At the same time, scientists have more and more evidence that an overabundance of deer takes a surprising toll not only on suburban pansies and azaleas but also on wild forestlands. From the Rocky Mountains to the Midwest and up and down the East Coast, white-tailed deer are vacuuming up acorns, herbs, flowering plants, woody shrubs and short saplings that make up the forest's "understory," plants from ground level up to about six feet. West Virginia University ecologists have reported that deer are a major threat to American ginseng, an increasingly rare herb.
Deer browsing transforms forest ecology. Harder to find are yellow spicebush blossoms (and the spicebush swallowtail butterflies that feed on them), highbush blueberries and white trillium. Invasive Japanese knotweed, multiflora rose and garlic mustard outnumber once common native orchids and ferns. Thrushes, warblers and other shrub- and ground-nesters have taken flight in search of choicer areas. Chipmunks, frogs and snakes are rarer because they have little to eat—as do the hawks and owls that prey on them. What's left is a "ghost forest," says Eric Stiles, the New Jersey Audubon Society's vice president for conservation and stewardship.
Stiles says that in the past 30 years the group's Scherman-Hoffman Wildlife Sanctuary, in the north-central part of the state, has lost dozens of species of plants and well over a dozen species of birds that nest on the ground or in shrubs because of the burgeoning deer population. For the first time in its 108-year history, the conservation organization recommended hunting in its sanctuaries as a wildlife-management strategy. But in many areas, including almost all suburbs, shooting deer is both unpopular and illegal. Which is why deer contraception may be an idea whose time has come.
That day in the Virginia forest, biologist McShea measures the doe's length (67 inches, from the tip of her tail to her nose) and girth (27 1/2 inches) and attaches a numbered tag to each ear. An assistant uses a portable ultrasound device, which looks like a computer monitor, to measure the fat on the deer's rump, an indicator of health.
"One, two, three," McShea says. "Go!" They release the animal. "That's the only dicey part. Whoever lets go last is going to get kicked." Researchers have gotten black eyes, bruises, gashes worthy of stitches, but no one is hurt this time. The doe prances off into the woods.
At the 3,200-acre campus of the National Zoo's Conservation and Research Center near Front Royal, Virginia, McShea has been overseeing a study of immunocontraception, or using a vaccine to prompt the animal's immune system to prevent conception. Thirteen years ago, McShea tested a drug called porcine zona pellucida, which comes from pig eggs and is known as PZP. He found that the drug prevented pregnancy, but only for one season. Since 2003 he has been testing a Canadian PZP-derived drug called SpayVac, which costs $110 per dose but may last a deer's lifetime.
Unlike human hormone-based contraceptives, which prevent ovaries from releasing an egg, PZP causes a female deer to produce antibodies that stick to the egg's surface. The antibodies block sperm from fertilizing the egg. The drug has to be given directly into the bloodstream, largely because researchers have yet to develop an oral version that can survive a trip through a deer's digestive system. The U.S. Food and Drug Administration still considers the drug "experimental." The manufacturer prohibits women, such as McShea's female co-workers, from handling the contraceptive—nobody knows if it could sterilize them too.
The drugs aren't perfect. McShea's team has injected 38 deer with SpayVac. None had fawns in the first year after treatment. But at least two treated does gave birth the second season.
Not even reliable contraceptives can, by themselves, solve the problem, McShea says. Before the SpayVac tests began in 2003, he had 232 deer culled from his 850-acre experimental area, leaving about 50 deer behind. Now the population in that area is growing, despite contraceptives, because new deer are immigrating to the property. He says widespread hunting will be needed to reduce deer populations to a sustainable level. McShea says he's "guardedly optimistic" that, eventually, improved immunocontraceptives will help control deer overpopulation. Until people learn to manage the deer numbers, McShea says, his advice is to "drive slower."