A heuristic underlies the search for relief in fruit flies

Humans rely on multiple systems of sensory information to make decisions. However, strategies that shorten decision-making time by taking into account fewer but more essential elements of information are preferred to strategies involving complex analyses. These "shortcuts to decision" are also termed "heuristics". The identification of heuristic principles in species phylogenetically distant to humans would shed light on the evolutionary origin of speed-accuracy trade-offs and offer the possibility to investigate the brain representations of such trade-offs, urgency, and uncertainty. During experiments on spatial learning, we acknowledged that the search strategies of the invertebrate Drosophila melanogaster, the common fruit fly, resembled a spatial heuristic. Here we show that the fruit fly applies a heuristic termed the "Nearest Neighbour Rule" to avoid bitter taste (a negative stimulation). That is, the fly visits the salient location closest to its current position to hopefully stop the negative stimulation. Only if this strategy proves unsuccessful, the animal uses other learned associations to avoid bitter taste. The acknowledgement of a heuristic in D. melanogaster supports the view that invertebrates can leverage on economic principles when making choices and that the existence of heuristics in evolution dates to at least 600 million years ago.