A few years ago, the central campus of Cornell University in Ithaca, N.Y., was facing a problem familiar to the Washington area. An infestation of white-tailed deer had pushed the community to the limits of its tolerance. Without natural predators, the deer reproduced until their numbers were limited only by the availability of food.

Gardens were being eaten. Lyme disease was spreading. Collisions with cars threatened human lives. Ecologists warned that the proliferation of deer was probably changing wild habitat enough to place songbirds at risk. Invasive plants began to fill the voids left by excessive grazing. By 2009, Cornell needed to find a solution that would satisfy everyone, including animal rights activists.

Typically, a deer boom is dealt with through hunting. Often, sharpshooting riflemen or archers are brought in to bait the animals into zones where shots can safely be taken. Cornell’s administrators took a different approach: They chose to experiment with sterilizing many of the wild deer on campus while allowing periodic hunting on nearby land — and the result was something that nobody anticipated.

Washington’s deer population has surged in recent years, particularly in Rock Creek Park, where last year an estimated 335 deer lived. Out of concern for overgrazed native plants, the National Park Service approved a plan to cull deer from the park using professional sharpshooters. Many District residents have criticized the program, demanding that nonlethal methods of population control be used, including the forms of sterilization and contraception that Cornell recently experimented with.

Surgical sterilization of wild animals has worked in some cases. Small bands of wild horses in Montana and Nevada have been reduced by choosing the most dominant stallions and performing vasectomies on them. Because horses maintain harems, in which a dominant male fights to maintain an exclusive ability to mate with many females, only a few stallions needed to be neutered to significantly reduce the birth rate. However, white-tailed deer do not maintain harems. Bucks and does form temporary pairs for a few days in order to mate. Bucks can travel for miles to seek out does in heat, which means that a large number of females would need to be targeted for sterilization.

By sterilizing dozens of does through tubal ligation, Cornell accidentally drew in new bucks from the surrounding area. (Bigstock)

The method of contraception chosen by Cornell was tubal ligation, in which a doe’s fallopian tubes are either blocked or severed. This prevents egg cells from reaching the uterus. Unlike chemical forms of birth control, tubal ligation is typically permanent and avoids the expense of capturing the same deer each year to maintain their infertility. At a cost of roughly $1,200 per deer, 77 does were captured and sterilized though tubal ligation. (Without the help of the Cornell University College of Veterinary Medicine, the costs would have been higher.)

A team of biologists led by Paul Curtis, an associate professor and extension wildlife specialist at Cornell, began to closely track changes in the deer population. Initially, the results looked promising: The birth rate went down. Yet the total number of deer remained steady over five years. Something strange was going on.

“Sterilization definitely did decrease fawn numbers, and doe numbers also declined,” Curtis said. “However, these population reductions were offset by increasing buck numbers. There were about 100 deer on campus when we started, and there were still about 100 deer [five years later].”

Something was attracting an abnormal number of mature bucks. Cornell’s biologists realized that the reproductive cycle of the ligated does was to blame.

Under normal conditions, all female whitetails go into heat within several weeks of each other and become pregnant at around the same time. This annual event is called the rut. However, if a doe is not impregnated during the rut, it will enter heat again the following month and again the month after that. Because the ligated does were unable to become pregnant, they continued to produce chemical signals of readiness to reproduce — signals that can attract bucks from miles away.

By preventing pregnancy in does, Cornell had accidentally invented a population of buck magnets that regularly drew in new deer from the surrounding area.

Perhaps something like this should have been expected, according to Bernd Blossey, chair of the Cornell Deer Research and Management Committee.

Two deer run across the San Francisco’s iconic Golden Gate Bridge on Friday, holding up the evening’s peak hour traffic. (The Washington Post)

“I’m an ecologist looking at the literature,” he said. “I thought that sterilization in an open population where things can move in and out won’t work. Maybe it was worth doing it in a sophisticated way to say we tried in the best possible way and it didn’t make a difference.”

After examining Curtis’s data, Cornell’s administration rethought its nonviolent approach to deer population control. The tubal ligation program was halted, replaced by a program of nuisance deer removal using a combination of professional trapping and hunting by volunteer archers.

“In winter 2013, our camera survey indicated there were 100 to 105 deer on campus. After the nuisance deer removal in 2014, the camera estimate was about 58 deer remaining on campus,” Curtis said.

“Because the bow hunters are volunteers, this program is essentially cost-neutral,” Blossey said.

Some of the meat from the culled deer is taken home by the hunters to be eaten; some is donated to anyone who wants it.

Cornell has begun experiments with ovary removal in deer, but Curtis’s team has already had a surprise.

“Three of the 77 tubal ligation deer gave birth to fawns,” Curtis said. “These three deer were recaptured and later were given ovariectomies. All three had ovarian anomalies, and at least one experienced tissue regrowth post-surgery.”

Even after the surgical removal of their ovaries, one of the three deer became pregnant again. It is not clear how this was possible. One supposition is that some ovarian tissue may have escaped the scalpel and regrown into a functioning ovary.

If some form of surgical sterilization or contraception is eventually successful among free-ranging populations of wild deer, it could change the way that urban and suburban communities deal with excessive deer numbers.

Tufts University in Massachusetts is studying the use of a contraceptive vaccine in white-tailed deer. The method involves stimulating the deer’s immune system to prevent pregnancy. Earlier experiments with captive deer have been successful. While it will be years until the results are clear, the ongoing challenges in the wild include high cost (it costs about $1,300 each time a deer is captured and given a shot) as well as the difficulty of monitoring large numbers of individual deer and getting them booster shots.

“It is nearly impossible to keep free-ranging deer on a booster schedule,” said Curtis, who has also been involved with immunocontraception experiments in deer. Without being restricted to an enclosure, deer that have been given shots can either hide or wander out of the study area. Deer from outside the study area will tend to move in and occupy the habitat opened up by a declining local population.

Landers is an author and freelance writer in Charlottesville.