Campuses:
By Becky Beyers 
They’re everywhere.
It’s hard to venture anywhere in Minnesota without seeing one of the state’s estimated 1 million whitetail deer bounding along a ditch or browsing for food in forest or grasslands. Every fall, hunters net about a quarter of the population, but the deer seem to come right back.
The ever-growing population has long posed questions for wildlife managers, especially in the northern woods: how many deer can a given timber stand feed, and how much of the timber can be harvested without starving the deer? In a bad winter, how do the deer avoid freezing or starving?
A 15-year study in the Chippewa National Forest led by Glenn DelGiudice, an adjunct associate professor in the Department of Fisheries, Wildlife, and Conservation Biology, answers many of those questions and provides first-of-its-kind insights into how deer survive and thrive. The study was sponsored by and conducted through the state Department of Natural Resources, where DelGiudice is a wildlife research biologist.
The study began in 1990 and was purposely designed to cover a longer term than similar research, DelGiudice says, in order to include a broader range of winter severity and other environmental factors.
DelGiudice’s study was a field experiment involving sites near Grand Rapids, Minn., where logging had been conducted midway through the study period on two sites, but not on two others. The team used 50 to 60 Clover traps—which involve a net hung inside a frame with a trap door that drops down when a deer enters and trips the trigger—so researchers could collapse the trap to physically restrain the deer for chemical immobilization and “handling,” which includes ear-tagging, radio-collaring, collecting blood and urine samples, using ultrasound to assess pregnancy and body condition, extracting a tooth and performing other techniques . Very high frequency (VHF) radio transmitters and eventually GPS tracking allowed the team to follow each deer’s movements within a few meters, which in turn revealed far more details about the life of a deer than previously had been known. 
DelGiudice’s team checked the traps every day for 12 weeks each winter; leading to about 1,000 captures and recaptures of male and female deer. Adult bucks were not part of the study and were immediately released, so the final numbers included about 450 individual does and their fawns. Over the years, about 145 graduate students from across the country and around the world—including several from CFANS’ conservation biology and wildlife biology programs—traded room and board for learning how to design research studies and how to capture, chemically immobilize and handle deer.
Michelle Carstensen was looking for a master’s degree project in 1998 when she enrolled in the College of Natural Resources while working full-time. She had already done research related to nutritional and reproductive physiology on domestic species and says she jumped at the chance to work with DelGiudice on the link between nutritional stress and body condition of deer during winter (with varying winter severities) and the survival of their fawns born the following springs. “Glenn is very well known and respected in the wildlife research community, and it was exciting to get an opportunity to work with a researcher of his caliber,” she says.
Differences in winter severity provided some of the study’s most interesting findings. The winter of 1995-96 was among the coldest and snowiest on record and a full 30 percent of the does in the study that winter died, much higher than the average 5 to 10 percent mortality rate.
But another rough winter followed the very next year. This time, though, only about 8 percent of the does died. That’s because the first bad winter weeded out the weakest deer—including bucks not included in the study—so more food was available for the survivors. Most of the surviving, but nutritionally stressed does in spring 1996 gave birth to small, underdeveloped fawns that died relatively soon. This meant that the mothers didn’t have to lactate and nurse these fawns all summer, which is a significant nutritional drain on them. Therefore, they entered the next winter in extraordinarily good condition, which enhanced their chances of survival.
So how do the adult deer survive a difficult winter? “It’s about conserving their body’s energy, mostly in the form of fat, and they’ve adapted in a number of ways…behaviorally and physiologically,” DelGiudice says. “The seasonal decrease in hours of daylight triggers a hormonal reaction in their brains that tells them to eat less, they move less, and their metabolism decreases. Everything slows down. On clear, really cold nights, they’ll use dense conifer stands, with 70 percent or more canopy closure, for thermal cover. Greater infrared radiation under these canopies than out in the open helps them to keep warm. But more importantly, these canopies also retain up to 45 percent of the snow load, making snow depth on the ground much shallower, which allows the deer to move about more easily and at a significantly lower energetic cost. A price they do pay, however, is there’s less food under there. If deep snow restricts them for only a day or two, they’re fine, but eventually they have to venture out to find more food.”
