Biologist connects particles and behaviour

Dawn Wiseman

Biologist Emma Despland studies the secret life of insects, and what their behaviour can reveal about us.

photo by rob maguire

Your behaviour may be nothing more than physics. Think about pedestrian traffic downtown, suggested biology professor Emma Despland.

“If it isn’t too busy, everyone follows their own path, but as the streets get busier — say, towards lunch time — we start to see collective behaviour like laning.”

No one individual decides eastbound traffic will take the inside of the sidewalk and westbound the outside; nevertheless, lanes rapidly and spontaneously appear as pedestrian density increases.

“Everyone is subtly reacting and adjusting to the movements of the people around them. For particles that are influenced by their neighbours, the single-propelled particle (SPP) model predicts random movement at low densities and cohesive movement at high densities.”

Biologists have predicted for a long time that SPP models could explain collective behaviour, but Despland and her colleagues at Oxford University were the first to demonstrate a firm connection between theory and real life.

“It was the first time any one developed a system where you could ask, ‘Is this really what’s happening? Is it really a good model?’” They made the connection by studying desert locusts.

Desert locusts are one of the most destructive forces on Earth — literally the stuff of plagues. Swarms contain billions of insects, each eating its own weight per day. During the worst outbreaks, they travel thousands of miles devastating commercial crops, natural vegetation and ruining subsistence farmers.

Most of the time, desert locusts are innocuous green solitary beasts. They move slowly and quietly from food source to food source minding their own business. Solitarious locusts actively avoid their neighbours, moving away from any who come within a body length. As long as food is plentiful, life is good.

The situation changes dramatically, however, when food becomes scarcer. In close proximity, locusts change into their gregarious form.

With yellow and black racing stripes, gregarious locusts both look and act remarkably different from the solitarious form. They move together in little pockets, which join together and grow. Eventually, they form aligned marching bands of locusts.

“As nymphs (or juveniles), they are limited to the ground,” explained Despland, but when they mature and sprout wings the ravenous swarms of the Bible and Qu’ran are a real potential.

Despland and her collaborators have discovered that locust density is key in whether these groupings become massive self-sustaining swarms or return to their solitarious form.

In the lab, they have found a critical density of just over 70 locusts per square metre. At this point, individual behaviour ceases to impact the group and swarming begins in earnest. Scaled up to real life, locust swarms rarely change direction except under the influence of fairly massive external forces, like wind storms or drought.

Despland hopes these results may help institutions like the Food and Agricultural Organization (FAO) of the United Nations decide how best to deal with outbreaks. But she acknowledged, “It’s a multi-dimensional, socio-economic-political issue,” of which her research forms only a small part.

Despland is interested in how results can be replicated with other group behaviours. She is looking at forest tent caterpillars, a North American species that feeds on the leaves of hardwoods such as sugar maple and aspen.

“These caterpillars hatch together, move together and eat together — I want to know how they all decide they’re hungry at the same time.”