Inside out: cannibalism, nutrition and swarm formation in locusts

It may be difficult to picture just one locust singled out from a swarm. But believe it or not, desert locusts—insects infamous for their contribution to plagues and famine—are naturally solitary creatures. So what causes the group uprising that farmers are so familiar with? Research has shown that the internal workings of a solitary locust can affect the swarming behavior of the entire group. As described in a Live Science article, rain and food are very important factors in swarm formation:

“Desert locusts live in barren regions that see rain only rarely. They eke out an existence alone when times are tough. When the rains come, they breed like crazy. Then things dry up and hoards of locusts are forced to gather around dwindling patches of vegetation.” The insects then transform from solitary to gregarious locusts.

“The gregarious phase is a strategy born of desperation and driven by hunger, and swarming is a response to [finding new pastures],” said Stephen Rogers of the University of Cambridge in the UK in the Live Science article. In his study, Rogers and colleagues found that the chemical serotonin spiked in desert locusts within a two hour period as they started to team up.

The locusts experienced elevated levels of serotonin—a neurotransmitter responsible for regulating mood and appetite in humans and other animals—as they rubbed thoraxes or were exposed to the sight and smell of other locusts, said Rogers and colleagues. In addition, the researchers used a series of drugs to inhibit serotonin production in the locusts—those locusts were less likely to switch to the gregarious phase or exhibit aggressive behaviors.

A study recently published online in Proceedings of the Royal Society: Biological Sciences added one more element to the process of swarm formation: cannibalism. As Sepideh Bazazi of the University of Oxford in the UK and colleagues wrote, “When locusts are in a group, internal nutritional conditions affect social interactions since other locusts provide a source of high-protein food.”

Perhaps, then, a single locust with a food source and dry locale could maintain even serotonin levels, keeping it solitary. On the other hand, a protein-deprived desert locust in a rainy environment could potentially turn cannibalistic. And, as any zombie movie would demonstrate, swarms of voracious cannibals can lead to a fast, unpredictable group.

As described in Bazazi and colleagues’ study, the locusts individually exhibited pursuit and escape behaviors, contributing to the rapid, random movement of the swarm. The researchers supplied desert locusts with low-, medium- and high-protein diets and recorded their behavior accordingly. They found that solitary locusts fed a balanced diet moved at slower speeds and spent less time in motion than those fed low-protein diets.

But in groups, locusts fed a low-protein diet moved approximately 40 percent faster than individuals fed a high-protein diet. And as time progressed, even locusts fed a balanced diet increased their speed within the group. That is, depending on how protein-deprived one locust was, it would pursue another locust as possible prey. The prey locust, then, would be escaping predation.

These fluctuations contributed to the overall rapid movement of the group, which increased speed as time progressed and proteins became more and more depleted. The result, it seems, is a nutritionally deprived, aggressive locust pursuing other locusts and causing a ravenous swarm.

Photo Credit: Paul Martin

Anstey, M., Rogers, S., Ott, S., Burrows, M., & Simpson, S. (2009). Serotonin Mediates Behavioral Gregarization Underlying Swarm Formation in Desert Locusts Science, 323 (5914), 627-630 DOI: 10.1126/science.1165939

Bazazi, S., Romanczuk, P., Thomas, S., Schimansky-Geier, L., Hale, J., Miller, G., Sword, G., Simpson, S., & Couzin, I. (2010). Nutritional state and collective motion: from individuals to mass migration Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.1447

Author: Katie Kline

Moderator of EcoTone and ESA's communications officer.

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