James Traniello doesn’t like to play favorites. For almost 40 years, Traniello, a professor of biology at Boston University, has devoted his life to the study of ants, investigating their extraordinary social lives. And like a father describing his children, he finds each species wonderful in its own way. But when pressed, he admits to having “a thing” for Oecophylla smaragdina, the Australian weaver ant. These green ants make their homes in rainforest canopies, building elaborate nests. Worker ants (all female) form living ladders with their bodies between adjacent leaves, pulling them together. When the leaves touch, the ants sew them together with silk squeezed from larvae like toothpaste from a tube. (The larvae don’t seem to mind.) The result is a durable nest about the size of a football, built in less than a day. The process is a remarkable display of the cooperative behavior that makes ants—by their sheer numbers and richness of species (about 20,000 at last count)—one of the most dominant social organisms on the planet. That’s part of the reason for Traniello’s crush on the weaver ant, but it’s not the whole story.
“They’re also renowned for their territorial behavior,” says Traniello. “If you look at them in the lab, they stop and look at you. They stand up, open their mandibles, raise their gasters, and threaten you.” Traniello pauses for a moment, considering an ant the size of an eyelash facing off against a human. “That’s pretty gutsy. It takes a lot of ovary to do that.”
Ant and human, eyeball to eyeball. We are linked with the ants in ways both surprising and profound. Humans, as you may have guessed, are the other dominant social organism on the planet. Both ants and humans divide labor and form complex social networks. Both work in groups to accomplish tasks—leaf nests, Mayan temples—that no individual could complete alone. Both raise children in families. Both use the same class of neurotransmitters—“biogenic amines” like dopamine and serotonin—to govern behavior. We both go to war.
There are differences, of course, and here’s a big one: humans build skyscrapers and societies with brains that are large relative to their body size. Ants weave nests, navigate dark forests, and even farm food with brains that are downright diminutive. This may not seem surprising at first, but consider this: only a measly two percent of insects—ants, bees, some wasps and termites—live in societies. Most insects are like fruit flies, buzzing around, doing their own thing, every fly for himself. So the ultra-social ants, operating with brains up to 600 million times smaller than humans, made even Charles Darwin step back in awe. “The brain of an ant is one of the most marvelous atoms of matter in the world,” he wrote in 1871, “perhaps more so than the brain of man.’’
How can ants do so much, with such tiny brains? That leads to the central question of Traniello’s research: How does collective intelligence influence brain evolution? And how does brain size and shape and neurochemistry relate to social behavior? In 2014, he and his collaborators received, along with collaborators Corrie Moreau from the Chicago Field Museum and Wulfila Gronenberg at the University of Arizona, a four-year, $1.44 million grant from the National Science Foundation to continue his work comparing the size and structure of ant brains and how they relate to their complex social organization. The grant will allow his research team to continue unraveling the natural history of the ant, which will ultimately lead to insights into the evolution and neurobiology of all animals that form societies. Including, maybe, us.
Let’s take a moment to consider the leafcutter ant, which BU PhD student Andrew Hoadley studies in Traniello’s lab. These tropical ants cut bits of leaves and flowers, mulch them, and use the mulch to grow fungus. The ants actively farm their fungus, feeding it with fresh leafy bits and keeping it free from molds and waste. The farm is organized by sharp divisions of labor, with certain ants specialized for leaf cutting, harvesting, and sharing the fungus. (As in all ant species, all of the workers are female.) The queen lays eggs in the fungus, which serves as both a home for the larvae and food for the colony. The elaborate farm has no centralized control, just a lot of ants with really small brains.
“You have teeny little workers who are only good at tending the fungus, and then huge muscular soldiers,” says Hoadley. “And you get this amazing agricultural system arising with no foreman—actually forewoman would be more accurate—directing individual workers what to do.”
Ants, unlike humans, live only as social animals. Removed from a colony, most ants will die within hours, lost and alone. This is because they divide tasks among specialists—the queen reproduces, the soldiers guard the colony, and so on. Ants also communicate continuously by secreting and sensing pheromones, chemical signals that are specific to each species and tell other ants to, say, attack an invader or follow a trail through the woods. While this way of life means that one ant alone will almost always die, it allows groups of ants to perform astonishing tasks with no centralized organization.
