It almost sounds like a playground mockery: You look and smell like bird droppings.
For the aptly named bird droppings crab spiders that inhabit the tropical rainforests of Southeast Asia, such looks and smells are essential to survive in a world of eat or be eaten.
“All spiders are predators, but they also have their own predators,” said Daiqin Li, a biologist at the National University of Singapore. The spiders’ glossy black-and-white patterns and foul odor are part of a Mephite masquerade that tricks predators who would otherwise want to eat the spiders — after all, birds tend to avoid ingesting what they’ve already fully digested.
But the mimicry of the bird droppings spiders has another role.
According to a study published last month in Current Zoology, the spider’s fecal facade attracts prey and wards off predators — the first disguised species described to use what researchers call aggressive mimicry to be active in their lunch. to lure.
Previous research hypothesized that the crab spiders’ masquerade could attract unlucky insects. But until now, no one had experimental evidence. Still, the idea made sense, because for many insect species, bird droppings are both an attractive source of nutrients and inviting homes to lay their eggs. Crab spiders are also sedentary predators, preferring to ambush unsuspecting prey that land on their leaves.
“They stay there for more than 12 hours or more,” said Dr. Li. “Sometimes they just stay there for the rest of their lives.”
To test the hypothesis, the researchers first videotaped spiders in the wild perched atop leaves and compared the resulting swarms of insects with those attracted by similarly sized bird droppings. (Dr. Li noted that they had to make sure the droppings were “wet enough” because dry droppings didn’t attract many insects.)
Insects visited both the spiders and bird droppings at significantly faster rates than empty leaves. The spiders attracted insects, especially flies, although the real scat attracted them at a faster rate.
Then to test whether the spiders’ distinctive color combination was key to fooling some insects, the researchers applied an odorless watercolor paint to manipulate the spiders’ colors. Spiders painted all white or all black were less attractive to insects than unpainted spiders or spiders painted the same color as they already were, meaning that looking like bird droppings was the key to the deception. (The paint was easy to wash off with drops of water once the researchers finished observing the spiders.)
The researchers also modeled what the insects would see in their visual systems, finding that the unlucky prey may not be able to tell the difference between a hungry spider and actual bird droppings.
Not that we humans will do much better.
“Many people wouldn’t even be able to tell a spider from 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 have a really, really good masquerade.”
The findings have raised new questions about how the manure trickery evolved. Other species of crab spiders have different patterns and proportions of white and black on their bodies, which can affect how convincing their disguise is to insects, said Dr. Li. (The more “typical” types of crab spiders are green and white, which makes them blend in with leaves. They also don’t smell like bird droppings and attract far fewer flies.)
Other animals have also evolved to disguise themselves as inedible or inanimate objects for protection from predators — early thorn moth larvae look like twigs and deadleaf butterflies look like, well, dead leaves. But researchers rarely investigate whether color tricks can have multiple functions in the same species. That could change, said Dr. Pekar.
“I think in the future we will see many more cases where both the coloring and the pattern will be both defensive and offensive.”