-competed.

What They Found

As hypothesized, transparency of the wings was a good predictor of detection, whereby the more transparent the wings of the butterfly the harder it was for a predator to detect them. This held true for both the bird and human predators. The birds did not appear to form a search image for the butterflies, as they didn’t seem to develop any preference for a type of clearwing over time. Interestingly, the humans did.

 

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Few examples better explain predator search image than the peppered moth. A species with light and dark forms, the dark forms stood out against their lightly-colored tree homes, making it easier for a bird to find and eat them. But when industrial pollution coated the trees in soot, the darker forms were now better-hidden and predators switched their search image for the light forms. (Image credit: Siyvula Education, CC BY 2.0).

Problems

The authors’ species of choice for the bird predator, Parus major, was a good choice in that these birds have similar visual abilities to the native predators of the clearwing butterflies. However, they are not native predators themselves, and thus any prey detection ability does not tell us much about the natural system in which these clearwing butterflies have evolved. Additionally, the butterflies were held in place during the trial and not allowed to perch or fly as they would naturally, perhaps affecting the results.

So What?

This study is a great example of scientists taking an idea that is widely assumed to be true (i.e. more transparent species are less likely to be detected by predators) and doing the work to determine if that is actually the case. Unlike our previous breakdown, the authors of this study found that the widely-held idea is actually supported with experimental evidence.

Although there are still many questions to be asked and explored within the world of clearwing butterflies (and the evolution of body color in general), this study has pulled back the curtain enough to allow scientists to ask more specific questions and better get to the bottom of it.

*It’s worth noting that these butterflies were models, and not the real thing as with the birds.

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