Humidity as a zeitgeber for circadian entrainment of insect systems

Humidity levels, like light and temperature, fluctuate daily yet are less predictable; however, whether humidity entrains circadian clocks and enables animals to synchronize behaviors to environmental variations remains unknown. Here, we investigate the circadian humidity entrainment in various insects. Multiple species robustly respond to humidity cycles, and when the humidity cue is removed, their rhythmic behaviors continue, suggesting that humidity-associated rhythmic activities are under circadian control. Moreover, the Drosophila clock and hygrosensation mutants lack rhythmic activities during and after humidity entrainment, indicating that the core clock components and hygrosensors are essential to circadian entrainment. Our findings identify that humidity serves as a novel zeitgeber for circadian entrainment for insects that could have broad applicability and importance among animal systems.