Deer Are Everywhere, but We Barely Know Them

On June 4, 2013, Buck 8917 did something weird, for a deer: He took a long, purposeful walk.

Researchers from Penn State had captured and put a GPS collar on the adult male that spring in Bald Eagle State Forest, about 15 miles northeast of State College, Pa. Put a tracker on most deer and you’ll find they stick pretty close to their home range, which was true for 8917. He sauntered, stopped to forage or bedded down for a nap mostly within an undulating square mile of forest full of towering hemlock and tangled rhododendron. But on that June day, he made a one-mile beeline, hiking to the top of a rocky ridgeline, where he seemed to while away the afternoon before walking directly home.

Then, in 2015, after two mating seasons, two hunting seasons and thousands of laps around his home range, Buck 8917 died — unsurprising given he was about 4 years old. It was where he died that surprised the researchers: that same ridge he’d visited just once in the two years he’d been collared.

Researchers don’t have a good explanation for Buck 8917’s odd visit, but it’s an example of the unexpected behaviors they observe while paying uncommonly close attention to the hoofed mammals, which are so frequently found across the North American landscape that we often take them for granted.

These discoveries are an outgrowth of the Deer-Forest Study, funded by the U.S. Geological Survey, the Pennsylvania Game Commission, the Pennsylvania Bureau of Forestry and Penn State. Now in its 10th year, the study has tracked more than 1,200 white-tailed deer around 100 square miles of Pennsylvania forest. It aims to be the most sweeping effort ever undertaken to understand North America’s most widespread large animals, as well as the impact they have on the vegetation and soil in our nation’s forests.

“It should be called the Forest-Deer Study because we’re really studying the forest,” said Duane Diefenbach, a Penn State ecologist and co-leader of the project.

On that front, Dr. Diefenbach and his colleagues have made some significant discoveries.

For example, the scientists have learned that Indian cucumber root, a flowering herb beloved by ungulates, won’t grow in soil high in manganese. That is a consequential finding because land managers often use the prevalence of the native plant as a way to measure deer population and set hunting quotas.

Research has revealed an interconnectedness between deer health and the fluctuating nutrients in forest vegetation. For instance, Canada mayflower makes up the bulk of a deer’s diet in the spring, when lactating does and antler-growing bucks need calcium and phosphorus and the plants contain extra doses of the nutrients.

But a decade of spying on deer has also yielded surprising revelations and quirky stories about the animals themselves. The scientists haven’t been shy about sharing this “serendipitous research,” as they call it, publishing more than 700 posts on the Deer-Forest Study blog. They’ve detailed everything from how much drool deer produce a day (two gallons) to what happens when a deer slinks back into the woods after a traffic collision (if it’s lucky, it limps but perseveres). Some entries, like Dr. Diefenbach’s account of Buck 8917’s mysterious death march, have attracted many readers.

“It just took off and we had no idea why,” said Jeannine Fleegle, a Pennsylvania Game Commission biologist who works with Dr. Diefenbach on the blog. “That’s when we realized this could really get a lot of attention on the project.”

Ms. Fleegle has blogged about one of the study’s most captivating characters, Doe 12866, in a series titled “The Real Does of the Deer-Forest Study.”

Like Buck 8917, this doe was remarkable for her get-up-and-go. Collared in Rothrock State Forest in January 2017, Doe 12866 was fitted with a vaginal implant transmitter that would notify the researchers when she gave birth, which she did the following May. To get to her “maternity ward,” as Ms. Fleegle called it, the very pregnant doe embarked on an all-night, six-mile hike to State College city limits, where she fawned in a patch of woods behind a housing development.

Does exhibit high birth-site fidelity — the tendency to return to locations where they had previous success raising their offspring — so it’s possible that Doe 12866 had given birth in State College before. It’s also possible that she left the woods for the city to avoid predators. The researchers saw Doe 16601 do something similar when she fawned near the intersection of two roads at the edge of a forest.

“Why would she choose to have her babies at the confluence of roads given the vast nothingness of the surrounding area?” Ms. Fleegle asked in a post. “Maybe 16601 is using us.”

As their populations increase, deer are living closer to humans than ever before — something many suburbanites with flower beds have probably noticed. But, Dr. Diefenbach explained, sharing space with us isn’t always easy for the deer. Living near humans means fewer predators, but it also means more commotion and heightened vigilance. “If you’re always looking out and being vigilant, you have less time to eat,” Dr. Diefenbach said.

In fact, Doe 12866’s fawn, which the researchers collared and nicknamed Rose, died less than a month after she was born. Her saliva contained high levels of the stress hormone cortisol, which the research team has found to be strongly correlated with fawn mortality. “Stress level does a better job of explaining survival than how many predators are around,” Dr. Diefenbach said.

And during hunting months, humans become predators.

Deer hunters play an important role in the study. Their hunting in designated areas of the forest, while staying out of others, helps researchers see how the landscape responds. Each year, participating hunters are asked to fill out a survey describing their experiences and observations. Over a decade of research, the team has gleaned new insights about how deer make it (or don’t) through hunting season, including how attuned they are to hunting pressure.

Take Doe 8921, also known as Hillside Doe. On the afternoon before rifle season, as humans tramped around the forest scouting out their hunting spots, Hillside Doe was looking for a spot of her own. She settled on the steepest (you guessed it) hillside in her home range, an inhospitable stretch of terrain covered with “boulders the size of suitcases,” Dr. Diefenbach said.

By 4 a.m. on opening day, Hillside Doe was bedded down in her safe space, as if someone had “texted her a message deer season was about to get started,” Dr. Diefenbach later wrote on the blog.

In the days that followed, the deer retreated there again and again, her behavior reflecting the schedule of the hunters looking for her. While they sat still in their deer stands, she sat still in her hiding place. Once the forest emptied out, Hillside Doe wandered among the hunting camps, feeding or perhaps, as Ms. Fleegle suggested, “doing reconnaissance.”

The study has also debunked old hunting adages. As it turns out, deer aren’t always more active in the morning and don’t seem particularly energetic during a full moon.

The “October lull” that some hunters swear by, when deer supposedly become extra elusive. It doesn’t really happen.

While cold and rain discourage a lot of hunters, foul weather doesn’t significantly hinder deer movement. “That throws out that excuse,” lamented Dr. Diefenbach, a hunter himself.

The blog posts also describe the work of the study’s field crew, who bait and monitor the traps placed around the forest. Field technicians work in crews that collar and tag each deer, without relying on sedatives. One trick of the trade: Wear a hockey helmet to prevent injuries caused by an ornery animal.

“Hockey is the most brutal sport, right?” Dr. Diefenbach said. “We couldn’t design anything better.”

While one technician tackles and restrains the deer, the other fits the monitoring equipment, takes measurements and obtains a DNA sample.

The Deer-Forest Study team plans to continue wrestling with deer and monitoring vegetation and soil conditions through 2026. While Dr. Diefenbach is excited about new technology that will allow the researchers to map understory conditions, Ms. Fleegle is looking forward to writing more blog posts.

“What good is the work we do if we can’t share it in a way that people understand?” she said. “Trees are boring. Vegetation and soil chemistry? Not very fun. So, we ride on the deer’s coattails.”