Most spiders are known for being loners that keep to themselves. But in rare instances, some close-knit spider communities rely on strength in numbers. From a young age, the spiders in these groups show selfless behaviors such as joining forces on hunts or sharing meals with one another.
A new study published in Integrative Zoology suggests that these spiders get along so well partly because of what’s inside of their brains. Researchers found that certain brain regions of social huntsman spiders and social crab spiders — both of which live in groups — display slight variations compared to spiders that live alone. The distinct structure of these regions may be the reason why some spiders develop strong social skills while others have a preference for privacy.
Social Spiders Are Rare
If you ever find yourself peeling back the bark of a eucalyptus tree or a dead acacia tree in Australia, you could be in for a surprise that’ll make you jump out of your shoes. Chances are, you’ll come across a hidden conclave of social huntsman spiders, which live in groups containing potentially hundreds of individuals.
Not all spiders have grand get-togethers like this. In fact, the behavior is extremely rare among spider species — only 0.1 percent of the world’s 53,000 spider species live in groups.
Most spiders are territorial and become aggressive toward others of their kind, even resorting to cannibalism at times. Social spiders, on the other hand, usually get along well with their kin.
It turns out that a mother’s love helps to foster their more extroverted tendencies. Young social spiders experience an extended period of maternal care and have delayed dispersal, meaning they stay home longer after their first moult. These conditions seem to discourage them from eating their peers and encourage them to instead share their meals.
Read More: Meet the Biggest Spiders in the World and Where to Find These Massive Arachnids
Staining Spider Brains
There may be a deeper explanation for social spiders’ communal lives, one that requires a look into their brains. Researchers involved with the new study sought to determine what could be so different about social spiders’ brains that makes them more outgoing than solitary spiders. To accomplish this, they used a technique called brain staining, which allowed them to visualize multiple brain regions.
“We use a complex staining process that fixes different tissues inside the brain so you can see contrast between different structures,” said study author Vanessa Penna-Gonçalves, a behavioral ecologist at Macquarie University, in a statement. “Without using staining that reacts to different tissues, everything inside the brain just looks grey.”
After completing the staining, the researchers produced images of the spiders’ brains with micro CT scanning. From these images, individual structures in the brain needed to then be mapped.
Brains and Social Intelligence
The study examined the brains of six species: social huntsman spiders (Delena cancerides), social crab spiders (Xysticus bimaculatus), and four closely related solitary species.
The researchers found that while the size of each species’ brain and central nervous system was all similar, the two social species had a few structural differences in certain regions.
Social huntsman spiders had larger “arcuate bodies” and “mushroom bodies,” areas of the brain that play a role in memory and cognitive processing. These enlarged structures are possibly related to social huntsmen behaviors like recognizing kin and group coordination.
Social crab spiders had similar cognitive regions to their solitary relatives, but had uniquely large visual processing areas. This may have to do with them living in low-light leaf nests, where they will collectively target prey like beetles and ants. The researchers also found that social huntsman spiders have smaller venom glands than solitary huntsman spiders, hinting at their reliance on cooperation.
“Because the social huntsmen share the prey, each of them only needs to use a little bit of venom to kill the prey,” said Penna-Gonçalves. “Creating venom is very energy-expensive for them, so this makes all of the spiders in this species better off.”
The intricate brains of these spiders reveal how they socialize, but their significance may extend even further; the researchers suggest that this study could be a stepping stone to observing how brains influence social intelligence all across the animal kingdom.
Read More: Spiders Do Sleep — Some Even Dream While Dangling From Webs
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