Just as humans use hidden codes for a variety of unique applications, so do black widow spiders, according to a new study conducted by researchers at Duke University. The findings reveal that the red hourglass of the black widow spider, notorious for their toxic bite and considered to be one of the most venomous spiders in the world, is actually a warning signal that can be perceived only by some animals.
The team, led by Nicholas Brandley, revealed that the unique red hourglass lets predators - such as birds - know that they would be better off avoiding them, while at the same time ensuring that this information is not conveyed to prey. This is accomplished by taking advantage of the unique differences in the way that birds and insects see color.
Brandley and his team first tested if birds responded differently to black widow spider models with red hourglass markings in comparison to those without by placing the fake spider models, which were designed for video game developers, onto bird feeders.
"Video game developers love to put black widows in their games because they're scary," Brandley said in a press release.
During the test on birds, half of the eight fake spiders were painted solid black, and the other half were painted black with a red hourglass to match real black widows. The results showed that birds avoided red hourglass models more when looking for a snack - birds were three times less likely to grab or peck at these models in comparison to the solid black models.
In addition, the team noticed that small, short-beaked birds were the most scared of the red hourglass models.
"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 the next phase of the study, the team estimated how the perception of the red and black hues used by the black widows varied in birds compared to insects such as crickets, beetles and ants, which are common prey for the venomous spiders.
"Insects can still detect red light, but not as well as mammals or birds," Brandley said.
Using a spectroradiometer to pinpoint the wavelengths reflected by two of the three black widow species found in North America, the team calculated how much light is absorbed by the eyes of birds and insects. The results revealed that black widow warning colors are more visible to birds than insects, with a color contrast more than two times greater for birds.
The findings were published in the Feb. 27 issue of Behavioral Ecology.
