Paul Cunningham, Associated Press A Pennsylvania Game Commission study on antlerless deer killed on highways provides some interesting results. Click photo for larger image.

If you live in Western Pennsylvania you have seen a road-killed deer. It's never a pleasant sight and sometimes downright revolting. At this time of year the visual impact is almost always accompanied by a potent odor, a brief sniff of which might leak in through the vents as we speed by at 60 mph.

That's the typical experience, but for the past four years, wildlife conservation officers and biologists with the Pennsylvania Game Commission have been experiencing much closer encounters with road-kill carcasses. They've been "getting their hands dirty" on the job, and that job has yielded some of the best evidence to date that deer numbers are ultimately controlled by the habitats where those deer live.

Starting in 2000, Game Commission personnel in the state began examining the reproductive tracts of road-killed does, year-round, to see what could be learned about whitetail reproduction. Researchers opened up (when necessary) car-casualty does to search for and examine embryos. They recorded the date of death and the number of embryos per doe, checked the sex and measured each fetus. More than 3,100 deer were examined and the results were released last week.

Since deer are known to have a gestation period ranging from 187 to 212 days, biologists interpreting the measurement data could determine the approximate date of conception. As suspected, they confirmed that the peak time of conception in adult does is mid-November. No surprise. But many of the deer examined in the study conceived much later, some as late as mid-January. These late-conceiving deer were primarily fawns, bred during their first winter.

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

Much more important, though, was the distribution of pregnant fawns around the state. Statewide, 91 percent of adult does examined proved to be pregnant, while only 26 percent of fawns carried embryos. The pregnancy rate among adults was fairly constant across various regions and wildlife management units, but that was not the case with fawns. In western and southeastern counties, and in the wildlife management units along the lower Susquehanna River valley, nearly half of all female fawns examined were carrying fetuses. In the north-central mountains less than 10 percent were pregnant. And both adults and fawns in the north-central region carried a lower average numbers of embryos.

Regions with the higher reproductive rates are characterized by agriculture, diverse landscapes and spreading suburbs where deer feed on gardens and ornamentals. The north-central mountains, where the research showed the poorest reproduction, are extensively forested but those woods were heavily browsed by abundant deer herds for many decades. In the past 20 years, deer herds there have apparently declined and some hunters blame the drop on the longer hunting seasons and higher antlerless license allocations authorized by the Game Commission.

That complaint is trumped by the Fawn Conception Study, which demonstrates that reproductive success is poor where food conditions are poor.

"The most reasonable conclusion to draw from this documented lack of reproductive parity is habitat," said Bret Wallingford, the Game Commission biologist who coordinated the study. "This isn't about over-harvesting deer, but rather soil fertility, agricultural crop availability, climate, over browsing and deer health."

Female deer weighing less than 75 pounds rarely conceive. Biologists believe that scarce food in the heavily browsed forests of north-central Pennsylvania prevents does from reaching that weight during their first breeding season, and some may never attain it.

"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 over-browsed big woods counties," Wallingford said.

All hunters like to see plenty of deer. But this new knowledge, gained by hands-on work in the field, proves again that poor habitats will not support big deer herds, even if doe hunting is stopped or curtailed. In deer management, trying to force a depleted range to carry high numbers of deer is like carrying water in a bucket without a bottom. You can try to pour more water in the top, but with no support at the bottom it's an exercise in futility.