These ants can shrink and grow their brains

For most ant colonies, there is a clear hierarchy: a single queen lays all the eggs, while a caste system of workers manages everything else: foraging for food, tending to ants, waging war, and so on. Only males and queens can reproduce and the rest of the ants are sterile. When the queen dies, so does the colony.

It is different for the Indian jumping ant, a species with pincer-like jaws and large black eyes that lives in the forests along the west coast of India. In these colonies, when the queens die, workers organize bizarre competitions in which the winner becomes the monarch – and is able to produce eggs. The triumphant female ant’s ovaries expand and her brain shrinks by up to 25 percent.

But new research shows these queens can be taken off their pedestal and returned to workers. This causes the ovaries to shrink again and the brain to regrow, an extraordinary feat previously unknown to insects.

“In the animal world, this level of plasticity — and especially reversible plasticity — is quite 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 this workers shift into queen-like reproductive mode, scientists call them gamergates (not to be confused with the online harassment campaign associated with 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’.

Any member of H. saltator can reproduce, but this can only happen if an individual wins a drawn-out series of dominance tournaments that take place after a queen dies. Like a small jousting championship, the ants take turns quickly stabbing each other with their antennae.

Half of the colony can get involved in these boxing matches, which can last up to 40 days, and all ants, except for the sole winner, remain workers.

Complex behaviors to resolve dominance are known in other insects; queen wasps, for 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 the workers who are fighting for the reproductive role, which is very neat.”

When a gamergate takes over, it undergoes a lot of internal changes. Most notably, his brain shrinks by a quarter, “which is just a massive loss of brain mass,” Penick says. The researchers also found that these queen-like ants stop producing venom and also undergo behavioral changes, hiding from intruders and stopping all hunting behavior.

To learn more about the plasticity of the ant’s brain, and see if these changes can be reversed, Penick and his colleagues chose 60 gamergates and painted specific colors to distinguish them from one another. Half of the ants were chosen at random and placed in isolation for a few weeks. The other 30 acted as controls. The isolation seemed to reduce the fertility of the queen ants, and when they were introduced back into the colony, they were immediately grabbed and held by other workers.

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

Scientists theorize that the stress of this situation triggers a cascade of chemical changes that return the gamer gates to a worker, usually within a day or so.

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

‘An entirely different thing’

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

“There are many insects with documented plasticity in all traits here, but I don’t know of any with this level of reversible plasticity,” said 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 work as workers, or move from foraging behavior to queen behavior. But shifting neural investments once, and then back again, is quite another.”

Adams says these kinds of reversible brain changes may not be as rare as we think — we just haven’t looked closely 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, such as Australian meat ants. When queens divide their labour, with some staying in the colony and others foraging, it can be associated with a corresponding difference in brain size or function, Adams says.

The more this issue of reversible plasticity is explored in all species, the more implications it may have for understanding human brains. “Very, very, very far downstream there could be insights into the way human brains develop,” Penick says.

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

“One may ask ‘why study this random ant species,’ but in the course of evolution they may have stumbled upon a fascinating mechanism of neural plasticity,” Snell-Rood says. “I think we can learn a lot from amazing neural adaptations in animals.”

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