“If we humans were to cooperate on something, we’d have a plan. We’d say, ‘We’re going to go have lunch. Where are we going to have lunch?’ and so forth,” says Traniello. “For ants to go raid a termite mound, it’s all based on communicating with pheromone trails. The intelligence emerges from collective organization. And in doing that, they’re able to achieve just incredible things through the emergent abilities of a colony.”
So how do they do it? Is there something special about their brains? “How do these individuals who are not very smart alone come together to complete these sophisticated tasks? Where does the collective intelligence come from?” asks Moreau, an associate curator at the Field Museum. “James’ passion is to understand the mechanisms behind what’s driving the behavior. One of the best ways to do this is to compare the brains of different organisms.”
On a cellular level, ant and human brains work pretty much the same: we all have nerve cells that collect sensory information, fire electric impulses, and communicate with chemical signals. “If you look at a human nerve cell or an ant nerve cell, they look pretty similar,” says Gronenberg, who is an associate professor of neurobiology. “Whole ant brains look different than ours, but they have structures that perform similar functions as the human brain.”
One of these structures, called a “mushroom body,” may be part of the ants’ secret to social success. The mushroom bodies—which do actually look like cremini mushrooms—are distinct areas in insect brains associated with learning and memory, much like the human cortex. “They’re the sensory and learning integration part of the insect brain,” says Darcy Gordon, a BU PhD candidate in ecology, behavior, and evolution and one of Traniello’s students. “All insects have them, but there’s a lot of variation among species.” Former BU postdoctoral faculty fellow Mario Muscedere, now an assistant professor at Hendrix College, recently found that the mushroom bodies of the ant brain are relatively large, even in the tiniest ant species. However, not all ants have an equal slice of the mushroom pie, so to speak. The more specialized a worker, the smaller the mushroom bodies. Some workers in the seed-eating species Pheidole pilifer, for instance, spend their days doing one thing and one thing only: grinding seeds for food with their giant mandibles. Their mushroom bodies are inordinately small. “There’s not much thinking going on,” says Traniello. “So you can see how brain anatomy can reflect behavior. You can see the imprint of natural selection.”
Gordon is taking the work a step further, comparing the brain anatomy among different ant species and among castes within a species. Gordon studies the brains of Pheidole rhea, one of the species of ant with three distinct types of workers, all of which look quite different: minors, majors, and supermajors. In this species, the minor workers are the go-to ants for the colony. They forage for seeds, maintain the nest, and care for the larvae. The majors are soldiers—they stay in the nest until there’s a threat (like rival ants approaching, or a grad student blowing on it); then, they come out to fight. And the supermajors help with foraging and fighting, but the really striking thing about them is their giant heads. “Oh yeah, they’re bobbleheads,” says Gordon, pointing one out as it scrabbles around its plastic box. “Their heads are so big that when they fall over, sometimes they can’t get up. They just wave their legs in the air and do that turtle thing.”
Only 8 of the 1,100 species of Pheidole have supermajors, and they’re all found in the American Southwest. “There must be some sort of ecological advantage to having supermajors,” says Gordon. “But it’s still a mystery.” Some scientists have seen Pheidole supermajors using their giant heads to block the entrance to the nest when it was attacked by army ants. But if the giant heads are just giant corks, what does this mean for their brains? When Gordon examined the brains of each caste under a microscope, she found that the supermajors had proportionally larger optic lobes, and smaller mushroom bodies, than their fellow workers. She’s not sure why yet, but it seems likely that behavior is mirrored in their anatomy. Gordon’s next step is to compare these ants with other species to see if the supermajors’ brains are unique, and if clear patterns emerge between brain anatomy and behavior. “It leads to interesting questions about development,” she says.
“Supermajors” had proportionally smaller mushroom bodies than their fellow workers. The question is: why?
The work may ultimately shed light on human evolution, since there are hints that the mushroom bodies may arise from the same genes as the human cortex, the part of our brain that handles most of our information processing and also plays a key role in memory, language, and consciousness. While this is a new and provocative idea, still being debated among scientists, Traniello says it holds merit.
