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When a Queen Dies, Wasps Know Who's Next in Line (and Next, and Next)


This post originally appeared in August 2012. Inkfish will return to its regularly scheduled wacky animals next week. 


The office of postmaster general to the United States used to come with a perk totally unrelated to mail. In the unlikely event that an accident wiped out the president, vice president, and every member of their cabinet, the postmaster general would become the leader of the country.

In reality, the line of succession has never gotten beyond the vice president. But there are 16 people lined up behind the VP to take over (a list that no longer includes the postmaster general and now culminates, less quaintly, with the secretary of homeland security). In the United Kingdom, the order of succession to the throne winds bafflingly through a giant family tree of princes, dukes, viscounts, and so on.

Wasps of the species Ropalidia marginata never have to argue about titles or families: when the queen dies or disappears, the other wasps in the colony unanimously agree on who her successor is. And if that queen disappears too, they know who comes after her. Though the ordering system is invisible to human eyes, the wasps adhere strictly to their line of succession and follow it all the way down (if necessary) to their equivalent of the postmaster general.

Alok Bang and Raghavendra Gadagkar, researchers at the Indian Institute of Science in Bangalore, have been determinedly assassinating wasp queens to try to figure out how the R. marginata system works. Until the researchers get to her, each nest's queen lives a peaceful life. She doesn't bother anyone, and no one bothers her as she pumps out new generations of fertilized eggs.

The queen's quiet lifestyle, like that of most royalty, is in stark contrast to the lifestyle of her subjects. All around their docile ruler, worker wasps live in continuous violence. Gadagkar says the wasps chase, bite, and "nibble" one another, pin each other in place by holding body parts in their mouths, and crash down on each other from above. These displays of aggression don't usually injure the wasps, but maintain a hierarchy of dominance among them.

When the peaceful queen dies, or is plucked from the nest by interfering scientists, things get shaken up. One worker wasp—and only one—suddenly becomes hyperaggressive. Within minutes of the queen disappearing, this worker begins attacking the wasps around her at 10 or even 100 times her usual frequency, Gadagkar says. She distributes her attacks evenly among anyone nearby, and no one fights back. It's all a show to announce that this wasp is the heir to the throne.

Over the following week or so, the heir's aggression dies down and her ovaries develop. She becomes another peace-loving, egg-laying machine.

The researchers believe that this successor is chosen somehow before the original queen disappears. Even though she's outwardly identical to the other wasps in the nest, she's predestined to be second in line to the throne. "The fact that there is invariably one and only one individual who becomes hyperaggressive" is one clue, Gadagkar says. That no one challenges this hyperaggressive individual is an even stronger clue. And in previous studies, the researchers have shown that the heir isn't simply the first wasp to get the news of the queen's death. The successor seems to know who she is ahead of time, and the other wasps know and respect it too.

If that weren't impressive enough, Bang and Gadagkar have now found that when they remove the first heir, a second one steps up just as quickly. In a new paper in PNAS, the authors say they've discovered a succession of at least five potential queens.

Each of these new queens jumps into action as soon as a the previous queen disappears, attacking any workers around her. Again, only one wasp steps forward, and no one challenges her. Within several days, this new queen starts laying her own eggs and maintaining the colony. In an entire nest of 20 or 30 individuals, the researchers say, there's no reason to believe the succession doesn't continue—maybe down to the very last wasp.

Having an agreed-upon order of succession makes sense for insects living in small colonies like R. marginata, the authors say. Unlike in a large honeybee colony, where queens are determined from birth and workers know they'll never lay their own eggs, workers in the termite colony actually have a shot at reproducing. Knowing where they are in the queen queue could help them decide whether to stay in their original nest or move out to start a nest of their own.

Even if it makes perfect sense for the wasps to have an orderly system of succession in place, that doesn't explain how on Earth they figure it out.

"That is the million-dollar question we are working on!" Gadagkar says. The researchers found that older wasps were more likely to be the immediate heirs to the throne, but the order doesn't go strictly by age. It also doesn't have anything to do with the dominance hierarchy in the nest.

"Perhaps it is something very subtle, related to the internal physiology of the wasp, that the wasps themselves can detect and which we have not yet discovered," Gadagkar says. Like obscure duchesses and earls, the wasps know their place in line—indecipherable as it may be to the rest of us—and wait for their day to step forward.


Alok Bang, & Raghavendra Gadagkar (2012). Reproductive queue without overt conflict in the primitively eusocial wasp Ropalidia marginata PNAS : 10.1073/pnas.1212698109

Image: Abhadra/Wikipedia

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