Hunger imprints it
A full stomach doesn't like studying - an empty one all the better? In the case of humans and other vertebrates, at least, good things leave deep traces in lean times. But what about simpler nervous systems?
Fresh after breakfast, the banana on the fruit plate is not very tempting - bananas fill you up, and that's what you are. It also seems a bit green, one more reason to ignore it. A few hours later, however, only one thing counts: to calm the rumbling stomach, and to do it quickly. What will you remember two days later - that the fruit tasted a bit bland? Or that you were then able to go on to the afternoon program well fed?
With regard to the learning effect, the second would be more important, since the memory meant that you would be able to reach for the banana again the next time you had a hunger pang. Oddly enough, some of them would even be left with something more filling. How come – vitamin considerations aside?
Apparently it has a particularly formative effect on later preferences when the very first experiences à la "banana fills you up" are internalized on an empty stomach. Numerous studies on humans, rats, starlings and the like have meanwhile shown that those affected, when faced with the decision, often prefer objects that are in some way perceived as expensive or objects that they have gotten to know in bad times, even to better alternatives. Why is still unclear.
Lorena Pompilio from the University of Buenos Aires and her colleagues at the University of Oxford investigated whether this complicated decision-making also occurs in simpler knitted nerve costumes with limited storage potential in desert locusts (Schistocerca gregaria). The animals were brought up both hungry and full to associate small bites of food with the scent of lemongrass or peppermint.
The highlight of this was that there was always the same amount of food, so the delicacies did not differ in quality or quantity. After successful training, the test followed: Again, both hungry and full, the six-legged friends should now decide on one of the scents. The previous experience was repeated for each animal: if it smelled of lemongrass on an empty stomach, this also applied to the test phase.
If the respective yield were decisive, the locusts should not show any special preferences - they had not experienced any difference in the amount of food. If they were primarily striving for the most pleasant state they had experienced, they would have to choose the scent that reminded them of the best of times – that is, the scent that promised them more bites on a full stomach. If the smells simply aroused the association "I felt like that before, and I smelled peppermint then", the grasshoppers are likely to choose exactly this alternative again.
But the choice fell clearly on the fourth possible variant: The majority of the animals decided on the smell that had signaled food to them when they were hungry - regardless of whether their stomachs were growling at the time or not. So, as in the vertebrate world, locusts appear to prioritize experiences that have benefited them greatly under poor conditions.
Two mechanisms could be responsible for this. On the one hand, it would be possible that the animals subjectively embellish their memories depending on their nutritional status. However, the researchers working with Pompilio consider it more likely that the perception of the locusts is distorted with the degree of hunger. According to previous work, the taste sensors on the insects' mouthparts become more sensitive to various key nutrients as the nutrient concentrations in the hemolymph decrease. Or the animals can get more ingredients from the food when there is a lack of food than when they are full - then the objective snack offer would not be decisive, but the subjective gain after eating. In any case, exactly as observed, the object that filled the stomach at hungry times aroused the most interest.
And what's the point of it all? The researchers suspect that this may be to speed up decisions. So it would be a lot more time-consuming to weigh the different options against the current nutritional status each time – how much easier it is to conclude that this or that has already filled the stomach, which always makes it a good choice. Although there is a threat that something better on offer will be disregarded, according to Pompilio and her employees, this is rarely the case. Or to put it another way: If the smell of pizza gets into your nose, you will probably postpone the banana until tomorrow without much hesitation.
