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The Distributed Brainpower of Social Insects

Attenborough-brainsHere’s David Attenborough, chilling out on a rock in the middle of Africa, with four lumps of plasticine. The smallest one on the far left represents the brain of a bushbaby, a small primate that lives on its own. The next one is the brain of a colobus monkey, which lives in groups of 15 or so. The one after that is a guenon, another monkey; group size: 25. And on the far right: a baboon that lives in groups of 50. “Were you to give a skull to a researcher who works on monkeys, even though they didn’t know what kind of monkey it belonged to, they would be able to accurately predict the size of group in which it lived,” says Attenborough.

That sequence, from The Life of Mammals, is a wonderful demonstration of the social brain hypothesis—a bold idea, proposed in the 1980s, which suggests that living in groups drove the evolution of large brains. Social animals face mental challenges that solitary animals do not: they have to recognise the other members of their cliques, cope with fluid and shifting alliances, manage conflicts, and manipulate or deceive their peers. So as social groups get bigger, so should brains. This idea has been repeatedly tested and confirmed in many groups of animals, including hoofed mammals, carnivores, primates, and birds.

What about insects? Ants, termites, bees, and wasps, also live in large societies, and many of them have unusually big brains—at least, for insects. But in 2010, Sarah Farris from West Virginia University and Susanne Schulmeister from the American Museum of Natural History showed that in these groups, large brains evolved some 90 million years before big social groups. If anything, they correlated with parasitic body-snatching rather than group-living.

“That got people thinking,” says Sean O’Donnell from Drexel University. “In recent years, there’s been a growing rumbling, almost subterranean movement arguing that social brain ideas may not apply to the social insects.” His new study is the latest addition to that movement.

O’Donnell’s team studied potter wasps, which lead solitary lives, and the closely related paper wasps, which live in colonies of varying sizes and complexities. They collected queens and workers from 29 species of these wasps, carefully dissected their brains, and measured the size of their mushroom bodies—a pair of structures in insect brains that control higher mental abilities like learning and memory.

And to their surprise, they found that as the wasp colonies got bigger, their mushroom bodies got smaller. Even within the social paper wasps, the team found that species with distinct queens and workers—a sign of a more complex society—have similarly sized mushroom bodies to those with no such castes.

“The pattern is so clear,” says O’Donnell. “Sociality may actually decrease demands on individual cognition rather than increasing it.”

“Here we have the first concerted evidence that costly brains aren’t needed to allow sociality, when you can do it other ways,” says Robin Dunbar, who first proposed the social brain hypothesis. “There are many routes to sociality.”

What other routes? O’Donnell notes that insects and (most) mammals build their societies in fundamentally different ways. Large mammal societies typically include individuals who are distantly related or even unrelated. Insect societies, by contrast, are basically gigantic families, where all the members are either queens (which reproduce) or their descendants (which do not). You could view these colonies less as groups of individuals and more as extensions of the queens.

As such, their members don’t particularly need to keep track of shifting relationships, or manage conflicts, or manipulate their peers, or any of the other social challenges that, say, a baboon or a human faces. They have less of a need for bigger and more sophisticated brains.

Social insects also benefit from swarm intelligence, where individuals can achieve astonishing feats of behaviour by following incredibly simple rules. They can build living buildings, raise crops, vaccinate themselves, and make decisions about where to live. In some cases, they make decisions in a way that’s uncannily similar to neurons—a colony behaves like a giant brain, and in more than a merely metaphorical way. They have a kind of ‘distributed cognition’, where many of the mental feats that other animals carry out using a single brain happen at the level of the colony.

Entomologist Seirian Sumner from Bristol University says that there are mammals, like meerkats and banded mongooses, which live in simple societies where adults cooperatively raise their young. These are often compared to primitively social insects, like paper wasps. “They share very similar family structures, group sizes and plasticity in behavioural roles,” Sumner says. It would be very interesting to see if the brains of these mammals follow the same patterns as those of O’Donnell’s wasps.

O’Donnell is all in favour of more studies. He wants to see if the same patterns hold in other insect groups that include both social and solitary species, including bees and cockroaches. And he’s intrigued by the naked mole rats—colonial mammals that have queen and worker castes, much like ants and wasps. “If our ideas are correct, we’d expect to see mole rats following a similar pattern to insects,” he says.

Reference: O’Donnell, Bulova, DeLeon, Khodak, Miller & Sulger. 2015. Distributed cognition and social brains: reductions in mushroom body investment accompanied the origins of sociality in wasps (Hymenoptera: Vespidae). Proc Roy Soc B. Citation tbc.

PS: The size of brain regions isn’t always the best indicator of intelligence. I asked O’Donnell about this, and he stands by his decision to focus on the mushroom bodies. “It’s definitely a blunt tool for studying brain evolution,” he says. “Brain tissue is metabolically very expensive, and even if it was just filler, tissue weight is a big deal, especially for a flying insect. We expect there to be really strong constrains on the size of the [mushroom bodies].”

5 thoughts on “The Distributed Brainpower of Social Insects

  1. Cool piece.
    I wonder — What about human brain size, evolution, and the size of human/human progenitor social group sizes?

  2. One of the other big disadvantages of the ‘ant method’ would be the myrmecophiles.

    There are tons of critters that exploit the heck out of the ants, whereas we mammals don’t usually have to worry about gigantic things that smell a bit like us setting up shop in our day cares and eating our babies.

  3. Question: Do meerkats and banded mongooses compete, within their social groups, for sexual partners?
    If so, we can expect the normal mammalian brain rules will apply.
    Applying the word ‘social’ to all fauna that live communally might not be as helpful as it first appears. Certainly, as far as primates are concerned perhaps, social/anti-social might be a more accurate descriptor. From reading Holldobler and Wilson’s amazing ‘Journey to the Ants’, it would seem that, with the exception of a little regicidal infanticide, agression is limited to those outside the social group.
    Communal mammals do not play nearly so nicely.

  4. As we humans struggle to extend the reach of our social circles, now through the Internet and the social networks it supports, we keep evolutionarily pushing for greater intelligence.
    Being addict to social networks may not be that bad after all, it will increase our intelligence! but its benefits will only become apparent in future generations.

  5. @Will Holz – Don’t be so sure. Human societies have their parasites, it’s just that we tend not to notice them (as and sprobably fail to understand the processes going on in the anthill). What else would you call people who take away your children at an early age and indoctrinate them to work or even die for the benefit of a select few (not a conspiracy freak here, just stating one of the many obvious parallels between human and insect societies)

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