Back before alarm clocks jolted us awake to greet the morning with bleary-eyed confusion, roosters performed that daily duty. Now, a new study shows that roosters don't need the light of a new day to know when it's dawn—rather, their internal clocks alert them to the time.

While researchers at Nagoya University in Japan were studying the genetic underpinnings of innate vocalizations—or nonlearned behaviors such as crowing—in chickens, they discovered that the male birds don't need external light cues to know when to start crowing. (Also see "Night Owls Stay Alert Longer Than Early Birds.")

"To our surprise, nobody [has] demonstrated the involvement of the biological clock in this well-known phenomenon experimentally," study co-author Takashi Yoshimura, who specializes in biological clocks at Nagoya University, said in an email.

Shedding Light on Roosters

During their experiments, Yoshimura and colleague Tsuyoshi Shimmura, also of Nagoya University, put PNP roosters—an inbred strain of chickens used often in laboratories because of their genetic similarities—through two different light regimens.

In the first experiment, roosters experienced 12 hours of light and 12 hours of dim light conditions for 14 days. The scientists found that the roosters would start to crow two hours before the onset of light—called anticipatory predawn crowing—consistent with observations in wild red jungle fowl. (See National Geographic's pictures of game birds.)

In the second experiment, roosters were kept under 24 hours of dim light conditions for 14 days. Yoshimura and Shimmura noticed that the animals started running on a 23.8-hour day and would crow when they thought it was dawn, according to the study, published March 18 in Current Biology.

When the scientists exposed the roosters to sound and light stimuli to test whether external cues would also elicit crows, they found that the animals would vocalize more in response to light and sound in the mornings than during other times of day. This means the roosters' internal clocks take precedence over external cues.

The researchers also found social rank among the birds affected the timing of when they crow.

"Crowing is a warning signal advertising territorial claims. Our preliminary data suggest that the highest ranked rooster has priority in breaking the dawn, and lower [ranking] roosters are patient enough to wait and follow the highest ranked rooster each morning," said Yoshimura.

Rooster Study a Long Time in Coming

Kristen Navara, a hormone specialist in poultry at the University of Georgia in Athens, said she isn't sure why no one has taken a closer look at this phenomenon before. (See pictures of rare chickens in National Geographic magazine.)

"I think many times we don't think to study what appears right in front of us," Navara, who wasn't involved in the research, said by email.

For instance, "we have definitely noticed in our own roosters that they begin to crow before dawn and have wondered why that was, but just never thought to test whether it was a circadian rhythm driven by an internal clock rather than an external cue."