WHEN IS THE RUT?
By Christopher S. Rosenberry, PhD, Wildlife Biometrician
Bret D. Wallingford, Wildlife Biologist

AS SUMMER gives way to autumn, white-tailed deer begin preparing for the breeding season. Bucks begin sparring and testing themselves against each other, and they begin scent marking by rubbing trees and creating scrapes. All of this is in preparation for the rut, when their stamina and dominance will be tested and they have a chance to pass their genes on to future generations of deer.

Historically, every year hunters have harvested approximately 75 percent of all bucks in Pennsylvania, and about 80-85 percent of the buck harvest has been yearlings. Because of the large difference between the harvest rates of bucks and does, there was concern that buck:doe ratios may be skewed enough to disrupt breeding.

Ideally, the vast majority of does should be bred during a short period each autumn. A short, properly timed breeding season provides a " flooding" effect when fawns are born the following spring. Having most fawns born over a short time reduces the effects of predation, because a large number of fawns are available. Most fawns quickly grow past their most vulnerable time of life before predators can have a high impact on the age class. A properly timed breeding season also results in fawns being born during warm weather and, in turn, gives fawns the optimum amount of time to grow before the upcoming winter.

But if, during the breeding season, there are not enough bucks, unbred does will repeat their monthly estrous cycles until they are bred. In this case, the fawning season is spread out from June into July, August and even longer. This increases the risk of predation, because young, vulnerable fawns are available over a long period of time. Also, fawns born late in the season have less time to grow and put on fat for winter.

When do deer in Pennsylvania breed, how long is the rut, and are there enough bucks to get does bred on time? To answer these questions, Game Commission personnel began collecting breeding date information from roadkilled does from February through May in 2000 and 2001. We determined the timing of the rut by measuring fetus length, which told us how old the fetus was, and then it was a matter of simply back-dating to find out when conception took place.

In addition, we can estimate when birth would have taken place if the doe had carried the fawn through to full term. Nearly 2,000 does were examined, allowing us to measure the breeding season in the fall of 1999 and 2000.

Overall, the rut ran from September into February, but 9 out of 10 does were bred from mid-October to mid-December. The peak of the rut occurred in mid-November (Figure 1). This peak in breeding resulted in a peak fawning time of late May to early June. The gestation period (time from breeding to birth) of deer is about 200 days.

Compared to adult does, female fawns were bred later. Peak breeding by fawns occurred in late November and early December, and nearly half of all fawn breeding occurred from December to February. Fawn breeding probably explains some breeding activity observed by hunters during the firearms season after Thanksgiving.

Overall, one of every four female fawns was pregnant, while nine out 10 adult does were pregnant. Fawns generally had single fawns and adult does usually had twins. Triplets were rare (Table 1).

From these results, we can conclude that Pennsylvania' s deer population contains enough bucks to breed does in a relatively short time period. Most adult does are being bred around November, with fawn breeding extending through December into February.

Late breeding by fawns may not be indicative of lack of bucks. Rather, it may be related to maturation of fawns, whereby they don' t become sexually mature until late November. Sexual maturity in fawns is related to body size, which is affected by quality and quantity of food as well as birth date. Consequently, fawns in the most productive areas of Pennsylvania may reach sexual maturity and become pregnant (Table 1). Fawns in other areas with poorer nutrition may not reach sexual maturity until the following year, when they are a year and a half old.

Because of the high harvest rate of bucks in Pennsylvania, most breeding has been done by yearling bucks, those in their first breeding season. Throughout the evolution of the whitetail, this has not been the normal situation. In a more natural situation, white-tailed bucks determined breeding privilege based on size and strength. Extremely high buck harvest rates modified this in the last 100 years.

Older bucks play a very important role in the social order of deer. They are the governors of the male side of the population, and typically do most of the breeding. Older bucks provide greater social stability during the breeding season, with yearling bucks participating less in the breeding. Reduced breeding activity allows yearlings to keep more of the body fat and weight they accumulated over the summer and fall, which can become critical during the winter.

Because of the breeding ecology of deer, increasing male competition for mating is desirable in Pennsylvania. This increased competition will force bucks to determine which animals will breed based on their physical fitness. Clearly, antler restrictions will increase competition for male breeding by moving more bucks into the older age classes. This will create a more normal breeding ecology than we had when the male yearling mortality rate was much higher. The presence of more older bucks can alter breeding activities and characteristics of a deer population. But will any changes be found after the new antler restrictions allow more bucks to reach the 2-year-old age class? To answer this question we will continue collecting conception data from roadkilled does as the deer population is modified by the new antler restrictions.