These ants can shrink and regrow their brains 

Troy Farah 

NATIONAL GEOGRAPHIC

4/13/2021

For most ant colonies, there’s a straightforward hierarchy: a single queen lays all the eggs, while a caste system of workers manages everything else—foraging for food, nursing baby ants, going to war, and so on. Only males and queens can reproduce, and the rest of the ants are sterile. If the queen dies, the colony usually does, too.

© Photograph by Clint Penick Ant with an ability to regrow its brain

OVARIES GROW BRAIN SHRINKS

Things are different for the Indian jumping ant, a species with forceps-like jaws and large black eyes that inhabits forests along India’s western coast. In these colonies, if the queens die, workers host bizarre competitions in which the winner becomes the monarch—and capable of producing eggs. The triumphant female ant’s ovaries expand and her brain shrinks up to 25 percent.

But new research shows these queens can be taken off their pedestal, reverting back to workers. This causes the ovaries to shrink again, and the brain to regrow, an extraordinary feat not previously known to occur in insects.

“In the animal world, this level of plasticity—and especially reversible plasticity —is pretty unique,” explains Clint Penick, the lead author of the study documenting this discovery, published Wednesday in the Proceedings of the Royal Society B: Biological Sciences.

Game of ants

Penick, an assistant professor of ecology, evolution, and biology at Kennesaw State University, in Georgia, has spent years studying Indian jumping ants, known as Harpegnathos saltator. When these workers shift into queen-like reproductive mode, scientists call them gamergates (not to be confused with the online harassment campaign tied to video games). The term gamergate comes from the Greek for “married worker” and was coined in the 1980s; The “gam” in gamergate rhymes with “ham.”

Every member of H. saltator can reproduce, but this can only occur if an individual wins a drawn-out series of dominance tournaments that take place after a queen dies. Like a tiny jousting championship, the ants take turns rapidly jabbing each other with their antennae.

Half the colony can become engaged in these boxing matches, which can last up to 40 days, and all the ants save for the sole winner remain workers.

Complex behaviors to sort out dominance are known in other insects; queen wasps, example, compete for the ability to produce offspring, says Rachelle Adams, who studies ant evolution and chemical ecology at Ohio State University. But “in this case, it’s workers that are fighting for the reproductive role, which is really neat.”

When a gamergate takes over, it goes through many internal changes. Most notably, its brain shrinks by a quarter, “which is just a massive loss in brain mass,” Penick says. The researchers also found that these queen-like ants stop producing venom and also change behaviorally, hiding from intruders and stopping all hunting behavior.

To learn more about the ant’s brain plasticity, and to see if these changes could be reversed, Penick and his colleagues picked 60 gamergates and painted them specific colors to tell them apart. Half the ants were randomly chosen and put in isolation for a few weeks. The other 30 acted as controls. The isolation seemed to reduce the queen-like ants’ fertility, and when they were introduced back to the colony, they were immediately seized and detained by other workers.

This is called being “policed,” Penick explains, which researchers think is how these ants prevent their colonies from having too many reproductive members. If a queen-like ant with partially developed ovaries is detected, other workers will bite and hold the ant for hours or even days, albeit without causing bodily harm. “It’s almost like putting them in ant jail,” Penick says.

Scientists theorize that the stress of this situation triggers a cascade of chemical changes that revert the gamergates back to a workers, usually within a day or so.

“Once we sacrificed them and did the brain scans, we found that they completely reverted in every trait,” Penick says. “Their ovaries shrunk down, they started producing venom again … and then their brain grew back to its original size.”

‘Another thing entirely’

Significant changes in brain size and complexity have been recorded in a few other species, such as hibernating ground squirrels and some birds. For example, white-crowned sparrows will grow as many as 68,000 new neurons when breeding season begins to help them learn new mating calls. By winter, when food is scarce, an equivalent number of neurons die back. When spring returns, the cycle repeats. But the phenomenon is new for insects.

“There are lots of insects with documented plasticity in all of the traits here—but none that I know of with this level of reversible plasticity,” says Emilie Snell-Rood, an evolutionary biologist at the University of Minnesota. “Many social insects show changes in these brain regions as they transition between phases of their worker life, or move from foraging behavior to queen behavior. But shifting neural investment once, and then back later, is another thing entirely.”

Adams says these types of reversible brain changes may not be as rare as we think—we just haven’t looked hard enough. “I wouldn't be surprised if we see more of this,” she says.

She suggests looking at ant species that can have multiple queens, one example being Australian meat ants. When queens divide their labor, with some remaining in the colony and others foraging, this might be accompanied by corresponding difference in brain size or function, Adams says.

The more this question of reversible plasticity is investigated in all species, the more implications it could hold for understanding human brains as well. “Very, very, very far downstream, there could be insights into like the way human brains develop,” Penick says.

Such research could, for example, teach scientists more about the genes related to neural plasticity and how they work.

“Someone may wonder ‘why study this random ant species’ but they may have, over evolutionary time, stumbled on some fascinating mechanism of neural plasticity,” Snell-Rood says. “I think we have a lot to learn from amazing neural adaptations across animals.”