With deer season quickly approaching, conversations increasingly turn to all things deer. And one of the commonly discussed topics is when the “rut,” or breeding season, will occur and what causes it.
The stimulus for determining when the breeding season occurs for a species must be very predictable. And, of course, it must affect both males and females. The breeding season for deer and most other animals is determined by changes in photoperiod.
Changes in day length, or photoperiod (the amount of light or darkness in a 24-hour period), unlike weather and many other environmental factors, are very predictable and change throughout the year. The shortest day of the year, for example, is on or around December 21, while the longest day is on or around June 21.
In mammals, the pineal gland, an endocrine gland located within the brain, is sensitive to changes in day length and regulates biological rhythms including reproductive biology. Changes in photoperiod cause changes in hormone production by the pineal gland that leads to changes in the production of sex hormones such as that resulting in the breeding season.
Although photoperiod may be the stimulus for the breeding season, the breeding season is timed such that the birth of the offspring occurs when conditions are conducive to their survival. For most big game in our area, the breeding season occurs during the fall, and most of the young will be born around mid-May through June. That gives them the optimum chance for fully developing before winter sets in.
Most of the breeding in white-tailed deer will occur within about a three-week period in November. Not all bucks and does will respond the same, of course, but the does will come into estrous for about 24-48 hours on a 21-30-day cycle during the breeding season.
Other factors will affect or modify the breeding season. For example, the nutritional status of a doe affects when she will come into estrous, with estrous delayed in nutritionally stressed does. Pheromones left by bucks on rubs and scrapes have been shown to have a stimulatory effect on does coming into estrous. And weather, hunting pressure and a variety of other factors may also influence deer behavior during the breeding season.
So, give some thought to all those physiological and environmental factors that influence the rut or breeding seasons of deer and other mammals. All is done to help ensure a successful next generation.