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Age Of Female Whitetail Deer Influences Timing Of Breeding

Four-year study also demonstrates habitat's effect on reproduction

Harrisburg, PA - Although Pennsylvania's whitetail rut may peak in mid November, breeding activity continues into the state's regular firearms deer

season, according to a recent four-year study conducted by the Pennsylvania Game Commission.

"Our research has confirmed that Pennsylvania's bucks still are actively pursuing does, particularly younger ones, at the outset of our firearms season," noted Dr. Gary Alt, Game Commission Deer Management Section supervisor. "The peak in breeding for adult females is mid-November, but for female fawns it occurs in early December.

"This peak in fawn breeding, however, doesn't represent a high point in overall breeding activity because there are fewer fawns in the population and a lower percentage of them breed. About 90 percent of the adult does checked in the study had conception dates ranging from Oct. 27 to Dec. 10; fawns, from Nov. 5 to Jan. 16.

"For years, we thought and reported Pennsylvania's rut peaked the week before Thanksgiving, when deer are provided closed season protection. That information was accurate, but it didn't tell the whole story. Now, thanks to this groundbreaking research, we have a clearer picture: Pennsylvania bucks aren't really cooling their jets in the pursuit of the opposite sex until well into December during our traditional firearms seasons."

Launched in 2000, the four-year deer conception study aimed to answer questions about pregnancy rates, the peak and range of the rut; when fawns are born, and the number of young carried per doe. Across the state, Wildlife Conservation Officers, biologists and other agency personnel collected data from road-killed deer. In all, 3,180 deer were examined.

The work involved determining the date road-killed deer died and measuring embryos found within the deer. While it wasn't a pleasant job for those gathering the data, the results have helped to clarify answers to important questions about Pennsylvania deer. For instance, the study showed 91 percent of the adult deer and 26 percent of the fawns examined statewide were pregnant.

"It would appear that our bucks are servicing an overwhelming majority of the available does in Pennsylvania," noted Bret Wallingford, the Game Commission deer biologist who coordinated the study. "But that interaction isn't guaranteeing the same reproductive success statewide. For instance, fieldwork showed the best deer reproduction was occurring in the state's southeastern and western counties, as well as in counties along the lower Susquehanna River. The worst rates were in the state's northcentral counties corresponding with some of the most overbrowsed habitat in Pennsylvania. The most reasonable conclusion to draw from this documented lack of reproductive parity is habitat. This isn't about over-harvesting deer, but rather soil fertility, agricultural crop availability, climate, overbrowsing and deer health.

Joe Kosack/PGC Photo

The study indicated habitat played a significant role in deer reproduction.
"Differences in reproduction were very noticeable in the number of fawns that were impregnated in their first year. For example, in northcentral Pennsylvania, less than 10 percent of the fawns checked were pregnant. In western and southeastern wildlife management units, however, the number of impregnated fawns approached 50 percent. That's quite a difference. But when you consider the vastly different environments these deer inhabit, it quickly becomes apparent just how important habitat is in defining a whitetail's life. Our highest reproduction came from agricultural settings, the worst, from our overbrowsed big woods counties."

The reproduction findings are significant because they suggest that in some places, at least, habitat conditions are adequate, and that agricultural areas and suburban sprawl can support plenty of deer. Of course, the downside of increased deer productivity and accelerated maturity is property owners are fueling it unwillingly, in many cases. Although these deer may not exceed the land's carrying capacity, they are impacting farmers' profit margin, landowner's landscaping efforts and motorist safety.

Timing of the rut may be affected by many factors including the length of daylight in a given day and age structure of the buck population.

"Some studies, especially those from southern states, suggest age structure of bucks in the population influences timing of the rut," noted Dr. Christopher Rosenberry, Game Commission wildlife biometrician. "Other studies in northern populations suggest length of daylight may have a greater effect than buck age structure. Given the young age structure of bucks in Pennsylvania at the time of this study and adult female breeding dates similar to studies in northern populations with older buck age structures, length of daylight appears more important than a young buck age structure.

Data collected in 2004 and subsequent years will begin to reveal the effects of current hunting regulations on reproduction, as well as habitat. According to Dr. Alt, "As we increase the amount of research on deer in Pennsylvania, we build a stronger base of information upon which management recommendations can be made. This helps keep the deer program defendable, which ultimately benefits our invaluable deer resource and Pennsylvania deer hunters. This research has provided some excellent information so far, and additional years of data will help answer even more questions regarding deer management regulations and reproductive success."



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