Like many other living things, mule deer are subject to photoperiodism. Photoperiodism is the physiological reaction of organisms to the length of day or night. The length of daylight affects the timing of migration, hibernation, and many other behaviors for a wide variety of animals.

For mule deer (and many other living things), their reproductive cycle is controlled by the length of day and night. In the fall, the days are getting shorter, and when they reach the right length, it triggers an increase in production of melatonin. That causes hormonal changes which start the breeding season, which is called rut. Those hormonal changes also determine when the males lose their antlers.