By: John Bowers, Georgia DNR Chief of Game Management and Charlie Killmaster, Georgia DNR state deer biologist; with info provided by Bobby Bond, Georgia DNR Wildlife Biologist
The timing of the rut, or breeding season, is of keen interest to many hunters and managers of white-tailed deer. Generalizations and errant conclusions are often made based on single observations of specific activities observed such as a buck chasing a doe, or a single deer visiting a scrape or making a rub. While these are certainly important reproductive related behaviors that can be observed, they are not objective assessments of determining the peak or length of the rut in white-tailed deer. It’s important to understand the basics of the rut!
The primary trigger for breeding in white-tailed deer is the photoperiod. Photoperiod refers to the duration that a plant or animal is exposed to light within a 24-hour period. In the fall, shortening day length reduces the photoperiod and this triggers several physiological and behavioral changes in white-tailed deer that initiate breeding. Yet, photoperiod is not exclusively responsible for the timing of the rut. Photoperiod opens a “window” during which breeding can occur. How long that window remains open is determined by other factors, such as nutrition and herd demographics. For example, does in poor condition from inadequate nutrition may delay or forgo breeding. Areas with imbalanced sex ratios (e.g., too many does to bucks) or skewed age structures (e.g., young buck age structure) can result in lengthy, or prolonged, breeding seasons. Prolonged breeding seasons can have several undesirable affects including reduced buck survival and a prolonged fawning season that can reduce fawn survival due to malnutrition, predation or exposure. Properly managed deer herds with balanced sex ratios and improved age structures have shorter breeding seasons with intense rutting behaviors, increased competition for breeding and enhanced hunting opportunities. Poorly managed herds with sex ratios skewed toward does and imbalanced buck age structures often lack these desired opportunities.
Wildlife biologists scientifically determine the breeding period for white-tailed deer using a fetus scale, which is a specially designed “ruler” that allows biologists and managers to determine fetus age in days. Deer fetuses are collected from does harvested late in the deer season after the rut has occurred. It is important to note the harvest date of the doe from which the fetus(es) was collected. Armed with the fetal age and the harvest date of the doe, a biologist can determine the date of conception. When this data is collected from numerous does within a hunting season, biologists can determine the range, or length, of the breeding season (determined by the earliest and latest bred does) and the peak of breeding (usually a 7-10 day period when the majority of does were bred). By “forward” dating, biologists can determine the length of parturition (birthing or fawning) and the peak of fawn drop.
This method allows biologists to scientifically determine the rut and this scientific information is an important consideration in managing white-tailed deer and informing policy and regulatory decisions for establishing biologically appropriate hunting seasons. It can also be important for hunting clubs and managers to add this data collection tool to their arsenal so they can “fine-tune” management decisions for the properties they manage and hunt.
This photo shows four fetuses collected by Ms. York of York’s Deer Processing in Barnesville (Lamar County). They are from a single doe that was harvested on January 1, 2014. Wildlife biologist Bobby Bond aged the fetuses on a fetus scale (shown in the photo). They were aged at 50 days old. Back dating from Jan. 1 returns November 12, 2013 as the date of conception, which coincides well with known rut for this region of the state (early-mid Nov.). Also, using the scale, the day of parturition would have occurred in about 148 more days or on May 29, 2014. Interesting, Mrs. York also previously provided us with a picture of 5 fetuses from a single doe the year before!
It should be noted that adult does in good condition typically give birth to twins. Occasionally, very healthy does can give birth to triplets. It’s rare that an adult doe will successfully carry three or more fetuses to full term. Even though a doe may conceive three (3) or more fetuses, one (1) or more of the fetuses are unlikely to survive in-utero and will be reabsorbed.