NAGOYA, Japan, March 18 (UPI) -- Roosters crowing at dawn aren't just reacting to the external stimulus of light, Japanese researchers say; they actually know what time of day it is.
In a study reported in the journal Current Biology, scientists at Nagoya University say there is indeed a clock in "cock-a-doodle-doo" -- a biological one.
"'Cock-a-doodle-doo' symbolizes the break of dawn in many countries," researcher Takashi Yoshimura said. "But it wasn't clear whether crowing is under the control of a biological clock or is simply a response to external stimuli."
While external stimuli such as a car's headlights can cause a rooster to crow at any time of day, the researchers say the morning call is a matter of their responding to their internal clock.
Yoshimura and his colleague Tsuyoshi Shimmura placed birds under constant light conditions and found when kept under round-the-clock dim lighting, the roosters were still crowing each morning just before dawn, proof that the behavior is linked to a circadian rhythm.
So whether it's "cock-a-doodle-doo" or "ko-ke-kok-koh" as they say in the research team's native Japan, roosters are reliable alarm clocks because they've got a clock of their own, the study found.