A Tiny Ant Brain Is Still Too Big for Reproduction

Indian jumping ants policing a female
Indian jumping ants "police" a female to force her to revert to worker status.Clint Penick / Kennesaw State University

For most ant species, nothing spells apocalypse quite like the death of a queen. A colony stripped of its monarch, the group’s only fertile female and the sole source of eggs, quickly unravels, then dies—an entire society snuffed out. The captain does not go down with her ship; the ship goes down with her captain.

Indian jumping ants do not abide by such dictatorial dramatics. They’ve evolved a work-around to indefinitely forestall their colonies’ demise. Within hours of their queen’s death, female workers will begin to joust, fencing with their antennae, and nipping at each other’s heads. These dominance tournaments can last for more than a month, until, at long last, a dozen or so champions triumph. While the losers slink away to resume their workerly duties, the victors cast aside their former peasant status and become a new class of pseudo-royals called gamergates (no, not that kind). The queen phase of the colony ends, and the gamergate phase begins: Monarchy transforms into oligarchy, and new gamergates step up as each generation dies. In this way, “colonies can be immortal, theoretically,” Clint Penick, an entomologist at Kennesaw State University in Georgia, told me. They have been, so far, in the labs where he’s studied them.

The consequences of these tiny tussles range from the sociopolitical to the molecular. In earning the title of gamergate (pronounced gamm-ər-gayt), a female ant gains nearly exclusive rights to her colony’s reproductive responsibilities; she is among the very few of her sisters that can fertilize their eggs with the sperm of their brothers, the only available males. The transition rewires worker ants, altering their behavior and physiology until they become docile, nursery-bound “egg-laying machines,” Penick said. Gamergates stop leaving the nest. They lose their food-foraging chops and the will to hunt, relegating themselves instead to the darkness of their underground chambers, where they churn out eggs. They feast exclusively on the paralyzed prey served to them by workers. Normally these ants leap at assailants when disturbed—the classic “jumping” behavior that earned the species its name—but when confronted by intruders, gamergates cower and hide. Even their bodies reprioritize. The ants’ life span extends from six or seven months to three years or more. Their venom glands recede, and their ovaries swell to about five times their original size; the ants become, in a sense, perpetually pregnant.

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In perhaps the most astounding change of all, the ants’ brain shrinks by about 20 to 25 percent in volume when they become gamergates. (For humans, that’d be the rough equivalent of losing a hunk of brain the size of two tennis balls.) The downsizing isn’t uniform: The insects appear to selectively jettison bits of their brain devoted to hunting, foraging, spatial mapping, and the anty equivalent of critical thinking—a move that likely reroutes precious bodily resources to the ovaries. Jumping-ant brains are already quite small, about a tenth of a cubic millimeter in volume. But brain tissue is “very energetically costly,” Floria Mora-Kepfer Uy, an entomologist at the University of Rochester, told me. And when the future of the colony is at stake, every calorie counts. “There’s a trade-off between reproduction and cognition,” Uy said. It is a heavy crown to wear atop a newly lightened head.

Given the triple threat of casual incest, sororal dueling, and what amounts to a self-inflicted prenatal lobotomy, the whole affair might sound a touch unpalatable to us humans. But in some ways, being a gamergate is downright cushy. These ants are the colony’s VIPs, and their social status is abundantly clear. When gamergates stroll among their subjects, they walk slowly and deliberately, standing high on their legs; workers scuttle out of their way to make room. “They have a royal attitude” about them, Jürgen Liebig, an animal behaviorist at Arizona State University and Penick’s former adviser, told me. “Once you know what to look for, it’s very conspicuous.”

And for any gamergate that’s not vibing her new digs, there is, under certain circumstances, an out. In a new paper published today in Proceedings of the Royal Society B, Penick, Liebig, and their colleagues report that the gamergate transformation is entirely reversible, down to the mind-boggling changes the ants’ brain tissue undergoes. When the researchers isolated gamergates from their colonies, depriving them of the social signals needed to maintain their über-fertile status, then reintroduced them to their peers, the ants rapidly regressed into workers. Their ovaries shriveled, leaving room for their venom glands to grow; their brains ballooned out. They reacquired their aggressive fighting acumen, and would once again jump when provoked. Functionally sterilized and juiced back up with intellect, they became once again indistinguishable from their commoner kin.

The wild flexibility of the ants’ brain is “especially cool,” Lindsey Lopes, a biologist at Rockefeller University who studies ant behavior, and wasn’t involved in the new research, told me. Scientists have found a small handful of other animals that can toggle through brain sizes. Songbirds will bloat their brain ahead of breeding season to help them learn sexy tunes for their mates, only to pare away the excess tissue once the deed is done; hamsters and shrews will cull brain tissue to help conserve energy during hibernation, then restore the lost matter when they awaken in the spring. But reversible changes are unprecedented among insects, which were previously known to add or subtract, but not both, in a single individual. In the insect realm, “I’ve never heard of something [like] a brain shrinking, and then reverting back to a bigger size,” Lopes told me. “That’s a lot of change.”

There’s probably a good reason reversible brain shrinkage isn’t terribly common. The act of pruning and then regrowing an organ, especially one as complex as the brain, is no small feat. But for these bizarre ants, who stubbornly refuse to let the death of a single royal topple their entire colony, it seems to make sense, Uy, of the University of Rochester, who was not involved in the new study, told me. And the change has to be reversible for the system to endure. The turmoil that ensues after the death of a jumping-ant queen can send up to 50 percent of the colony’s workers into a tizzy; the transitions to gamergate can begin almost immediately. “That means half the workers are starting to shunt resources from their brains to their ovaries,” Penick told me. “But only a fraction wins.” With their physiology on such a sliding scale, workers that lose the tournament can simply turn the dial backward, and resume business as usual. (The alternative might be nightmarish: getting stuck in pseudo-gamergate limbo, unable to fully forage or reproduce. Penick tells me that other ants don’t take kindly to ants that temporarily enter this hybrid state, and will chomp at them until they flip back to worker status—an act called “policing.”)

Even some fully developed gamergates may revert in the wild. Liebig, of Arizona State, said he suspects that those cases are more rare, because most faux queens assume the title for life. Late-career transitions can happen, though, when the colony is destabilized by unexpected tragedy—a mass death, an environmental disaster, an influx of foreign male ants—that forces the gamergates to abdicate.

Among ants, females always rule the nest. (Generally speaking, males exist solely to manufacture sperm, then promptly drop dead after sex.) Even in this company, the Indian jumping ants’ fluidity is especially empowering and delightfully unpredictable. Among these ants, every little girl actually can grow up to be a queen. But thrones can always be usurped.