Seasonal regulation of deer reproduction as related to the antler cycle - a review

The antler cycle in male deer is closely associated with the photoperiodic regulation of its reproduction. In most boreal cervids antlers grow during the late spring and early summer, they mineralize before the rut and are cast thereafter. A cascade of events involving several hormones such as melatonin, prolactin (PRL), luteinizing hormone (LH) and testosterone (T) mediates the primary effect of the photoperiod. The pineal gland serves as a neuroendocrine transducer of circadian and circannual variation of photoperiod, producing melatonin in response to the onset of darkness. Peak melatonin levels occur in December while lowest levels are observed in June. Declining melatonin concentrations in the blood after winter solstice unblock the production of PRL which increases until the peak levels are reached in mid June. High PRL levels block LH receptors on the testicular Leydig cells and thus prevent them from producing T too early, at a time not yet suitable for reproduction. The decline of PRL after June unblocks the LH receptors and allows ever stronger stimulation of T production by LH. Experimental manipulations of the photoperiod, exogenous administration of melatonin or blockade of PRL secretion by bromocriptine induce a distinct seasonal shift of reproductive patterns and the antler cycle. In addition to T, the female sex hormone estradiol appears to be also involved in the development of antler bone by being involved in the formation of the ivory compact bone.