Calcium is something of a double-edged sword. Too much of the essential element is as dangerous as too little, either case adversely affecting health in animals from humans to mice to fruit flies.

Sensing calcium at all can be crucial. Though it doesn't fit into the five established tastes the tongue's receptors can identify  sweet, sour, salty, bitter and savory (umami)  humans can taste it, and describe it as slightly bitter and sour.

New research conducted by scientists at UC Santa Barbara and colleagues in Korea has established that calcium taste also exists in fruit flies (Drosophila melanogaster). The team also uncovered a unique class of gustatory receptor neurons (GRNs) necessary for calcium taste in these model organisms. Surprisingly, given that some calcium is necessary to sustain life, the flies were indifferent to low calcium and averse to high levels. The findings appear in the journal Neuron.

The investigators used petri dishes with one side containing solely sugar (fructose) and the other a mix of sugar and a high level of calcium. Normal flies that rejected the high-calcium side and ate only pure fructose survived. Mutant flies  those in which any one of the three newly found GRNs was removed  were unable to distinguish the two halves of the petri dish. As a result, they consumed enough of the calcium to cause viability problems and, over time, died.