Yes, deer do have a secondary breeding phase, often called the “second rut.” The term “rut” describes the primary, most intense breeding season, characterized by high levels of aggressive buck activity and widespread doe estrus. This later activity follows the initial peak and is driven by the female deer’s reproductive cycle and the delayed maturity of some younger animals.
The Biological Clock Driving Deer Reproduction
Deer are classified as short-day breeders, meaning their reproductive cycle is regulated by the amount of daylight they receive. As the days shorten in late summer and fall, a biological mechanism is triggered, ensuring the species breeds at the correct time of year. This mechanism is the influence of photoperiod on the deer’s hormonal system.
The pineal gland in the brain detects the increasing hours of darkness and responds by increasing the production of the hormone melatonin. Rising melatonin levels initiate a cascade of hormonal events, including the release of luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These hormones drive the doe into estrus and increase testosterone production in the buck, preparing them for the breeding season.
This precisely timed biological clock is necessary to ensure that the gestation period, which lasts approximately 200 days, results in fawns being born in the late spring or early summer when forage and weather conditions are most favorable for survival.
The Secondary Breeding Phase: Timing and Intensity
The timing of the secondary breeding phase is a direct consequence of the doe’s estrous cycle. If a mature doe is not successfully bred during her initial estrous period, she will recycle and become receptive again approximately 28 days later. This consistent physiological interval dictates when the secondary breeding activity will occur, typically shifting the action from the peak rut in mid-November into the middle or end of December.
While this phase marks a renewed interest from bucks, the overall intensity is lower than the primary rut. Fewer does are in estrus simultaneously, resulting in less widespread chasing, sparring, and aggressive behavior among males. The secondary rut is often less visible to observers because the activity is less synchronized and more spread out, involving only the small percentage of does who failed to conceive during the peak.
Participants in the Late Season Breeding Activity
Two main groups of deer drive the activity seen during the late breeding phase. The first group includes the mature does that failed to become pregnant during their first estrous cycle. These does are biologically programmed to enter estrus again roughly one lunar cycle after their initial window, maximizing their chance of conception.
The second group includes the doe fawns, young females born that same spring, who reach sexual maturity later than the adults. A doe fawn must attain a critical body weight, generally between 70 to 90 pounds depending on the geographical location and nutritional quality of the environment, before her reproductive system is fully functional.
In areas with excellent nutrition, these fawns achieve this threshold weight weeks after the adult breeding peak, coming into estrus for the first time in December or January. Breeding these late participants is an important factor in maximizing the overall reproductive success and productivity of the entire deer herd.