Roosters greet the rising sun with [crowing sound effect]. But they also crow at other times. So are they responding to the light? Or do they simply know that it’s morning? New research says the latter: roosters crow because of internal time cues. The finding is in the journal Current Biology. [Tsuyoshi Shimmura and Takashi Yoshimura, Circadian clock determines the timing of rooster crowing]

Scientists controlled the light levels in rooster habitats. For two weeks, the birds experienced 12 hours of light followed by 12 hours of dim light. Consistent with the pre-dawn noises observed in wild fowl, the roosters began to crow about two hours before their rooms lit up.

Then, for two weeks, the roosters lived in constant dim light. Yet they continued to crow about once a day—at intervals of 23.7 hours to be precise. Even without morning light, their circadian rhythms told them when dawn should be breaking.

The birds also crowed in response to sudden light, and to the sounds of other roosters. But they were more likely to react when those stimuli occurred near dawn. Showing that you can’t really keep a rooster in the dark about the time.

—Sophie Bushwick

[The above text is a transcript of this podcast.]