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."
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.
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.
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."