Analysis of arrhythmic mutants uncovers strong clock-independent temperature-driven regulation of daily activity. The daily activity profiles of flies lacking circadian clock function are readily driven to synchrony in response to daily temperature gradients. Relative to the behavioral profiles of temperature-entrained wild-type controls the mutant data show reduced bimodality (per
01, and cyc
01) and a more gradual decrease in activity with decreasing temperature (cyc
01). Daily locomotor activity profiles (mean ± S.E.M.) in the presence of a daily temperature gradient were determined from the median activity records for groups of male and female flies carrying arrhythmic per
01, or cyc
01 mutations in a y w genetic background (white lines with black shading) and directly compared with that of y w controls (white lines with gray shading). The dotted lines in the activity profiles as well as the shaded bars underneath represent the daily temperature gradient (see Z7 profiles in Additional file 2).