Photoperiodic control of antler cycles in deer. IV. Effects of constant light:dark ratios on circannual rhythms

Yearling sika deer, when exposed to constant unequal photoperiods since the autumnal equinox, replace their antlers the next spring and at irregular intervals thereafter. On equatorial light cycles (12L: 12D) they retain their original antlers indefinitely. The changeover occurs between constant day lengths of 12 3/4 and 13 hours. Since yearlings held on 6L:6D:6L:6D are able to replace their antlers on schedule, the failure to do so on 12L:12D is not due to light:dark parity, but to the 12-hour durations of the days and/or nights.