Nocturnal male sex drive in Drosophila.

Many behaviors and physiological processes including locomotor activity, feeding, sleep, mating, and migration are dependent on daily or seasonally reoccurring, external stimuli. In D. melanogaster, one of the best-studied circadian behaviors is locomotion. The fruit fly is considered a diurnal (day active/night inactive) insect, based on locomotor-activity recordings of single, socially naive flies. We developed a new circadian paradigm that can simultaneously monitor two flies in simple social contexts. We find that heterosexual couples exhibit a drastically different locomotor-activity pattern than individual males, females, or homosexual couples. Specifically, male-female couples exhibit a brief rest phase around dusk but are highly active throughout the night and early morning. This distinct locomotor-activity rhythm is dependent on the clock genes and synchronized with close-proximity encounters, which reflect courtship, between the male and female. The close-proximity rhythm is dependent on the male and not the female and requires circadian oscillators in the brain and the antenna. Taken together, our data show that constant exposure to stimuli emanating from the female and received by the male olfactory and other sensory systems is responsible for the significant shift in intrinsic locomotor output of socially interacting flies.