Extended Data Fig. 4: Responses to CO₂ are strongest at 5% concentration and are unaffected by social dynamics.

a, Control and 5% CO₂ responses for individual flies. For these experiments, we starved a single two-day old wild-type (HCS) female fly for either 24 h or 3 h before starting the experiment. In every other way, the data are plotted as in Fig. 4. The data shown were collected from n = 29 individual flies, in which each fly was subject to a 20-h long experiment with n= 14 5% CO₂ stimuli and n = 10 control stimuli. b, CO₂ responses exhibited by flies to three concentrations of CO₂. For these experiments, we starved groups of 10 flies for 24 h before starting the experiment. Flies were presented with 0%, 1.7% or 5% CO₂ in one set of experiments, and 0% or 15% in another set. Data are plotted as in Fig. 4. n = 20–170 trials per condition. To explain the complex dynamics of the approach behaviour under the different CO₂ concentrations, we made a very simple agent-based model with the pseudocode shown in c; see Supplementary Information for additional discussion. d, Dynamics of the CO₂ attraction of flies can be explained by the simple agent-based model described in c. Preference indices are shown for the results of n = 100 iterations of the model, under three different CO₂ concentrations. The data are plotted in the same manner as b. The key insight offered by this model is that although our agents were programmed to exhibit the same behaviour towards 1.7% and 5% CO₂, the decreased likelihood of them detecting the lower concentration CO₂ in conjunction with the long-term aversion results in an apparent indifference towards low concentrations of CO₂. e, To show that flies are indeed attracted to the low (1.7%) concentration of CO₂, we used a different analysis that calculated the number of times that flies approached the CO₂ source during the course of each 10-min stimulus. Pairwise statistics were determined with the two-sample Kolmogorov–Smirnov test (test statistics were 0.57, 0.83 and 0.41 for comparisons between 0% and 1.7%, 5%, and 15%). f, Time course of the number of times that flies approach the CO₂ source, in 5-min intervals. In each panel, the shading shows the bootstrapped 95% confidence intervals around the mean.