Japanese researchers say they have proof that an internal biological clock -- the so-called Circadian rhythm -- plays a role in letting roosters know when it's time for their morning crow.
The sound of a rooster's familiar morning greeting -- known to many as "cock-a-doodle-doo!" -- often occurs like clockwork as the rising sun lights up the eastern horizon at the start of a new day.
But how do roosters know when it's time to perform? Do they have an internal sense of the time of day, functioning like nature's alarm clocks, as some experts believe? Or are they just reacting to what's going on around them, prompted to crow by the sunlight or other environmental cues?
"'Cock-a-doodle-doo' symbolizes the break of dawn in many countries," says Takashi Yoshimura of Nagoya University. Writing in the journal Current Biology, Yoshimura added it's not clear whether crowing is under the control of a biological clock or is simply a response to external stimuli.
Yoshimura notes that roosters don't only crow at dawn, but also at other times of the day, which suggests that external factors, such as the stray glare of a car's headlights, or the sound of another rooster crowing nearby, can motivate the bird's vocalizations.
To find out the degree to which internal and external factors prompt the morning crowing, Yoshimura and his colleague Tsuyoshi Shimmura exposed a group of roosters to constant "dawn" light and then turned on their recorders so that they could watch and listen to them.
Kept under this steady simulated twilight, the roosters initially maintained their schedule of crowing just before dawn each morning, suggesting that the behavior is linked to a Circadian rhythm, a natural synchronization many plants and animals -- including humans -- have with the Earth's 24-hour day-night cycle.
The researchers noticed that while the roosters could be spurred to crow throughout the day by external factors, the intensity of their crowing was greatest at the dawn hour.
Over time, however, the daily crowing became more scattered, suggesting that the birds' Circadian rhythm was weakened by their regimen of perpetual twilight.
The researchers believe these behaviors indicate that the roosters' internal Circadian clock not only governs their morning crowing, but also moderates their response to external stimulation.
Yoshimura and Shimmura say that this study is just the beginning of their efforts to learn more about roosters' natural vocalizations, which they say are not learned like most other bird songs or human speech.
"We still do not know why a dog says 'bow-wow' and a cat says 'meow,'" Yoshimura says. But there is interest in the mechanism of genetically controlled behavior and they believe that chickens provide an excellent model.