When it comes to encounters with black widow spiders, birds are warned by their vision while insects are doomed by theirs.
That was the implication of a study out of Duke University that sought an answer to the following conundrum: How is it that the black widow spider's world-famous red hourglass scares off predators, such as birds, while not frightening away things the venomous spider wants to eat, such as insects?
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First, lead author of the study Nicholas Brandley and co-authors Matthew Johnson and Sönke Johnsen needed to find out if birds reacted differently to spiders with a red hourglass as opposed to those without.
They placed 3D-printed, plastic black widow spider replicas at live bird feeders and tested feathered fliers' reactions to two versions of fake spider: one with the haunting red hourglass and one without.
They found the birds were three times less likely to make overt moves – pecks or grabs – against the fake spiders that had the hourglass.
"The birds would see a spider model with red markings and get startled and jump back, like ‘Oh no man, get me out of here,'" Brandley said in a statement.
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Next Brandley and his colleagues wanted to check out how well birds see the spider's markings compared to insect victims, such as crickets, that tend to get trapped in a black widow's web.
Using a spectroradiometer, the scientists determined the wavelengths reflected by two species of North American black widow spider and then calculated how much of the light could be absorbed by a bird's eyes vs. an insect's.
The calculations showed that birds' photoreceptors in their eyes are simply better equipped than those of insects to see the spider's "go away!" warning marks. The widow's warning is more visible to birds -- brighter, with higher contrast -- and keeps the spider alive to spin its webs another day. Bugs, meanwhile, don't get a sufficient visual warning and face a grim fate.
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Black widow spiders, then, may have evolved their coloration signals to "be more conspicuous to their vertebrate predators than to their insect prey," the researchers wrote.
"Potential eavesdroppers may be an under-appreciated force shaping these signals," said Brandley, noting that relatively few studies examine animal warning colors for their ability not to frighten prey.
The Duke team's study was released in the Feb. 27 issue of the journal Behavioral Ecology.