We all have the luxury of knowing what foods are good for us. Our parents' telling us to eat our greens allows us – however unwillingly – to find the nutrients we need to survive. But how do other animals find out what foods to eat, without being able to rely on the advice of others?
Taste plays a major role in this process. Sweetness indicates the presence of sugars, which can be nutritionally valuable. However, this indicator is unreliable because not all sugars can be metabolised, and to make matters worse, not all nutritious sugars are sweet. So how can an animal improve on this? Two teams of researchers have addressed this question by asking whether the fruit fly Drosophila melanogaster is able to assess and remember the nutritional value of sugars independently of how they taste.
Both teams first wanted to see which sugars are actually nutritious to the flies. To do this they looked at how long flies survived when placed in a vial that contained a source of a particular sugar.
They found that sugars commonly found in the flies' natural food source of soft rotting fruit were able to sustain the flies very well, but other sugars were not. However, a compound called sorbitol also sustained the flies, even though flies do not perceive it as sweet. The teams therefore showed that flies eat things that they do not consider tasty in order to survive. So do flies actually learn to eat tasteless food, or do they simply eat everything they can find in the hope of getting lucky? In order to find this out the teams performed a series of elegant behavioural experiments to test which compounds the flies prefer to eat.
Both teams indeed found that, even though flies like sugars that are sweet, they will prefer to have a nutritious meal. This shows that flies are able to assess the nutritional value of sugars. Their experiments also showed that flies very quickly acquired a memory of the nutritional value of a sugar and retained it for at least a day. However, when the researchers repeated the experiments using flies that have defective synapses, the flies failed to learn the nutritional value of a sugar. This showed that the system that assesses and remembers nutrient quality must be found within the brain.
The authors of the two studies have shown that fruit flies are able to assess and remember the nutritional value of sugars in a process that occurs after the food has been ingested. This mechanism involves cells in the brain telling the fly that, whatever it is eating, it is doing the fly good. The next step – presumably already underway – will be to locate these cells and find out how they are connected within the brain.
