Extended Data Fig. 1: Drosophila prefer early fermentations, at peak CO₂ production.

a, Alcohol by volume for apple juice and sugar fermented with champagne yeast over the course of two weeks, measured with a hydrometer. CO₂ production was calculated from the stoichiometry of fermentation (1 sugar molecule yields 2 ethanol and 2 CO₂ molecules), corresponding to the derivative of alcohol by volume. n = 4 independent ferments; the results were very consistent. b, Trap assay. c, Preference index exhibited by flies in three two-choice assays, using traps shown in b. Flies were presented with two traps: one was a completed 14-day-old ferment that had been stored in the refrigerator, the second was a fresh ferment aged 2, 7 or 12 days old. The positive preference index indicates a preference for the fresh ferment. The red line shows the linear regression (P < 0.001, r² = 0.28). n = 12 trials per condition. The mean and standard deviation of the total captured flies for each trial was 105 ± 59. d, CO₂ concentration in 500-ml fly-rearing bottles under common laboratory conditions. n = 6 trials per condition. e, Measurement setup for the data shown in d. f, Time course of CO₂ concentration measurement for three bottles filled with different concentrations of CO₂. n = 3 per calibration gas. g, Peak measured CO₂ concentration versus actual CO₂ concentration for the calibration gases (black). Coloured lines show the measured peak concentrations for the actual fly-food bottles, and the resulting CO₂ concentrations shown in d. In all panels, shading indicates the bootstrapped 95% confidence intervals around the mean.