Strange things happen when individuals aggregate. That is, a solitary sparrow does not reveal how large flocks move. Likewise, fish and schools;bison and herds; ants and anthills; people and maddening crowds; and cars and traffic jams. Recent work, from physics, has focused on similarities across these systems. The result is a class of models based on so-called`self-propelled particles,' in which individuals power themselves and interact with neighbors according to a set of rules and over some distance. A key prediction is that as density increases, the group undergoes a rapid transition from disorganized movement to highly ordered collective motion. In physical terms, the group should undergo a phase transition, much like water molecules do when they align during freezing.
Is this prediction supported by data? An international group, led by Jerome Buhl of the University of Sydney, tested the prediction using the desert locust, Schistocerca gregaria, the most widespread and devastating of the plague locusts. The insect's secret lies in a remarkable Jekyll- and-Hyde duality. Individual juvenile locusts (called nymphs) normally are, like Dr Jekyll, shy and retiring. But if compelled to socialize, for example by competition for scarce food, an individual can rapidly transform into a wild Mr Hyde (a process called gregarization), who prefers the company of other locusts and may join together with millions of his fellows to form roving bands of destruction. Band formation, however, requires that locally gregarized groups move in a coordinated fashion into other areas from which they recruit more locusts. In locust terms, the key questions are: do locusts undergo a rapid transition from random to ordered movement with increasing density and what is the critical density?
Buhl and colleagues put different numbers of locusts (5-120) into a ring-shaped arena and filmed their motion for 8 hours. The team then processed the movies with software that kept track of the orientation and movement of each individual, and found that locust movement depended dramatically on density. At low densities (2-7 moving locusts) individuals were rarely aligned. At medium densities (10-25 moving locusts), individuals were highly aligned for most of the 8 h test period, but showed rapid and spontaneous changes in direction. At high densities (30 or more locusts), the locusts almost immediately became aligned and did not change direction thereafter during the test period (see online movie). These results conformed closely to the output of simulated `self-propelled particle' models encoded with known locust biology, suggesting that the locust experiment represents a special case of a more general process influencing the movement of other animal aggregations.
The team's work sheds welcome light on a devastating insect problem. Previous field work has shown that the lower density in marching bands is about 20 locusts per m2, which corresponds to 8 locusts in the ring-shaped arena. Buhl and colleagues' experiment confirms this threshold density but also adds interesting, and sobering, detail. Above the threshold,locusts are almost always aligned although mediumdensity locusts often change direction while high-density locusts do not; they seem to have more directional inertia. This result suggests, counter-intuitively, that the movement of high-density swarms may be easier to predict and control.