July 3, 2022

It almost sounds like a playground mockery: you look like bird droppings and you smell like it too.

For the aptly named bird droppings crab spiders that live in the tropical rainforests of Southeast Asia, such looks and smells are essential to surviving in a world where it is all about eating or being eaten.

“All spiders are predators, but they also have their own predators,” says Daiqin Li, a biologist at the National University of Singapore. The glossy black and white pattern and foul smell of the spiders are part of a Mephite masquerade that trick predators that would otherwise try to eat the spiders – after all, birds tend to ingest what they have already fully digested.

But the mimicry of the bird spiders also fulfills another role.

According to a study published in Current Zoology last month, the spider’s faecal facade attracts prey while repelling predators – the first masked species described to use what researchers call aggressive mimicry to be active within it Lure lunch.

Previous research hypothesized that the crab spider masquerade might attract unfortunate insects. But no one had experimental evidence until now. The idea still made sense, because bird droppings are both an attractive source of nutrients and an inviting home for many insect species to lay their eggs in. Crab spiders are also sit-and-wait predators and prefer to ambush unsuspecting prey that land on their leaves.

“They stay there for 12 hours or more,” said Dr. Li. “Sometimes they just stay there all their lives.”

To test the hypothesis, the researchers first filmed spiders sitting in the wild on leaves and compared the resulting swarms of insects with those attracted by bird droppings of similar size. (Dr. Li noted that they needed to make sure the droppings were “wet enough” because dry droppings didn’t attract many insects.)

Insects visited both the spiders and the bird droppings much more frequently than empty leaves. The spiders attracted insects, especially flies, although the real droppings attracted them more.

Then, to test whether the spiders ‘distinctive color combination was key to fooling some insects, the researchers used an odorless watercolor paint to manipulate the spiders’ colors. All white or all black painted spiders were less attractive to insects than unpainted spiders or those painted the same color they already had, meaning that looking like bird droppings was the key to deception. (The paint was easily washed off with drops of water when the researchers finished observing the spiders.)

The researchers also modeled what the insects would see in their visual systems, and found that the hapless prey may not be able to tell the difference between a hungry spider and actual bird droppings.

Not that we humans are much better off.

“Many people couldn’t even tell a spider from a bird droppings,” said Stano Pekar, a zoologist at Masaryk University in the Czech Republic who was not involved in the study, and said the results were impressive. “I mean, they really have a very good masquerade.”

The results have raised new questions about how the crap deception developed. Other types of crab spiders wear different patterns and proportions of white and black on their bodies, which can affect how persuasive their disguise is to insects, said Dr. Li. (The more “typical” species of the crab spider are green and white so they can blend into leaves; they also don’t smell like bird droppings and are far less likely to attract flies.)

Other animals have also evolved to disguise themselves as inedible or inanimate objects to protect themselves from predators – early thorn moth larvae appear like twigs and dead-leaf butterflies look like, well, dead leaves. However, researchers rarely investigate whether coloring tricks can serve multiple functions in the same species. That could change, said Dr. Pekar.

“I think in the future,” he said, “we will see many more cases where both the coloration and the pattern will be both defensive and offensive.”