In bushland near Melbourne last year, Dr Vanessa Penna Gonçalves was collecting certain 'social' species of huntsman and crab spiders for her PhD research, and peeled back a strip of eucalyptus bark to uncover a remarkable sight — a society of huntsman spiders living together in social harmony under a single piece of bark.
Field discovery: Dr Vanessa Penna-Gonçalves, above, collects social huntsman spiders from under eucalyptus bark, where colonies of up to 100 individuals live together.
"When I opened the bark, I was so surprised -— there were over a hundred spiders living in their little social community," says the Macquarie University data scientist and PhD candidate.
New findings from Dr Penna-Gonçalves ongoing research into spider brains and behaviour, just published in Integrative Zoology, shows while social and solitary spiders have similar overall brain sizes, the internal structures tell a completely different story.
The findings represent the first robust test of a theory called 'The Social Brain Hypothesis' in non-web-building spiders, examining whether animals living in groups need bigger brains to manage complex social relationships.
Social spiders scurry against the trend
Sociality in spiders is extraordinarily rare — only 0.1 per cent of the world's 53,000 spider species live in groups. Most spiders are hostile territorial loners, who, given the chance, will cannibalise their neighbours.
"Spiders are typically very aggressive towards other individuals," says Dr Penna-Gonçalves.
Spiders' social tendencies appear to stem from what Dr Penna-Gonçalves calls being 'lazy to launch', where young spiders stay home longer instead of dispersing after their first moult.
"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," she says.
Her laboratory experiments revealed striking behavioural differences between social and solitary species when presented with large prey such as a live grasshopper.
"Social spider species babies will collaborate to hunt, or one individual might hunt the prey, but the others come by and stay to share the meal," she says. "I didn't see this behaviour in the solitary babies."
Mapping microscopic minds
To study the inner workings of spider brains, Dr Penna-Gonçalves developed new techniques that pushed the boundaries of neuroscience technology, spending almost two years perfecting brain staining methods so she could get meaningful images via micro CT scanning.
Social connections: Professor Marie Herberstein, who supervised the spider brain research, says understanding neural differences helps reveal how social behaviour evolved.
"We use a complex staining process that fixes different tissues inside the brain so you can see contrast between different structures," she says. "Without using staining that reacts to different tissues, everything inside the brain just looks grey."
Some samples required more than 100 days of staining before they were ready for micro CT scanning, in a partnership with the University of Melbourne.
Spider's brains are particularly tricky to study because of their location.
"Most insects and animals have a separate head that contains their brain, but the brain of a spider sits inside of its fused head and thorax, the cephalothorax, surrounded by muscle and many other tissues, so it's difficult to isolate," she says.
Once scanned, each spider brain image required painstaking manual mapping of individual structures — a process that can't yet be automated.
"We don't have AI to do these things, we don't yet have enough data and the edges of the structures of spiders' brains requires a trained eye to be able to distinguish, so it takes a long time," says Ms Penna-Gonçalves.
Size isn't everything
The team compared brains from six species: social huntsman spiders (Delena cancerides), social crab spiders (Xysticus bimaculatus), and four closely related solitary species.
To their surprise, they discovered that the overall size of the brain and central nervous system showed no significant differences between social and solitary species.
However, internal brain structures revealed important differences. Social huntsman spiders had distinctly larger brain areas called 'arcuate bodies' and 'mushroom bodies', both regions of the brain that are linked with memory and cognitive processing.
Dr Penna-Gonçalves believes these enlarged structures probably support the complex social behaviours observed in huntsman spider colonies, such as recognising kin or friendly spiders from their own society, and group coordination.
Another interesting finding was the quite different patterns emerging from within the brains of social crab spiders, which had enlarged visual processing areas but similar cognitive regions to their solitary relatives.
"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," she says.
Sharing the load
One of the study's most intriguing discoveries was that social huntsman spiders have smaller venom glands compared to their solitary cousins — direct evidence that cooperation within the species gives them an evolutionary advantage.
"Because the social huntsmen share the prey, each of them only needs to use a little bit of venom to kill the prey," says Dr Penna-Gonçalves. "Creating venom is very energy-expensive for them, so this makes all of the spiders in this species better off."
This finding supports the idea that cooperative behaviours reduce the investment that individuals need to make in costly biological systems, such as venom production.
While collecting specimens across different seasons, Dr Penna-Gonçalves discovered that social structure varies throughout the year.
"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," she says.
Do spiders dream?
Recent research suggests that some spiders may even sleep and dream.
"Discovering that spider brains can support sophisticated social behaviours challenges assumptions about intelligence in invertebrates," says supervising author Professor Marie Herberstein.
She says that Dr Penna-Gonçalves' mapping of the neural landscapes of tiny spider brains along with observations of their behaviours, could deliver insights beyond spider biology to help understand how social intelligence evolved across the animal kingdom.
"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," says Dr Penna-Gonçalves.
Future research could employ new techniques like 'brain soup' — literally dissolving brains to count individual neurons — which may provide more accurate measures of cognitive capacity than brain volume alone. The team also plans to test how social experiences during development shape brain structure.
Dr Vanessa Penna Gonçalves is a behavioral ecologist and research assistant in the School of Natural Sciences, Faculty of Science and Engineering.
Marie Herberstein is a Professor in the School of Natural Sciences, Faculty of Science and Engineering.
