Serotonin Boost Turns Locusts Into Social Swarms
A flow chart of the impact of serotonin on the locust. - Click to see how serotonin affects the locust's behavior. (Alyson Hurt/NPR / Steve Rogers/University of Cambridge/Science)
A common brain chemical that affects mood in people has been linked in new research to the devastating swarming behavior of locusts, the insect that has plagued farmers since biblical times.
Elevated levels of serotonin, the same chemical targeted by some anti-depressant medications, appear to have a transformative effect on the insects' social behavior, according to a report published this week in Science.
Desert locusts are a kind of grasshopper; they migrate in great swarms that can contain billions of insects and stretch for miles. These swarms eat everything in sight — and have recently devoured crops in places like Africa, China and Australia.
(Alyson Hurt/NPR / Steve Rogers/University of Cambridge/Science)
But even though these massive swarms are the reason locusts are so notorious, "the swarming is something that locusts only do very occasionally," says Steve Rogers, a researcher at the University of Cambridge in England.
"In fact, locusts can spend many generations in a form that not only doesn't swarm but is actively repelled by other locusts," he says.
These mild-mannered, solitary grasshoppers can change, however, if something like a drought forces them to crowd together at a dwindling food source, if they start to smell and see other locusts and touch them.
"This is exactly the point at which behavior changes dramatically," Rogers says.
In Crowds, Locusts Transform
The locusts go through a kind of "Dr. Jekyll and Mr. Hyde" transformation. They become gregarious, start flying around and seek out others so they can migrate as a group.
This change in behavior occurs quickly, after just a few hours of forced crowding, says Rogers.
He and his colleagues wanted to know what actually happens inside the insect during this transformation. They learned that inside the insect's nervous system, there is a steep rise in a brain chemical called serotonin. The more serotonin an insect has, the more gregarious it is.
Testing Serotonin In The Lab
To find out if serotonin was actually responsible for the insects' change in behavior, the researchers decided to manipulate serotonin levels by giving the locusts certain drugs.
"It was absolutely startling to see the effect that these drugs had," says Michael Anstey, a researcher at the University of Oxford.
Drugs that block serotonin made solitary locusts stay shy, even in conditions that would normally make them attracted to others. And drugs that boosted serotonin made solitary insects suddenly act more gregarious.
"This really was the 'Eureka!' moment," says Anstey.
It's not clear yet whether this research could someday be used to develop new methods of pest control. But Rogers says it is surprising to see just a single chemical having such a huge effect.
"Using models like bees and wasps, we know that as they change behavior, as they leave the nest and things, there's a whole stew of different chemical changes going on," he says. "So, I think what we've found is genuinely quite unusual."
Still, serotonin is known to affect mood and behavior in other species, including people.
"Serotonin is implicated in a lot of aspects of mood and well-being in humans, as well as causing this change in locusts," says Rogers. He notes that some antidepressant drugs, like Prozac, boost serotonin levels.
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ROBERT SIEGEL, host:
If you take anti-depressants, you might be interested in some new research on locusts. That's right, locusts, these swarming grasshoppers that have plagued farmers since biblical times. Scientists say that they have found the chemical trigger that makes locusts gather together and it's the same brain chemical affected by Prozac. NPR's Nell Greenfieldboyce has more.
NELL GREENFIELDBOYCE: A swarm of locusts can have billions of insects and stretch for miles. Back in 2004, my NPR colleague, Richard Harris, went to West Africa and stepped out of his car into a cloud of the big bugs. Listen closely.
(Soundbite of locusts swarming in West Africa)
RICHARD HARRIS: They carpet a rice field and an adjoining pasture, and they even land on our clothes. Locusts don't vocalize, but we hear their wings and we hear the sound they make as they eat these plants down to their bare stems.
GREENFIELDBOYCE: This kind of voracious swarm is why locusts are famous. But, it's actually not their usual routine.
Professor STEVE ROGERS (Zoology, University of Cambridge, England): The swarming is something the locusts only do very occasionally.
GREENFIELDBOYCE: Steve Rogers studies locusts at the University of Cambridge in England.
Prof. ROGERS: The fact locusts can spend many generations in a form that not only doesn't swarm, but is actively repelled by the other locusts.
GREENFIELDBOYCE: He says locusts are usually solitary creatures, just harmless mild-mannered grasshoppers. But, if something like a drought forces locust to crowd together at a dwindling food source, and they start to smell and see other locusts and touch them...
Prof. ROGERS: This is the exactly the point to which behavior changes dramatically.
GREENFIELDBOYCE: The locusts go through a Dr. Jekyll and Mr. Hyde kind of transformation. They become gregarious. They seek out others so they can migrate as a group. Roger says this change happens quickly after just a few hours of forced crowding. He and his colleagues wanted to know what was actually happening inside the insect to cause such a dramatic change.
First, they found that inside insect's nervous system there was steep rise in a brain chemical called serotonin. But was this causing the change? Michael Anstey is a researcher at the University of Oxford. He says they decided to find out and this week, in the Journal Science, they report on what happened when they gave locusts drugs that alter serotonin levels.
Mr. MICHAEL ANSTEY (Zoology, University of Oxford): You know, it was absolutely startling to see the effect that these drugs had.
GREENFIELDBOYCE: He says drugs that block serotonin made solitary locusts stay shy, they never became gregarious, even in conditions that would normally make them swarm.
Mr. ANSTEY: This really was the Eureka moment.
GREENFIELDBOYCE: And drugs that boosted serotonin made solitary locust be suddenly attracted to other insects.
Mr. ANTSEY: Watching those locusts actually go and behave as if they'd been in a crowd, or were born in a crowd, is absolutely remarkable. So, it's quite exciting and truly - truly fun to do.
GREENFIELDBOYCE: Besides being fun, researchers hope this new insight might lead to new forms of pest control. That's a long way off. But Steve Roger says it is surprising to see just a single chemical making such a difference.
Prof. ROGERS: Using models like these in wasps, we know that as they change behavior, as they leave the nest and things, there is a whole stew of different chemical changes going on. So the thing that we found is generally quite unusual.
GREENFIELDBOYCE: Still, serotonin is known to affect behavior in other species, and boosting serotonin is what drugs like Prozac do.
Mr. ROGERS: Serotonin is implicated in a lot of aspects of mood and well-being in humans as well as causing this change in locusts.
GREENFIELDBOYCE: A solitary locust may seem far removed from a depressed person, but Roger says at a very, very deep level there may be some interesting similarities. Nell Greenfieldboyce, NPR News.
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