Why Are There Annual Cycles In Antler Growth?
Believe it or not, the 23 degree tilt of the Earth's axis is the ultimate
cause for the annual cycles in deer antlers. This tilt is what causes
Earth's annually recurring seasons. Deer have adapted their physiology and behavior to these seasonal changes, including antler growth. The
environmental cue that regulates antler growth is the amount of day
length, or photoperiod.
The physiological cue is the male hormone testosterone. The way this works is complicated, but changing day lengths are sensed by the eyes, which send this message, via the optic nerve, to the pineal gland. The pineal gland - a pea-sized organ at the base of the brain - produces many different hormones. One hormone produced is luteinizing hormone, which controls the amount of testosterone produced in the testes.
The antler cycle lags behind the changes in day length because the
hormonal changes take time. During fall, decreasing day lengths cause
melatonin production to increase, resulting in decreased production of
both luteinizing hormone and testosterone. Decreasing testosterone levels then cause the antlers to shed.