Extended Data Figure 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 2-day old wildtype (HCS) female fly for either 24 hrs or 3 hrs prior to starting the experiment. In every other way, the data is plotted as in Fig. 4. The data shown were collected from N = 29 individual flies, where each fly was subject to a 20-hour 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 hrs prior to 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 behavior under the different CO₂ concentrations, we made a very simple agent-based model with the pseudocode shown in c, see Supplemental Materials for additional discussion. d, Dynamics of flies’ CO₂ attraction can be explained with 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 behavior 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, which 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 2-sample Kolmogorov-Smirnov test (test statistics were 0.57, 0.83, 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-minute intervals. In each panel, shading shows bootstrapped 95% confidence intervals around the mean.