Notes from Underground

It’s an unlikely starting point for neurobiology research, but James Traniello is in the woods in Concord, Mass., striding along paths trod by Thoreau. Sweet fern perfumes the air and the ubiquitous Queen Anne’s lace pokes up its white flowers along the side of the trail, even as the Route 2 traffic noise buzzes through the woods. Traniello finds what he is looking for, tell-tale signs of the ant species Pheidole pilifera: tiny piles of seed hulls at nest entrances. It’s warmer on the sun-drenched trail than it is in the woods, so ants build their homes underneath it.

Traniello (CAS’74), a College of Arts and Sciences biology professor, comes here with his students and colleagues to dig for living colonies of P. pilifera to take back to his Cummington Street lab. They excavate the nest, suck the ants up with an aspirator, then sort them out, making sure they have the queen, essential to keeping a colony alive.

Almost all ants in a colony are sterile workers and sisters, with assigned roles from nurse to nest keeper to forager to soldier. Traniello wants to know what happens in ant brains that leads to behavioral specialization. Until recently, scientists didn’t have the tools to even begin answering this question. But with new imaging techniques and neurochemical analyses, the sociobiology of ants is becoming clearer.

Now, with help from a nearly half-million dollar grant from the National Science Foundation, Traniello and his colleagues are trying to discover the neurological basis of ant behavior, and by looking at several different species at the same time, they also hope to understand the evolution of the structure of ant brains. These aren’t just idle questions: Edward O. Wilson, one of Traniello’s doctoral advisors at Harvard, has famously suggested that ant behavior is helpful in understanding the behavior of certain upright biped creatures as well.

Ants are probably Earth’s most successful species. Some 100 million years ago, they branched off from the wasp family, became entirely social, and never looked back. With more than 12,000 species, ants now inhabit every continent except Antarctica, and it’s estimated that they make up 15 to 25 percent of all animal biomass in some regions of the planet.    

But they get a bad rap. Look carefully at an ant hole in a lawn or a crack in the sidewalk, and they seem to be working like, well, ants, steadily and mindlessly digging or lugging crumbs and tiny dead bugs into their nest, relentless in their efforts, automatons on a preprogrammed mission.

Look more closely, though, when pavement ants swarm on sidewalks in the summer: that’s a full-pitched battle between two colonies for local dominance. Workers from adjacent colonies pair off and grapple with their mandibles — jaws — though few apparently are harmed. In other ant species, female soldier ants rip their enemies to pieces, leaving thousands dead. In some species, such as P. dentata, workers aren’t big enough for successful warfare: instead they act as sentries on patrol. If they bump into an enemy ant — perhaps a fierce army ant or a fire ant on a scouting mission — they scuttle back to the colony, recruit soldiers equipped with powerful jaws, and rush back to kill the enemy before they can call in reinforcements.

Then there are the army ants themselves. Instead of living in established colonies, they collectively attack virtually everything in their path, even other insect colonies, eat their young, and move on. Another species of ants located in Concord doesn’t produce workers. Their solution? They enslave other ants to do their work.

It’s not completely a Hobbesian world, though. Some prefer farming to fighting. Leafcutter ants forage for leaves, which specialist workers macerate and convert into mulch. A particular fungus grows on the mulch garden, the sole food for the colony, which in the tropics can hold up to two million ants.

Seen from afar, it’s hard to think of ants as having brains. But they do, and their brains, like the brains of mammals, comprise discrete areas used for specific functions. Their societies are likewise complex, with castes and subcastes. “Ants are not these genetic little robots that do everything on cue and don’t learn anything and are deterministically driven to do things,” Traniello says. “The social environment is very rich — a lot of stimuli are directing behavior — and causing changes in behavior as needed.”

An ant colony starts with a queen, one who has flown from the colony in which she was born, mated with one or more males from other colonies, and dug an underground nest of her own. Soon she starts pumping out eggs. Those become larvae, then pupae, and finally fully formed adult ants. All are sisters, except for the few unfertilized eggs that become males, whose short lives have a sole purpose: a brief tryst with some adventurous queen. After several years, a colony of P. pilifera (they range from Maine to Florida) will consist of 800 to 1,500 ants. As they mature, the females split into two castes: minors, who specialize in the care of the brood, and majors, much bigger than the minors, who head out to forage for food and when necessary, fight off invaders, usually another ant colony.

“What’s going on in the brain that leads to that division?” asks Mario Muscedere (GRS’09), a doctoral student in Traniello’s lab. “What is it about their brains that is causing them to do these different things?”

To find out, Traniello and his students are closely examining the ants’ brains. In particular, they are looking at the brain volume changes as the ants age, and at amines, neurotransmitters like serotonin and dopamine, which regulate behavior.

Traniello pulls up slides on his computer showing scans of the brains of the three different species he’s working with: the P. pilifera from Concord, P. dentata from Florida, and P. morrisi from Long Island. All are of the same genus, but they are quite different: the pilifera are smaller, and when frightened will feign death. Bother the morrisi, though, and they will bite. And in dentata those characteristics are mixed, with soldiers especially adapted to dealing with specific threats.

Their brains look different too, both between the minors and the majors within each species and across species. In particular, there’s a brain area known informally as mushroom bodies, which in fact looks a bit like Shitake mushrooms. Called corpora peduncula, they are the centers of learning, memory, and sensory integration. The difference between a fully mature minor’s mushroom body and that of a major is remarkable — a major’s is almost twice the size, perhaps because of the larger repertoire of activities majors perform out of the nest, even though they’re sisters from the same queen mother.

How do Traniello and his colleagues know these things, given how tiny ant brains are? It turns out they’ve perfected the art of dissection, using fine watchmaker’s forceps. “They are really tiny, very sharp,” says Muscedere. Ants have a head capsule, an exoskeleton with no bones inside. “You pull pieces of the head off until you see the brain, and sort of flip it out. It’s about the size of a pinhead. It’s not invisible, but it’s very small.” With the help of confocal microscopy — an imaging technique that enables 3-D reconstruction — researchers can study the brain structure, even to the level of individual neurons.

“We’re seeing an extraordinary amount of brain differentiation,” says Traniello, “even at this minuscule scale.”

The BU researchers are the first to work with ant brains at this level of detail. “I think a lot of people have steered away from doing neurobiology on something with a brain a hundredth the size of the honeybee brain,” Traniello says. “But once you get by the size issues, then you’ve got this incredibly rich group to work with.”

Part two of “Notes from Underground” will appear tomorrow on BU Today.

Taylor McNeil can be reached at tmcneil@bu.edu.