“Some researchers believe that the genetic toolkit that eventually was involved in the evolution of the vertebrate cortex was similar to the genetic toolkit that arthropods use to make the mushroom body,” says Traniello. The genetic similarities “suggest that the relationship is pretty deep.”
Angry Ants on Drugs
For both ants and humans, brain anatomy is only part of the story behind social behavior. Another piece of the puzzle is brain chemistry. “There are many ways to get to the bottom of complex behavior. One way is to measure brain structures and compare them, another way is to get down to the functional components. Which neurons are connecting with which, and what chemicals are involved?” asks Moreau. “It’s a level of detail on a much finer scale, and it speaks to James’ innovative approach. You could just look at the structure, but he wants to go further and find the mechanism. What’s really going on?”
This brings us back to the Australian weaver ants that have so smitten Traniello. BU PhD candidate Franne Kamhi is equally awed. “They have a personality. They’re very aggressive, very visual—you can see them watching you,” she says. “I’m just fascinated by the fact that a small brain can accomplish so many cool things, like building chains out of their bodies. It’s just fun to watch them in the field and then be able to study what’s actually going on in their brains.”
Kamhi also studies social brain evolution, from gross anatomy to synaptic connections. And as a neuroscientist, she’s interested in the role that neurotransmitters play in shaping the brain. She’s focused on a group of molecules called the biogenic amines, like dopamine and serotonin, which are also important in human behavior. Dopamine plays a critical role in pleasure and satiation, and altered serotonin levels can make a person depressed or aggressive. There’s also a lesser-known molecule, octopamine, found mostly in insects. (It’s called octopamine because it was first discovered in the octopus.) Humans also have traces of octopamine, but its role in our bodies is unclear. However, the human stress hormone norepinephrine, responsible for our “fight-or-flight” response, has a similar structure to octopamine and plays a similar role in our bodies.
Kamhi began by watching the weaver ants in the lab and monitoring them for aggressive behavior. Weaver ants have two castes of workers: majors and minors. The majors weave nests and defend the colony, while minor workers generally stay in the nest and care for the brood. Kamhi found that major workers were far more aggressive than minors: more mandible flaring, head lunging, biting, and other nasty behavior.
Was this aggression tied to brain chemistry? Kamhi measured the levels of dopamine, serotonin, and octopamine in both castes of workers, and found something unexpected. While both minors and majors had similar levels of dopamine and serotonin, the majors had much higher levels of octopamine, even when scaled for differences in brain size. “We were surprised,” says Kamhi. “We thought it would be serotonin, since that’s been linked to aggression in many other species, though octopamine is implicated in aggression in crickets and fruit flies.” Kamhi is now experimenting to see if blocking the octopamine receptor in majors will make them less aggressive, or increasing octopamine levels in minors will make them more so. She’s also studying the weaver’s less-socially adept sister species Formica subsericea to understand the interplay of anatomy, chemistry, and behavior.
What might this mean for us? “There’s no way to say, ‘it works this way in the ant, so it will work this way in humans,’ but I think it could lead to some insight,” says Kamhi. “It’s the same neurotransmitters—or similar, in the case of octopamine—they work on similar receptors, the neurons function the same way in ants as in vertebrates. So I definitely think you can learn, in general, how our brain functions by studying ants. And it’s a smaller brain, so you have a better chance of actually figuring out what’s going on.”
The Incredible Shrinking (Human) Brain
All this research comparing ant brains may ultimately yield insight into one of the big mysteries in human evolution: why our brains are shrinking. Although our brains are pretty big, about 30,000 years ago they began to gradually get smaller. No one is exactly sure why our brains got so big in the first place, but many scientists presume it’s because early humans started living in larger groups and engaging in more complex social interactions.
But then as society grew even more complex, our brains weirdly began to shrink. The average male human brains have decreased from a peak of about 1,500 cubic centimeters to the modern average of about 1,350 cubic centimeters. (The trend is similar for human females.) That’s a loss of 150 cubic centimeters—a volume about the size of a tennis ball. That seems like a lot of brain to just disappear.
