To become queen, these ants shrink their brains and balloon their ovaries (then, they reverse it)

An Indian jumping ant.
An Indian jumping ant. (Image credit: Clint Penick)

Even among ants, royal status is mostly an inherited affair. But for Indian jumping ants, a shot at wearing the crown is worth losing a bit of your brain for — especially as you'll always be able to grow it back later.

Unlike other ant species, Indian jumping ants (Harpegnathos saltator) do not die with their queens. Rather, select females participate in monthlong antenna-boxing matches to decide who gets to be the new matriarch. The victorious female then expands her ovaries and shrinks her brain to three-quarters of its original size. 

So far, so bizarre, but scientists have discovered another surreal twist to the storied lives of the forest-dwelling, black-eyed, forcep-jawed critters — If a female is deposed from her queenly throne, she will revert back to being a worker, shrinking her ovaries, regrowing her brain and resuming her previous duties.

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"We found that their brain returns completely to its previous size within a month after reverting back to a subordinate worker," lead study author Clint Penick, an associate professor at Kennesaw State University, told Live Science. "This was pretty amazing, and it's the first time reversible changes in brain size of this scale have been reported in an insect."

As with most other ant species, colonies of Indian jumping ants are strictly hierarchical. A queen is responsible for laying eggs — queens of some species, like the army ant, can produce up to 300,000 eggs a day — and the workers protect the colony, raise the larvae and hunt for food. 

Where they differ is what happens once the queen is dead. Most ant colonies slowly dwindle away after their leader dies, the workers die one by one and the royal offspring leave to become queens of their own colonies. 

But in an Indian jumping ant colony, the death of the queen is cause for more than half of the colony's females to enter into a monthlong tournament of fierce dueling with their antennas. The workers capable of activating their ovaries while delivering and receiving constant antenna jabs to the face are then chosen to be the next queens, the researchers said. Once the ritual is over, the triumphant new queens release a pheromone to alert their fellow ants of their royal status. 

Indian jumping ants antenna-boxing to decide who will become queen. (Image credit: Clint Penick)

In a colony of 100 ants, around five to 10 females will become new queens, according to the researchers. 

Only after they have acquired queenly status do these newly minted ant-queens undergo some more radical changes. Changes in gene expression and a cascade of hormones, driven by a burst of dopamine, cause their ovaries to balloon to five times their original size and their brains to shrink down by 25%. The life spans of these new queens stretch out from six months to five years. 

"The biggest changes to the brain occur in the optic lobes and central brain," Penick said. "Ants that win the tournament essentially become egg-laying machines, and they will generally never leave their nest or see daylight again. They also no longer need to hunt, take care of larvae or defend the nest. All of their needs are taken care of, so they don't need the same level of cognition required to perform complex tasks."

To test whether this metamorphosis was reversible, the researchers used a sample of 30 colonies, marking two new queens from each colony. One of the queens from each colony was kept as a control and allowed to do their normal royal duties, while the randomly chosen others were sent into solitary confinement for a month — where they were fed and kept in complete isolation from their fellow ants. Soon enough, the isolated new queens stopped laying eggs and had gone back to behaving like workers.

Upon return to their colonies, the reverted workers were seized and detained by their fellows for a few hours, likely because of their partially-developed ovaries. Once they were released, they returned to their duties as queens. Later dissection, performed six to eight weeks later, showed the ovaries of the reverted ants had shrunk and their brains had returned to their full size.

The researchers suspect that this bizarre plasticity may have evolved because the species experiences a higher than normal queen-mortality rate in their natural nests in the Indian jungle, but they aren't certain.

Their study isn't the only research that shows animals morphing their brain structure in extreme ways, according to Penick. Species of songbirds do it too, regrowing the part of the brain involved in song-learning before the breeding season.

"The typical wisdom I heard growing up was that once you lose brain cells they never grow back," he said. "Now that we know this occurs in the Indian jumping ant, it's possible it may be common in other species. At the very least, this research shows that even the brain of an ant has the tools to regrow itself, and many of the genes and regulatory networks involved in this are likely to be similar in other animals, even vertebrates."

The researchers published their findings April 14 in the journal Proceedings of the Royal Society B.

Originally published on Live Science

Ben Turner
Staff Writer

Ben Turner is a U.K. based staff writer at Live Science. He covers physics and astronomy, among other topics like tech and climate change. He graduated from University College London with a degree in particle physics before training as a journalist. When he's not writing, Ben enjoys reading literature, playing the guitar and embarrassing himself with chess.