By Joe Rogerson, Division of Fish & Wildlife Game Mammal Biologist
While bucks are biologically and physiologically capable of reproduction the moment they shed their antler velvet, does are not ready for mating at that same time. The onset of the rut, or deer mating season, is triggered by a reduction in the number of hours of daylight within a given day. Through thousands of years of evolution, the timing of the rut within a given year is set such that it creates a situation in which, when fawns are born about seven months later, they arrive during the peak of the year’s nutrition cycle. Some researchers have suggested that the rut is triggered by lunar cycles, but the currently-accepted theory finds that the rut comes when daylight diminishes as fall and winter progress.
In categorizing the rut, researchers have divided it into four parts. It should be noted, however, that these periods can and will overlap, so distinguishing each individual phase may be difficult.
Pre-Rut: After bucks shed their antler velvet they often begin sparring matches among themselves. These are not life-and-death fights but simply a preliminary testing of their competitors. During this time bucks are capable of breeding, but again, the does aren’t ready yet.
Bucks during this time of year can typically be found commingling with other bucks. These groups are commonly referred to as “bachelor groups.”
Chasing Phase: In Delaware this period typically takes place during the latter part of October and the beginning of November. At this time, bucks are eagerly attempting to breed with does, but the does aren’t physiologically ready to accommodate them. As a result, bucks begin to follow the does around and often will chase them. Scraping and rubbing activity on trees and shrubs often begins to peak during this time period. At first bucks follow the does from a distance, often shadowing them. As the doe nears her full estrus cycle the bucks’ chase becomes more intensified. Hunters often refer to this time period as the rut, but the true definition of the rut is the time period in which does are actually bred.
The Rut: The estrus period, when a doe is most fertile, only lasts about 24 hours. The doe will now stand still for the buck, rather than run away from him the moment he tries to get very close to her. She will now tolerate the buck’s mounting attempt. After breeding, the buck will stay with that doe throughout her estrus period before he goes off to find a new estrus doe, commonly referred to as a “doe in heat.” Bucks typically breed several does in a very short time frame. Not all does come in heat on the same day, which enables this promiscuity.
It’s during this time that hunters often complain about seeing very few deer afield. This is a result of many bucks already having paired up with does in estrus, and thus not having to roam the woods (when they would be more visible to hunters) in search of a doe.
If a doe has not been bred the first time, she will come in heat again after 28 days. While researchers have found that some does are physiologically capable of six to seven estrus cycles, in reality the great majority of does are bred during their first cycle.
Post Rut: Any does that haven't been bred during their first estrus cycle will cycle back in again 28 days later, and this second estrus is what causes the post-rut.
The post rut is the same as the rut but much less intensive. Bucks are still wandering about and checking out doe feeding and bedding areas in pursuit of the last unbred doe.
Delaware white-tailed deer data: From January through April 2006, the Division of Fish & Wildlife conducted a white-tailed deer reproductive survey. This survey produced previously unknown information regarding the reproductive status and potential of Delaware’s white-tailed deer population. From the survey, it was learned that peak breeding had occurred from Nov. 10-20, 2005.
By November 20, 80 percent of the female white-tailed deer in Delaware had already been bred. This data signifies that nearly all does are bred the first time they come into estrus.
From additional data the survey yielded, it can be deduced that the buck-to-doe ratio in Delaware is relatively close. Because it was determined almost exactly when the does were bred, the white-tailed deer gestation period (approximately 6.5 months) could be added to the conception date to determine the parturition (fawning) date.
With a gestation period of about 200 days, it was determined that the peak fawning period was the last week of May and the first week of June. As far as breeding success among age classes, 96 percent of all deer older than 1.5 years were pregnant, while only 7.9 percent of the fawns (six-month-old deer experiencing their first estrus) were pregnant.