What exactly enables an individual to change? In pursuit of an answer to this question, scientists from the Universities of Leicester, Sussex, and Cambridge, discovered that environment dramatically influences the capabilities of an individual, and any radical shift in environment must be accompanied by — at least in the case of locusts — a modification of both memory and learning ability.

“Our research shows how animals that undergo a profound transformation in their life style also adapt their learning and memory capabilities to cope with the new environment in which they find themselves,” Jeremy Niven, Ph.D., University of Sussex, stated in a press release.

Individuals and Groups

In terms of horror value, the vision of a swarm of locusts, which may appear as a dark cloud against the sky just before they descend, is rivaled by the persistence of their collective undulating sound. Yet, desert locusts do not only move in swarms, rather they alternate between two separate lifestyles based on available food. When food is readily accessible, they live and feed alone in what is known as their “solitary” phase; when food is scarce, these same solitary insects enter a “gregarious” phase in order to search for food in a dreaded swarm. What phase a locust is in not only impacts appearance but also behavior, especially with regard to what they choose to eat. For instance, solitary locusts are muted in color, a natural camouflage that helps them evade predators, and they are also slightly larger as they avoid eating toxic plants. Gregarious locusts, however, must eat toxic plants in order to survive and so they are smaller than their solitary counterparts; infused with toxins, however, their smell deters predators.

What fascinated the team of English researchers is not just the fact that locusts are able to fluidly move from solitary to gregarious behavior, but they accomplish this transformation within a matter of hours. It must be difficult for solitary locusts, who have learned that certain plants are toxic, to “unlearn” what they already know in order to become gregarious and survive. Considering how radical a change this is in practice, how exactly do locusts retrain themselves in so short a time?

"Locusts should consider toxic food 'bad' while they live alone but 'good' when they are in a swarm, which made us wonder, how do swarming locusts learn that 'bad is the new good?'" Dr Swidbert Ott, from the University of Leicester, stated in the press release.

What the researchers discovered is this shift is a matter of adapting memory and learning abilities to the new environment; in short, the locusts’ external change is matched by a revolution of mind.

Selecting Memories

The researchers found themselves focusing on memory. If a solitary locust has already learned that a particular plant smells toxic, and then the individual finds itself in a swarm and must eat toxic plants, what happens to this memory if they must forget what they know in order to survive?

"Desert locusts aggregate into swarms when they run out of food — the crowding is driven by hunger and competition for the last few plants in the desert,” Patrício Simões, Ph.D, of the University of Cambridge and Champalimaud Neuroscience Program, stated in the press release. “They are pretty desperate when they transform into the gregarious phase, so they will give the toxic plants another try."

Wanting to understand what transpires at this moment of giving the toxic plants a second try, the scientists attempted to replicate one aspect of the conditions in the field by training individual locusts in their lab. For their experiment, they gently blew vanilla or lemon odor at restrained locusts, while they spoon-fed them with artificial food. Next, the researchers "asked" the locust if they preferred vanilla or lemon odor — this was accomplished by walking each locust on a rod that bifurcates into two arms: one arm has lemon odor, the other vanilla. All of the locusts naturally preferred vanilla.

“But if you pair nutritious food with lemon during the training, they will go to lemon,” the researchers said. “And if you train solitary locusts with toxic food and vanilla, they will also go to lemon."

The researchers discovered that the locusts retained their earliest food associations, including aversive memories to toxin-containing food, while they were transitioning despite the fact that suchs memories were no longer ecologically appropriate. “A relearning mechanism that comprises the selective blockade of aversive memory formation coupled with hunger and competition for food could enable transiens locusts to assign an appetitive value to an odor they previously learned to avoid,” the authors wrote in their study. Because they were unable to change their memories, the locusts changed their learning abilities — during the transition, they become able to form new positive (appetitive) memories but they also became unable to acquire any new negative (aversive) memories.

In essence, one simple, selective mental block enables solitary locusts to become gregarious locusts and fit into their radically different environment. "Simply crowding a locust won't change its mind about the odor being 'bad'," the authors noted: "In the circumstances that they need to re-evaluate the 'meaning' of an odor, it takes only a simple modification of the rules by which they learn: turn off learning 'bad' but keep learning 'good', and the locust can retrain itself.”

Source: Simões P, Ott S, Niven J. Phenotypic Transformation Affects Associative Learning in the Desert Locust. Current Biology. 2013.