As the organ responsible for maintaining equilibrium in the body and the most energy-demanding of all the organs, the brain takes a lot of the body's energy allocation. So when food is in short supply, the brain is the organ that is fed first. But what happens when there isn’t enough food to fulfill the high-energy needs of the brain and survival is threatened?

The brain does not simply self-allocate available resources on the fly; instead it “trims the fat” by turning off entire processes that are too costly. Researchers from CNRS in Paris created a true case of do-or-die, starving flies to the point where they must choose between switching off costly memory formation or dying. When flies are starved, their brains will block the formation of aversive long-term memories, which depend on costly protein synthesis and require repetitive learning.

But that doesn't mean all long-term memories are shut down. Appetitive long-term memories, which can be formed after a single training, are enhanced during a food shortage.

In order to test the idea that the neurons responsible for long-term memory are disabled during starvation, the researchers trained starved flies to associate certain odors with jolts of electricity. They combined this training with a manipulation that activated the two neurons responsible for long-term memory formation. When these neurons were activated in starving flies, their lives were shortened by about 30 percent, suggesting the activity pushed these neurons past their energetic limits.

Although the researchers used fruit flies in their study, many of the features at play in the fly are also involved in the regulation of long-term memory in mammals.

Science, 2013. DOI: 10.1126/science.1226018 (About DOIs).