In the forested areas where the study took place, deer tend to eat mostly beaked hazel, mountain maple, dogwoods and a handful of other native shrubs and plants. In winter, especially when snow gets deeper than about 40 centimeters, the deer face a two-edged problem: Food becomes scarce and less nutritious, and moving through the deep snow to find food is difficult, especially for fawns. In a winter like 1995-96, the weaker deer, many fawns and the oldest deer simply couldn’t survive.
Deer have an amazing fertility rate, DelGiudice says: Nearly 100 percent of does between ages 2.5 and 15 are pregnant each spring, and they almost always have twins.
“Once we began trying to catch newborn fawns, we realized how difficult the task was in the forest zone,” Carstensen says. Her research expanded to include assessing habitat characteristics of birth sites and how that might influence fawn survival. She also tracked movements of newborn fawns in relation to their does, trying to understand more of their fawn rearing behavior, social dynamics, and vulnerability to predation.
The research team was able to track does’ pregnancies and to capture and tag new fawns thanks to relatively new technology, a vaginal implant transmitter, which allowed them to tell when a doe was giving birth. The implanted transmitter has a temperature-sensitive trigger, so when she gave birth and the transmitter was expelled and exposed to an ambient temperature lower than 95oF, the transmitter pulse rate increased from 45 to 120 beats per minute. Homing in on the transmitter’s rapid pulse rate, researchers could then quickly find the doe’s birth-site and capture her fawns for handling, data collection, and radio-collaring. 
Not all fawns survive; about half die in their first 12 weeks. But most mothers still have a remaining twin. By the next spring, about 35 percent to 45 percent of the original fawn crop has survived and the population continues to grow.
“So what’s happening is they thrive even though most does lose at least one fawn by the next spring, because many still have one to add to the population as yearlings,” DelGiudice says. “The reproductive potential of these deer is incredible. Biologically they’ve adapted to conditions and they’re very resilient.”
Newborn fawns don’t move much in their first week or so, so they’re susceptible to predators, especially the black bear. Most fawns are born in late May, right about the time bears are waking up from hibernation. “People don’t realize that the bear is one of the biggest predators of newborn deer,” DelGiudice says. “They’re omnivores, so when there’s an opportunity to kill a fawn, they will.” Wolves and bobcats also will prey on deer fawns, especially when their other food sources are limited.
“The volume of data generated by the long-term, comprehensive study will take some time to unravel,” DelGiudice says. “There’s so much data here, well beyond the average study. Importantly, we were able to calculate deer ages by extracting last incisors and examining their roots under the microscope.” This reveals cementum annuli, similar to the growth rings of a tree, which allows researchers to study aspects of deer life in an age-specific way. “For example, what we found is that the risk of death for these deer is highest when they’re born, decreases markedly during their first two years of life, remains low and stable through about 6 years of age, then steadily increases through 18 years. Wolf predation is the primary natural cause of death for all deer in the northern forests of Minnesota, having their greatest impact on fawns in their first winter and deer older than 6 years old.” 
DNR officials commissioned the study to learn more about deer and improve management of the state’s populations, so hunting policies are naturally one of the first issues to be addressed.
“Most of the hunting pressure is on bucks,” DelGiudice says. “But when local or regional populations are too high, an effective tool of management to bring the population more in line with the habitat’s carrying capacity is to issue more antlerless permits. This removes more females, which frequently represent three individuals (including twins the doe would have birthed). By way of their survival and reproductive capacities, they have the greatest potential influence on population size, both for increasing and reducing it. But as we found in this study, the carrying capacity of any area varies with winter severity and its specific effects on the deer’s biology, survival, and reproduction—these effects must be considered and incorporated into management strategies each year.”
While the study has already generated 20 scientific publications, there’s much more to be examined, he says. “The next step would be adaptive management research. I’d like to see us make some predictions, relative to some of the managers’ activities, based on the study and then collect before-and-after information and see if the predictions were right.”
Similar studies also could be created for different geographic areas of the state, to develop data that could be compared to the original areas near Grand Rapids.
“People think of deer on an individual scale,” DelGiudice says. “But wildlife managers have to think of them on a population scale and landscape-wide. That’s why these data were so important.”
