Readers know by now that I love mimicry. This is for many reasons, but I suppose foremost among them is that it shows the power of natural selection to “mold” an animal to closely resemble something else. (That’s a metaphor, of course, for natural selection is not something “outside” that “molds” an animal or plant, but simply a process of the accumulation of genes that, in this case, help keep their carrier from being killed.)
A common “something else” is bird droppings, which of course are unpalatable to predators like birds and wasps. Ergo, many insects and spiders, as well as vertebrates like frogs, have evolved to resemble bird droppings, hiding themselves from predators. This would of course be favored by selection, for any resemblance to a dropping reduces your chance of being nommed, and increases the chances of passing on your genes. Over time, genes would accumulate that would make you look, within developmental and ecological constraints, as close to a bird dropping as possible.
According to a paper in Nature’s Scientific Reports by Min-Hui Liu et al. (reference below; free access), this has happened in the spider Cyclosa ginnaga from East Asia.
It’s long been known that this spider weaves decorations into its web that look like droppings, but some spiders of this genus have also evolved to look like droppings. When a spider sits in its decoration, the combination looks remarkably like a bird dropping, thereby avoiding attention from predators. Here’s a photo from the paper showing the spider sitting in the decoration it’s woven in its web (a), along with a normal bird dropping (b). The scale bar is 5 mm. (about 1/5 inch). Note that the spider’s abdomen, and perhaps the rest of it, resembles bird droppings as well.
Here’s another photo, from a Smithsonian article about the paper, showing the similarity between bird droppings alone (first and third rows) and spiders on their webs (second and fourth rows). The spiders are oriented on a vertical web, just as many bird droppings are on vertical surfaces:
The authors wanted to test the hypothesis that the mimicry was not only something that deceived the eyes of wasp predators, but also reduced predation on the spiders. To do this, they first did a spectral analysis of the spiders on their decorations and compared their “colors” to a computer model of the sensitivity of the hymenopteran eye (wasps, along with bees, are hymenopterans). They found that the spiders were indistinguishable in this spectrum from both their web decorations and from real bird droppings—again using the model of what a wasp eye could see.
The authors then changed the colors of spiders and their webs using a black marking pen to darken the spiders’ bodies, and black carbon powder to darken the web decorations. There were three treatments: darkened spider on darkened decoration; unmarked, light spider on darkened decoration; darkened spider on unchanged light decoration, and the control: unmarked spider on normal, undarkened decoration. They then used video cameras to record incidents of wasp attacks.
Only one of the treatments significantly increased the predation rate on spiders: the normal, light spider sitting on a decoration that had been blackened with carbon powder. None of the other treatments affected predation. Here’s a graph from the paper showing the elevated predation in the normal spider/blackened decoration treatment (fourth bar to the right) compared to the control and the other two treatments. Predation was increased more than fourfold, a significant selective force:
It’s not hard to understand why a black spider on a blackened decoration (first bar to the left on graph above) isn’t spotted by predators so easily, because it’s camouflaged. But it’s more puzzling why the blacked spider on an unblackened decoration (third bar from the left) didn’t experience higher predation. Nor do the authors (or the Smithsonian piece) discuss this anomaly. Perhaps the spider is sufficiently hidden by being on a larger spot that looks like a bird dropping, or perhaps the blackness of the spider simply looks like a bird dropping, too, for droppings contain black bits. But that doesn’t explain why the spider itself would evolve body markings resembling bird droppings. Why would that happen if its body color was unimportant compared to the color and design of the web decoration it makes?
There are possible answers, including the presence of other predators (like birds) that weren’t detected. And the effect of hiding yourself from your prey wasn’t considered. What the paper does show is that the color of the spider’s decoration helps hide it. What it doesn’t show is that the spider itself, sitting in that decoration, is part of the mimicry. The authors’ conclusion that “C. ginnaga‘s decoration and body coloration forms a bird dropping masquerade”, then, seems a bit dubious to me.
Perhaps readers have other explanations.
Reference: Liu, M.-H., S. J. Blamires, C.-P. Liao, and I.-M. Tso. 2014. Evidence of bird dropping masquerading by a spider to avoid predators. Scientific Reports 4:10.1038/srep05058.