Gil Menda, Hoy Lab, Cornell University
Gil Menda, Hoy Lab, Cornell University

Jumping Spiders Can Hear Just Fine, Thank You Very Much

Gil Menda, Hoy Lab, Cornell University
Gil Menda, Hoy Lab, Cornell University

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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Animals
How Bats Protect Rare Books at This Portuguese Library
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Visit the Joanina Library at the University of Coimbra in Portugal at night and you might think the building has a bat problem. It's true that common pipistrelle bats live there, occupying the space behind the bookshelves by day and swooping beneath the arched ceilings and in and out of windows once the sun goes down, but they're not a problem. As Smithsonian reports, the bats play a vital role in preserving the institution's manuscripts, so librarians are in no hurry to get rid of them.

The bats that live in the library don't damage the books and, because they're nocturnal, they usually don't bother the human guests. The much bigger danger to the collection is the insect population. Many bug species are known to gnaw on paper, which could be disastrous for the library's rare items that date from before the 19th century. The bats act as a natural form of pest control: At night, they feast on the insects that would otherwise feast on library books.

The Joanina Library is famous for being one of the most architecturally stunning libraries on earth. It was constructed before 1725, but when exactly the bats arrived is unknown. Librarians can say for sure they've been flapping around the halls since at least the 1800s.

Though bats have no reason to go after the materials, there is one threat they pose to the interior: falling feces. Librarians protect against this by covering their 18th-century tables with fabric made from animal skin at night and cleaning the floors of guano every morning.

[h/t Smithsonian]

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Honey Bees Can Understand the Concept of Zero
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The concept of zero—less than one, nothing, nada—is deceptively complex. The first placeholder zero dates back to around 300 BCE, and the notion didn’t make its way to Western Europe until the 12th century. It takes children until preschool to wrap their brains around the concept. But scientists in Australia recently discovered a new animal capable of understanding zero: the honey bee. According to Vox, a new study finds that the insects can be taught the concept of nothing.

A few other animals can understand zero, according to current research. Dolphins, parrots, and monkeys can all understand the difference between something and nothing, but honey bees are the first insects proven to be able to do it.

The new study, published in the journal Science, finds that honey bees can rank quantities based on “greater than” and “less than,” and can understand that nothing is less than one.

Left: A photo of a bee choosing between images with black dots on them. Right: an illustration of a bee choosing the image with fewer dots
© Scarlett Howard & Aurore Avarguès-Weber

The researchers trained bees to identify images in the lab that showed the fewest number of elements (in this case, dots). If they chose the image with the fewest circles from a set, they received sweetened water, whereas if they chose another image, they received bitter quinine.

Once the insects got that concept down, the researchers introduced another challenge: The bees had to choose between a blank image and one with dots on it. More than 60 percent of the time, the insects were successfully able to extrapolate that if they needed to choose the fewest dots between an image with a few dots and an image with no dots at all, no dots was the correct answer. They could grasp the concept that nothing can still be a numerical quantity.

It’s not entirely surprising that bees are capable of such feats of intelligence. We already know that they can count, teach each other skills, communicate via the “waggle dance,” and think abstractly. This is just more evidence that bees are strikingly intelligent creatures, despite the fact that their insect brains look nothing like our own.

Considering how far apart bees and primates are on the evolutionary tree, and how different their brains are from ours—they have fewer than 1 million neurons, while we have about 86 billion—this finding raises a lot of new questions about the neural basis of understanding numbers, and will no doubt lead to further research on how the brain processes concepts like zero.

[h/t Vox]

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