Spider teamwork sparks surprising brain changes

When most people think of spiders, they picture a lone predator spinning a web in a corner – not a crowded social club. But some spiders break the mold, living together peacefully and even sharing meals. A new study has taken a deeper look – literally – into their brains to figure out why.

This research shows that spiders don’t need bigger brains to live in groups, but social ones are wired differently. Their brains are set up to help them remember, recognize others, and work as a team.

Inside a spider social club

Last year in the bush near Melbourne, something unexpected turned up under a strip of eucalyptus bark: more than a hundred huntsman spiders living together like a tiny society.

These spiders were part of a study comparing brain structures between social and solitary species. The researchers wanted to know whether group-living spiders needed bigger or more complex brains – a question tied to the “Social Brain Hypothesis.”

This theory suggests that animals living in groups need more brainpower to keep track of relationships, hierarchies, and social cues.

Social spiders are rare. Out of about 53,000 spider species, only about 0.1 percent live in groups. Most are loners – and not just any loners. They’re often aggressive and territorial, with a habit of cannibalizing neighbors. So finding spiders that live together peacefully is already unusual. But it turns out their brains are unusual too.

From spider loners to families

The spiders in the study didn’t start out social in a traditional sense. Instead, they seem to become social by staying put. Normally, young spiders leave the nest after their first molt, but in some species, they hang around longer.

“In those spider species who show an extended maternal care timeframe for young, we found the young don’t eat each other, and they start sharing prey,” said Dr. Vanessa Penna-Gonçalves, a Macquarie University data scientist and Ph.D. candidate.

In lab tests, baby spiders from social species even teamed up to hunt large prey like grasshoppers. Sometimes, one would take the lead, while others joined in to share the meal. That kind of behavior didn’t show up in solitary spiders.

Peering into spider brains

To understand what was going on inside their heads, the team had to get creative. Spider brains are difficult to study because they’re buried inside the fused head-and-body structure known as the cephalothorax. Muscles and other tissues crowd the space, making it hard to obtain a clear image.

The team used a special staining process to highlight different brain tissues. Some samples had to soak in stain for more than 100 days before they were ready for scanning.

“We use a complex staining process that fixes different tissues inside the brain so you can see contrast between structures. Without staining that reacts to different tissues, everything inside the brain just looks gray,” said Dr. Penna-Gonçalves.

After scanning the brains with microCT machines, the team manually mapped each structure – a tedious and slow process.

“We don’t have AI to do these things. We don’t yet have enough data, and the edges of spider brain structures require a trained eye to distinguish, so it takes a long time,” Dr. Penna-Gonçalves added.

Teamwork rewires spider brains

The study examined six spider species: two that live in groups – Delena cancerides (a social huntsman spider) and Xysticus bimaculatus (a social crab spider) – and four that live alone.

At first glance, the brain sizes were similar, which was surprising. But a closer look revealed key internal differences..

Social huntsman spiders had larger arcuate bodies and mushroom bodies – two regions tied to memory and thinking. These areas likely help them remember who’s a friend, coordinate with others, and manage life in a shared space.

The social crab spiders told a slightly different story. Their cognitive brain regions weren’t much different from those of their solitary relatives, but their visual areas were larger. That aligns with how they live – in small, dark nests with close-knit family groups where eyesight might play a bigger role.

“This makes sense when you think about their different social structures, because crab spiders live in small family groups in dark leaf nests, while huntsman groups can be complex communities with multiple adult females and males,” said Dr. Penna-Gonçalves.

Less venom, more energy

Another twist: social huntsman spiders had smaller venom glands than solitary ones. That matters, because making venom costs energy.

“Because the social huntsmen share the prey, each of them only needs to use a little bit of venom to kill the prey. Creating venom is very energy-expensive for them, so this makes all of the spiders in this species better off,” said Dr. Penna-Gonçalves.

The spiders’ social lives also changed with the seasons. “In the summer, we found more spider mothers with babies, but we also found more groups of females together and more eggs, which shows the society is more complex than we first thought,” said Dr. Penna-Gonçalves.

Spiders, brains, and teamwork

It might sound strange, but some spiders may even sleep – and dream. If spiders really do have dream-like states, that would suggest their brains are doing more than simply running on instinct.

“Discovering that spider brains can support sophisticated social behaviors challenges assumptions about intelligence in invertebrates,” said supervising author Professor Marie Herberstein.

The researchers say that spider brain structure may offer clues about how intelligence evolved in animals in general.

“We’ve shown that the volume of the brain is not a key indicator for intelligence, but rather intelligence reflects the amount of neurons and their connections in the brain,” said Dr. Penna-Gonçalves.

In the future, they plan to use a method nicknamed “brain soup,” which involves dissolving spider brains to count neurons. That might provide a clearer picture of how much brainpower spiders – and perhaps other small animals – really have. They are also interested in how social experiences affect brain development.

What began as a simple surprise under a piece of bark turned into a new way of thinking about intelligence, memory, and how far a little teamwork can go – even for spiders.

The full study was published in the journal Integrative Zoology.

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