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The Incredible Shrinking And Growing Brains Of Indian Jumping Ants

Apr 18, 2021

New research on ants has shown a first in insects: the ability to shrink and then regrow their brains in a big way.

It relates to how these particular ants, called Harpegnathos saltator, or the Indian jumping ant, reproduce.

"In most ants, the queen is the only member of the colony that lays eggs," says Clint Penick, an assistant professor of biology at Kennesaw State University in Georgia. "The workers just do all of the hunting and take care of the babies and all of the chores in the colony. But the queen is the only one who reproduces. And when she dies, the colony dies."

Not so for this type of ant, native to India. Their worker ants have the ability to mate and reproduce. So when the Indian jumping ant queen dies, "it actually triggers a dominance tournament. And they'll fight each other over a month to decide who's going to be the next ant to replace the queen."

(These are all females we're talking about. The males really aren't involved in anything here except mating and dying.)

The queen doesn't have an exact replacement. A handful of tournament winners — called "gamergates" — all assume queen-like duties of laying eggs.

Penick and his colleagues found that when the ants take on the role of gamergate, their brains shrink by 19% on average. The shrinkage likely happens so that they can save energy to focus on producing eggs. Hormones trigger additional changes in the ants, including larger ovaries, less venom production and much longer lifespans.

The fact that the ants' brains shrink to reproduce wasn't new.

"But what we didn't know was if they had the capacity to regrow them back to their previous size," Penick tells NPR's Lulu Garcia-Navarro on Weekend Edition. "The typical wisdom is that once you lose brain cells, they don't come back."

Penick and other scientists plucked these gamergates away from their colonies for a few weeks. The thinking was that the lack of social interaction and care would make them revert back to their previous worker status.

It worked. After several weeks, the ex-gamergates were back to being lowly workers — with their bigger brains regrown.

"When we started the study, we didn't know if any insect was actually capable of this," Penick says.

"We are now starting to find that there are animal species that are capable of shrinking their brain and then regrowing them, even humans to some degree, but not nearly on the scale to what we see here."

Still, there are potential implications for humans.

"It tells us that brains are a lot more plastic and have a lot more abilities to change back and forth between their size than we knew," Penick says. "And ants, their brains have some shared traits with humans, believe it or not. So now we're looking at digging into some of the genetic and other neural mechanisms that are underlying these brain changes."

Penick and his co-authors' study is published in Proceedings of the Royal Society B.

Peter Breslow and D. Parvaz produced and edited the audio interview.

Copyright 2021 NPR. To see more, visit https://www.npr.org.

LULU GARCIA-NAVARRO, HOST:

Expanding brains, shrinking brains, the ability to increase lifetimes by a factor of 10 - no, this isn't some B-grade 1950s sci-fi film. It is the life of the incredible Indian jumping ant. Usually, once the queen of an ant colony dies, the colony falls apart, but not with these ants. Their workers can battle it out for up to a month to try to become new insect royalty. And along the way, the newly crowned queen's brain shrinks, and her ovaries expand. And the losing combatants' brains expand back to their original pre-combat size while their ovaries shrink. This is all according to a new study led by Clint Penick. He's an assistant professor of biology at Kennesaw State University in Georgia, and he joins us now. Hello.

CLINT PENICK: Hi, Lulu. Good to be here.

GARCIA-NAVARRO: This is super weird.

(LAUGHTER)

GARCIA-NAVARRO: Let me just start by saying that. First, tell us about these ants. I'm told that they actually do jump.

PENICK: Yeah. So what these ants do - they - when they leave their nest to forage, they walk around the leaf litter in the jungles of western India, searching for a little insect prey. And when they see one, when they see, like, a cricket or a spider, what they'll do is they'll wait. And almost like a leopard or a lion, when the cricket gets close, they'll pounce and jump on them and sting them and then bring them back to the nest.

GARCIA-NAVARRO: So why these shrinking and expanding of brains and ovaries?

PENICK: Yeah. An unusual trait about this species is that the workers have reevolved the ability to mate and reproduce. So in most ants, the queen is the only member of the colony that lays eggs. The workers just do all of the hunting and take care of the babies and all of the chores in the colony. But the queen is the only one who reproduces. And when she dies, the colony dies. But in the Indian jumping ant, what happens is that once the queen dies, because the workers can mate and reproduce, it actually triggers a dominance tournament, and they'll fight each other over a month to decide who's going to be the next ant to replace the queen.

GARCIA-NAVARRO: Sounds very medieval.

(LAUGHTER)

PENICK: Yeah.

GARCIA-NAVARRO: Do they all have ovaries, or do the male ants just kind of stand back and watch the female ants go to battle?

PENICK: Males don't play a large role in any ant colony of any species. They have one purpose, and it's to leave the nest and mate. And so after they mate, they die, and they don't contribute to the colony after that. And all of the workers in a colony of ants is actually female, and so they do have ovaries.

GARCIA-NAVARRO: And how in the world did you figure this particular trait out? And how do you measure the brain of an ant anyway? That sounds like it might be a hard thing to do.

PENICK: Yeah. So, you know, I already knew that these ants - that their brains shrunk once they became reproductive. So this was known previously. But what we didn't know was if they had the capacity to regrow them back to their previous size. The typical wisdom is that once you lose brain cells, they don't come back. I know that's what my parents told me when I was growing up. But, of course, we are now starting to find that there are animal species that are capable of shrinking their brain and then regrowing them - even humans to some degree but not nearly on the scale to what we see here. And - but when we started this study, we didn't know if any insect was actually capable of this. And so what we had to do is figure out a way to take this royalty and convert them back to a typical worker. And once we were able to do that, then, yeah - then it came right down to having steady hands and working under a microscope and being able to dissect out their brains to take a picture of them.

GARCIA-NAVARRO: So let's talk about this life-extension thing because, of course, that's fascinating. If a worker wins the combat and becomes queen, its life span expands from around six months to five years?

PENICK: Yeah, their lifespan dramatically increases. Once they win one of these tournaments, it triggers, like, a hormone cascade and a whole suite of gene changes. And all of those result in them living up to five years.

GARCIA-NAVARRO: I mean, how many tournaments have you witnessed?

(LAUGHTER)

GARCIA-NAVARRO: I mean, it must be, like, amazing to see these battles between these ants sort of take place.

PENICK: Yeah. When - so we actually maintain over 200 colonies of this species in the lab. And so on any given day, usually one of them has cycled and is doing one of these tournaments. And yeah, in grad school, we actually - one of my jobs was to paint every worker in an ant colony. So they all got a colour code, and each individual was labeled, so we knew their names. And I would sit there for hours and hours and hours, just watching them duel each other and bite each other and write down what everyone did.

GARCIA-NAVARRO: God bless you. What is the practical application of all this? I mean, does it give us insight into our own brains?

PENICK: It tells us that brains are a lot more plastic and have a lot more abilities to change back and forth between their size than we knew. And ants - their brains have some shared traits with humans, believe it or not. So now we're looking at digging into some of the genetic and other neural mechanisms that are underlying these brain changes.

GARCIA-NAVARRO: Astonishing. Indian jumping ant expert and assistant professor of biology at Kennesaw State University Clint Penick, thank you very much.

PENICK: Thank you. It was great to be here.

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