In this study, we discovered that the highest-ranking rooster crows first every morning, followed by its subordinates in descending order of their social rank (Fig. 1c–e). Although anticipatory predawn crowing was usually observed approximately 2 hours before light onset, the timing of the top-ranking rooster’s first predawn crow varied to some extent (Fig. 1a). Crowing order, which reflects social rank, is strictly conserved even when the timing of the top-ranking rooster’s first crow is advanced or delayed each day (Fig. 1f). We also observed that lower-ranking roosters crowed less than higher-ranking roosters (Fig. 1b,e). Previous studies reported that the presence of a dominant rooster suppresses subordinates’ crowing15,16. By contrast, when we examined the effect of external stimuli (light stimulus or crowing sound stimulus) on crowing behavior, the percentage of first crowing (Supplementary Fig. S2a,b) and the number of crows (Supplementary Fig. S2c,d) were independent of social rank. These results suggested that although lower-ranking roosters do have the potential to crow, predawn crowing by subordinates is repressed by the presence of dominant roosters. This idea is also supported by the experiment in which we removed the top-ranking rooster. In the presence of the top-ranking rooster, the crowing of second ranking rooster was suppressed (Fig. 1c–e). However, once the top-ranking rooster is physically removed from the group, the second-ranking rooster behaves as if he is the top-ranking rooster (Fig. 2).

Because the timing of subordinates’ crowing was closely related with that of the top-ranking rooster, it was reasonable to speculate that the crowing rhythms of subordinates were entrained to the crowing rhythm of the top-ranking rooster. Indeed, the free-running periods of subordinates’ crowing were coincident with that of the top-ranking rooster under dimLL conditions (Fig. 3a,b). When the top-ranking rooster was removed from groups, the second-ranking rooster took its place and the free-running periods of the remaining three individuals were altered (Fig. 3c,d). Notably, the lower-ranking rooster sometimes crowed first in this situation (Fig. 3c), probably because the second-ranking rooster was not as able as the top-ranking rooster to repress lower-ranking roosters (Supplementary Fig. S3). We then analyzed the free-running rhythms of body temperature rhythms, but found that they differed among individuals (Fig. 4). In addition, anticipatory predawn crowing did not entrain to the timed sound stimulus of other roosters’ crowing (Fig. 5). This result was in marked contrast with the circadian clock of songbirds, which can entrain to a sound stimulus17,18. All of these results suggested that the circadian clocks of subordinates are not entrained to that of the top-ranking rooster and an identical free-running period observed within a group appeared to be a “masking response”, i.e., a direct response to environmental stimuli.

Because the top-ranking rooster always started to crow first each morning (Fig. 1, 2, Supplementary Fig. S1), it was also reasonable to speculate that the free-running period of the top-ranking rooster is shorter than those of its subordinates, or that the rooster whose free-running period is the shortest would become the highest-ranking rooster. However, both of these hypotheses are unlikely to be true, because the top-ranking rooster did not always have the shortest free-running period (Fig. 3e, 4). However, interestingly, the top-ranking rooster showed similar free-running period in both crowing and body temperature (Fig. 4b), which suggest that top-ranking rooster start crowing on his own timing. On the other hands, although the subordinate rooters also have their own free-running rhythm of body temperature, they appear to keep a pace with the top-ranking rooster. Taken together, the data suggest that subordinate roosters have the potential to crow, but they are patient enough to wait for the top-ranking rooster’s first crow.

In summary, in this study, we demonstrated that the highest-ranking rooster has priority to announce the break of dawn, based on his own circadian clock and that subordinate roosters compromise their clocks for social reason and wait for the top-ranking roosters’ first crow every morning.