Testosterone induces "splitting" of circadian locomotor activity rhythms in birds.

Under the influence of testosterone, the free-running circadian rhythm of locomotor activity of the starling, Sturnus vulgaris, tends to "split" into two components which temporarily run with different circadian frequencies: "splitting" occurred in intact birds whose testes grew, and in castrated birds that were injected with testosterone. Since "splitting" most probably reflects the temporal separation of two (or two groups of) circadian oscillators, these results suggest that testosterone affects the mutual coupling of circadian oscillators controlling locomotor activity.