“It’s a real mystery,” says Jeremy DeSilva, an assistant professor of anthropology at BU who has been studying the question with Traniello. “If you talk to the average person on the street, they think our brains have just been getting bigger and bigger. Sometimes you even see these projections of what the human brain will look like in an thousand years, and they look like these monstrous science fiction brains. But for the last 30,000 years, they’ve been getting smaller. And not only that, they’ve been getting smaller at a faster rate than they got bigger. At the same time, most people agree that we’re not getting stupider, so what’s going on?”
A clue may come from the ants. Maybe, says Traniello, because ants have these complex social networks, each individual ant needs less brain. “Collective intelligence is producing evolutionary selection for smaller brains, saving a lot of energy because brain metabolism is very expensive,” says Traniello. “Individuals are partitioning tasks, specializing. They need smaller amounts of brain to do that, and overall there are payoffs to the colony.”
And—aha!—maybe this is why our brains are shrinking, too. Since brain tissue consumes a whopping 20 percent of our metabolism, the less brain we have to feed, the better.
“James proposed this idea from the ant world,” says DeSilva. “When you have a critical number of individuals, it leads to emergent intelligence—society is complex enough that not everybody needs to know how to do everything, we can turn some tasks over to others. It’s a fascinating idea, but it’s really difficult to test in humans. So social insects can provide a wonderful model for the evolution of complex organisms like us.”
E. O. Wilson, one of the first scientists to look at the biological origins of social behavior and one of Traniello’s thesis advisors, has long said that ants may teach us about ourselves. “Scientists have begun to turn to the ants and other social insects for a new kind of wisdom,” wrote Wilson in his book The Social Conquest of Earth. “Although these small creatures are radically different from us in many ways, their origins and history shed light upon our own.”
Scientific Name: Atta cephalotes
Common Name: Leafcutter Ant
Distinguishing Feature: Evolved complex agriculture and the use of antibiotics millions of years before humans. Foragers cut leaves, bring them to the colony, and use the mulch to grow edible fungus.
Worker Size and Behavior: Highly size-specialized; the smallest ants are gardeners, while the largest defend the colony.
Colony Size: > 1,000,000 workers when mature.
Habitat: Tropical forests in Central and South America.
Temperament: Watch out for their mandibles!
Ecofact: Leafcutter ants are the dominant herbivores in neotropical forests.
Less-Socially Adept Sister Species: Mycocepurus goeldii, which forms very small colonies, has little worker-size variation, and uses caterpillar dung to culture their fungus.
Scientific Name: Pheidole dentata
Common Name: Big-Headed Ant
Distinguishing Feature: The worker caste is typically divided into two distinct sizes, minors and majors, that specialize in different tasks.
Worker Size and Behavior: Head width of minors (nurses, foragers) is 0.6mm; head width of majors (defenders) is 1.15mm
Colony Size: 1,000 to 5,000
Habitat: Ground and wood nesting, common in the southeastern US.
Temperament: Very active, mildly aggressive
Ecofact: There are more than 1,100 species of Pheidole, comprising approximately 10% of all known ant species.
Less-Socially Adept Sister Species: Cephalotes varians, turtle ants, which have a major worker caste with saucer-shaped heads that functions as a “living door” to the nest.
Scientific Name: Oecophylla smaragdina
Common Name: Australian Weaver Ant or Green Tree Ant
Distinguishing Feature: Workers build leaf nests in the rainforest canopy by forming “chains” from their bodies to pull and anchor leaves that are sewn together using silk secreted by larvae.
Worker Size and Behavior: Head width of minors is 1.15mm and head with of majors is 1.6mm, with some workers of intermediate size
Colony Size: Up to 500,000, spanning territories of up to a mile!
Habitat: Tropical forests in Southeast Asia and Australia
Degree of Task Specialization: Minor workers nurse the young inside the nest and majors perform most other colony tasks such as foraging and territorial defense
Temperament: Highly aggressive—quick to bite and spray acid!
Ecofact: Green Tree Ants play an important role in Aboriginal culture, including indigenous farming, folklore, and medicinal uses like contraception and curing headaches.
Less-Socially Adept Sister Species: Formica subsericea, which has much smaller colonies and seemingly low collective intelligence.