Swarming locusts need larger brains

Although desert locusts are infamous for their swarming behaviour -- when they migrate en masse and consume everything in their path -- they usually occur in a solitary form, living alone and actively avoiding fellow locusts.

Working with colonies of swarming (gregarious) locusts, Dr Swidbert Ott and Dr Stephen Rogers of the University of Cambridge, converted some of them into the solitary phase by keeping them in isolation for three generations.

When they then compared the size and shape of the locusts' brains, they found that extraordinary changes had taken place.

Despite being smaller than solitary locusts, swarming locusts developed brains that were 30% larger. Not only that, regions of the brain that are dedicated to different tasks had very different proportions in the two phases.

In the solitary locust the parts of the brain that deal with vision and smell are proportionately larger, possibly helping them to detect faint or distant stimuli, whereas in the swarming locust huge increases in size occur in the parts of the brain associated with learning and processing complex information.

Swarming locusts need these larger brains to cope with the challenges of a life on the move that can take them across continents in a milling mass of other locusts, and to deal with the intense competition that dominates their existence, Dr Ott believes.

"Their bigger and profoundly different brains may help swarming locusts to survive in the cut-throat environment of a locust swarm. Who gets to the food first wins and if they don't watch out, they themselves become food for other locusts. In a nutshell, you need to be brainier if you want to make it in the mayhem that is a locust swarm," he says.

He adds: "As swarming locusts move through the landscape, they face much more of a challenge in finding and assessing potential foods, which may be something new that they have never encountered before."

The challenges of living in large groups and of finding varied and unpredictable food sources are also believed to be key factors in explaining why some vertebrates have evolved particularly large brains.

Because desert locusts can transform between a solitary and a swarming phase, which look and behave very differently, they provide ideal opportunities for studying the relationship between an animal's lifestyle, its behaviour and its brain structure.

The results are published May 26 in Proceedings of the Royal Society B.

Dr Ott is supported by a Royal Society University Research Fellowship, and his laboratory is funded through a grant from the Biotechnology and Biological Sciences Research Council.

About locusts

Desert locusts (Schistocera gregaria) are one of the most devastating insect pests, affecting 20% of the world's land surface. Vast swarms containing billions of locusts stretching over many square kilometres periodically devastated parts of the USA at the time of the settlement of the West, and continue to inflict severe economic hardship on parts of Africa and China. In November 2008 swarms six kilometres long plagued Australia.

Locusts belong to the grasshopper family but unlike their harmless relatives they have the unusual ability to live in either a solitary or a gregarious state, with the genetic instructions for both packaged within a single genome.

Locusts originate from barren regions that see little rain. While unforgiving conditions prevail, locusts eke out a living as solitary individuals with a strong aversion to mingling with other locusts. When the rains come, the amount and quality of vegetation expands and the locusts can breed in large numbers.

In deserts, however, the rains are not sustained and food soon becomes more and more sparse. Thus large numbers of locusts are funnelled into dwindling patches of remaining vegetation where they are forced into close contact with each other. This crowding triggers a dramatic and rapid change in the locusts' behaviour: they become very mobile and they actively seek the company of other locusts. This new behaviour keeps the crowd together while the insects acquire distinctly different colours and large muscles that equip them for prolonged flights in swarms.