When Mother Nature closes a door, she opens a window, allowing each species to develop its own adaptations to make up for its deficits. Like jumping spiders: Scientists now say the tiny arachnids may have developed super-sensitive hearing to make up for their web-less lifestyle. A report on the spiders’ senses was published in the journal Current Biology.
Jumping spiders (family Salticidae) are big-eyed, fuzzy, and adorably small. The males of many species sport jewel-bright colors and engage in absurd exotic dancing to impress potential mates. Unlike most other spiders, jumping spiders don’t construct webs; instead, they stalk their teeny prey like teeny lions, then pounce.
Considering all this—plus the spiders’ lack of ears—scientists figured Salticidae species were mostly visual animals. Then researchers at Cornell University’s Hoy Lab stumbled onto some pretty compelling evidence to the contrary. They were researching jumping spiders’ visual processing abilities by hooking a single spider up to a brain activity monitor that made a popping noise each time the spider’s brain cells fired.
Co-author Paul Shamble recalls the day it happened. “[Researcher] Gil [Menda] was setting up one of these experiments and started recording from an area deeper in the brain than we usually focused on," he said in a statement. "As he moved away from the spider, his chair squeaked across the floor of the lab … and when Gil's chair squeaked, the neuron we were recording from started popping. He did it again, and the neuron fired again.”
Menda fetched Shamble to show him, and the two began talking about how spiders hear. “To demonstrate,” he said in the statement, “Paul clapped his hands close to the spider and the neuron fired, as expected. He then backed up a bit and clapped again, and again the neuron fired." Then they backed up again. And again. The neurons kept popping. The scientists were amazed.
Some real experiments were in order. First, they set spiders back up in the brain monitors and played a series of noises at various frequencies and distances. Next, they let the spiders roam around inside a mesh cage and videotaped their reactions to the same noises. Finally, they used a finely calibrated robot to vibrate a hair on the spider’s foreleg at the same frequencies while measuring the spider’s brain response.
The results of all three experiments agreed: The little spiders could hear just fine, even at distances of up to nearly 10 feet. They responded most strongly to low-frequency noises; when the scientists played the tone at 80 Hz, the spiders consistently froze in place like hunted animals. As it so happens, 80 Hz is also the frequency of the wingbeats of the predatory wasps and parasitic flies that prey on jumping spiders.
Previous studies have shown that freezing in place—part of the fight/flight/freeze survival response—may help animals focus their other senses and figure out what to do next. “All this might be particularly important … for species with vagabond lifestyles such as jumping spiders,” the authors write, “where the location of the threat and the subsequent route to safety may not be immediately apparent, instead requiring rapid and flexible decision-making.”
Stroking the spiders’ leg hairs also lit up acoustic pathways in their brains, which suggests that that’s how they’re taking in sound, even without eardrums.
The researchers say the spiders’ super-hearing may make up for their lack of webs, which serve as homes, safe spaces, defenses, and alarm systems for other spiders.
"In the movies, Spiderman [sic] has this strange, additional 'spidey sense' that helps him sense danger,” Menda said. “it turns out the real-life spidey sense of spiders might actually be hearing!”
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