Locusts pick swarms as lesser evil

You are starving, a prolonged drought means the nearest remaining food lies across miles of desert full of hungry predators ready to gobble you up: what do you do?

If you are a Desert locust you undergo a Jekyll & Hyde style transformation and turn from a shy, solitary individual into a gregarious, swarming migrant.

As reported in (among others) BBC Online, The Times and The Independent scientists including Oxford's Mike Anstey have finally identified the specific changes in the nervous system that affect this transformation: in particular the role of the brain chemical serotonin.

Previous research at Oxford had tracked down the physical stimuli that kick-off the change in lifestyle - a tickle of the locust's back leg was found to be the most powerful stimulus.

Part of Mike's work involved pinpointing the timeframe for the change in locust behaviour - something which happens before a stimulated insect changes body shape and colour. Once that was established the researchers could examine a range of chemicals in locusts to see which ones change as a result of the physical stimulation.

'Surprisingly, the only chemical that increased in the two to four hour timeframe it takes for the locusts to become gregarious was serotonin,' Mike told me. 'Although we had no idea that serotonin would play such a critical role in the process, it makes sense that serotonin is the culprit since it is well-known as a chemical found in the nervous system of other animals (including humans) which mediates behavioural changes towards others.'

In locusts an increase in serotonin causes them to be attracted rather than repelled by other locusts. Essentially the food shortage makes them 'switch' to the sort of safety-in-numbers strategy used by other animals in which a large pack of individuals moving as one confuses predators, enabling them to migrate in greater safety.

So why don't locusts live in swarms all the time?

'Although they are safer than predators they are at high risk of being cannibalised by other members of the swarm, which happens quite often!,' Mike explains. 'Switching to the gregarious phase is really the lesser of two evils - individuals will certainly die of starvation if they do not migrate and are only quite likely to die by migrating.'

The findings are evidence that serotonin is responsible for behavioural changes that occur as a result of interactions with others across a wide range of very different animals: from insects to humans.

Dr Mike Anstey completed the work whilst at Oxford's Department of Zoology.