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Study: Lack of Social Interactions Could Lead Spiders to Become More Aggressive

Many researchers believe that spiders become more aggressive as they grow, which drives them to avoid each other

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The researchers hope to learn what happens to the loners that makes them more likely to attack other spiders they encounter. Pixabay

Baby spiders like to mingle, but adult spiders tend to eat each other. New research published in PLOS Biology found that adult spiders seem to forget how to behave with each other after being alone too long, which causes them to become aggressive. These findings could help researchers understand why some spider species like to hang out together their whole lives but most would eat another spider if given the chance.

Regardless of how you feel about spiders, they’re an important part of many ecosystems. Despite that, they are often misunderstood, said Violette Chiara, a graduate student at the University of Toulouse, France, who led the study.

“Spiders are not just aggressive, cannibalistic monsters,” Chiara said. “There are spiders that are social at the beginning of their lives, and there are also some species that remain social during their whole lives.”

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FILE – A spider is seen on a flower. New research published in PLOS Biology found that adult spiders seem to forget how to behave with each other after being alone too long, which causes them to become aggressive. VOA

A friendly start in life

Baby spiders, known as spiderlings, begin their lives cozied up to their siblings — sometimes as many as several hundred. But when they grow up, they tend to live alone. Of the more than 40,000 known spider species, all but 30 lead solitary lives in adulthood.

It’s not clear why so few spider species remain social their whole lives. Many researchers believe that spiders become more aggressive as they grow, which drives them to avoid each other. Chiara decided to test which comes first: aggressive behavior or social isolation.

Chiara and team members Felipe Ramon Portugal and Raphael Jeanson studied labyrinth spiders, which are common in France. They observed that baby labyrinth spiders started to move away from each other five days after emerging from their eggs, which the researchers initially thought pointed to a natural increase in aggression.

However, they found that even spiderlings raised alone started to move around more after five days. In other words, the spiders weren’t fleeing from their siblings because they were worried they’d get eaten, they were just stretching their (many) legs and becoming more mobile as they grew.

If an increase in aggressive behavior doesn’t happen naturally as the spiders age, something must cause it. To test this, the researchers raised some spiders alone and others in groups. They then brought together pairs of spiders that weren’t familiar with each other to see if they reacted peacefully or violently.

Spiders that were raised in groups almost never tried to eat the unfamiliar spider, but those raised alone went on the attack 40% of the time. The more time they had spent alone, the more likely they were to try to eat the unfamiliar spider. The researchers concluded that isolation causes spiders to become aggressive and not the other way around.

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Spiders that were raised in groups almost never tried to eat the unfamiliar spider, but those raised alone went on the attack 40% of the time. Pixabay

Questioning assumptions

Jonathan Pruitt, an evolutionary ecologist who was not involved in the research, said that the study was a good example of “going back and scrutinizing what a lot of people have assumed but don’t even realize that they’ve assumed, and questioning it and finding a very different story.”

The researchers hope to learn what happens to the loners that makes them more likely to attack other spiders they encounter. “We know that they’re more aggressive, but from a cognitive point of view, what is the change?” said co-author Raphael Jeanson.

One possibility is that when spiders spend too much time away from other spiders, they forget how to read social cues — in this case, chemicals in their “skin” that help them recognize each other. The researchers hope to explore this possibility by studying a species that is closely related to labyrinth spiders, but lives in groups all their lives.

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Leticia Aviles, a specialist in social spiders who was not involved in the study, agreed that the lack of social interactions could lead spiders to become more aggressive.

“When they remain together, they are familiar with each other, they have these chemical cues that they can read from each other, so they remain tolerant. But when they have been isolated, the familiarity is lost, and that’s what leads to this intolerant and aggressive behavior,” Aviles said. “I think that has implications for all kinds of systems, not just spiders.” (VOA)

Next Story

Electric Fields Used By Spiders To Take Flight: Research

Charles Darwin remarked on the behavior when tiny spiders landed on the HMS Beagle, trailing lines of silk.

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A spider sits in her web. Researchers from England think spiders might be sensing and using electrostatic fields to become airborne.
A spider sits in her web. Researchers from England think spiders might be sensing and using electrostatic fields to become airborne. VOA

Since the 1800s, scientists have marveled at how spiders can take flight using their webbing. Charles Darwin remarked on the behavior when tiny spiders landed on the HMS Beagle, trailing lines of silk. He thought the arachnids might be using heat-generated updrafts to take to the sky, but new research shows a totally different cause may be at play.

Erica Morley and Daniel Robert from the University of Bristol in England were interested in exploring a second explanation for the spiders’ ability. They thought spiders might sense and use electrostatic fields in the air.

“There have been several studies looking at how air movement and wind can get spiders airborne, but the electrostatic hypothesis was never tested,” Morley told VOA.

Some observers suggested electrostatic fields might be the reason the multiple draglines some spiders use to float don’t get tangled with each other. Biologist Kimberley Sheldon from the University of Tennessee at Knoxville, who was not involved in the new research, pointed out that “though these spiders will have five or six draglines, those strands of silk do not get entangled. So we’ve known for a while that electrostatics probably [are] at least interacting with the spider, with the silk lines themselves, to keep them from getting tangled.”

Morley and Robert created a box with a grounded metal plate on the bottom and a plate on the top that they could pass an electrical current through. The scientists placed spiders in the box and turned on the voltage, watching as the creatures reacted to the electric field.

Reaction to current

When the electric field was on, the spiders lifted their abdomens into the air and started tiptoeing by raising up on the very ends of their legs. Morley told VOA that spiders only tiptoe right before they release silk draglines to fly away, in a process called ballooning.

And when the spiders did balloon and rise into the air, turning off the electric current caused them to drop.

Sheldon compared it to taking a balloon and rubbing it against your clothing. “If you hold the balloon [near your head], your hair stands on end. That’s kind of what’s happening with the spider silk.”

Spiders Use Electric Fields to Take Flight: Research
Spiders Use Electric Fields to Take Flight: Research. Pixabay

Clearly the spiders were able to sense the local electrostatic field and respond appropriately by releasing silk, but Morley and Robert wanted to know how.

“As a sensory biologist, I was keen to understand what sensory system they might use to detect electric fields,” said Morley. “We know that they have very sensitive hairs that are displaced by air movements or even sound. So I thought that it’s possible that they might be using these same hairs to detect electric fields.”

This was exactly what she observed. The small hairs along the spiders’ legs react not only to physical experiences like a breeze but also to the electric field. In nature, it makes sense for spiders to sense both the electrostatic field around them as well as wind conditions. Spiders probably use both when taking off and navigating the skies.

Mathematician Longhua Zhao from Case Western Reserve University in Cleveland has made computer models of how spiders balloon. She told VOA, “I think that both the electrical field and the fluid mechanics [of air flow] are important. They definitely play very important roles. However, we don’t know at this point which is the dominant factor.”

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Lead researcher Morley pointed out that spiders aren’t the only invertebrates to balloon. “Caterpillars and spider mites, which are arachnids but not spiders, balloon as well.” Morley hopes to see others follow up her research to see if these other animals respond in a way similar to the spiders. (VOA)