Washington, May 9 : Locust migrations have been viewed as one of the most spectacular events in nature. In seemingly spontaneous fashion, around 10 billion critters can suddenly throng the air and carpet the ground, bringing destruction. What makes them do it was a question unanswered up till now.
According to a team of scientists led by Iain Couzin of Princeton University and including colleagues at the University of Oxford and the University of Sydney, such a phenomenon is due to cannibalism.
"Cannibalism," said Couzin, an assistant professor of ecology and evolutionary biology at Princeton.
Writing in the May 8 online edition of Current Biology, the research team said that the collective motion of locusts is driven by "cannibalistic interactions."
"Cannibalism is rife within marching bands of locusts," said Couzin.
Desert locusts usually feed on vegetation, but individual locusts have been observed to feed on other live locusts or cadavers. This behavior and its effect upon the group, however, have not previously been studied.
"No one knew until now that cannibalistic interactions are directly responsible for the collective motion exhibited by these bands," added Couzin, whose graduate student, Sepideh Bazazi, is the lead author on the paper.
In zoology, cannibalism is defined as occurring when any species consumes members of its own kind.
Young locusts are pressed to eat others when the food supply necessary for supporting the population starts to dwindle. Starved for essential nutrients such as protein and salt, young locust "nymphs" will nip at each other.
Those under siege react by running from the aggressors. Others get jittery and simply seek to put space between them and any locust approaching from behind. That's how one aggressive interaction can lead to another and collectively start a vast migration, Couzin said.
And the activity intensifies, as the biting and ominous approach of others increases both the propensity to move and the forward momentum of individual locusts.
The researchers reached their conclusion by studying immature, flightless locusts. They developed computerized motion analysis to automatically track the insects marching in an enclosed arena.
In nature, Couzin said, these locust nymphs can gather in large mobile groups called bands. They can stretch over tens of miles, devouring vegetation as they march. They inevitably precede the flying swarms of adult locusts.
"Once they take flight, locust control is extremely expensive and ineffective. So understanding when, where and why the bands of juvenile locusts form is crucial for controlling locust populations," Couzin said.