Photoperiodic control of antler cycles in deer. III. Decreasing versus increasing day lengths

Deer were exposed for three years to photoperiods which increased or decreased two hours every four months, starting at 4L/20D or 20L/4D, respectively. Under both sets of conditions, antlers were repeatedly shed and replaced, usually in synchrony with every other time the day lengths were changed. On decreasing days, antler cycles were omitted as the photoperiod passed the equinox (12L/12D). On increasing days, the equinoctial photoperiod induced prolonged episodes of antler growth. The tendency for the antler replacement cycle to lock onto alternate changes in artificial photoperiods is consistent with the seasonal growth of antlers every other time the day lengths change in the natural environment. It is suggested that antler replacement is triggered neither by shortening nor lengthening days, but by the alternation of such changes irrespective of the direction of the shift in the photoperiod.