A six-year study of more than 600 deer in Wisconsin may have found ways to improve management of fatal chronic wasting disease (CWD) by studying hourly and daily group and individual movements and the use of their habitat throughout the seasons.

Patterns of habitat use in cervids like deer, elk and moose have important implications for where and when diseases are transmitted and the project's researchers hope results of the study will provide important clues on mitigating CWD.

Little is known about how individual habitat use translates to actual sites of overlap for individuals that spend time in or near more than one herd. This gap leaves uncertainty in how habitat shapes the risk of direct or environmental CWD transmission. Prions that cause the disease can live on for years after the ungulate dies.

The study team aimed to fill this "knowledge gap" by using movement data from the hundreds of individual white-tailed deer to determine how habitat use and selection varies by sex, age, season, and social environment, and to quantify variations in habitat composition in areas of between-group overlap, especially in the broader context of individual habitat selection. The report, "White-tailed deer habitat use and implications for chronic wasting disease transmission," was written by Marie L. J. Gilbertson, Alison C. Ketz, Matthew A. Hunsaker, Daniel P. Walsh, Daniel J. Storm and Wendy C. Turner and published by the Wildlife Society.

"Between-group interactions may be more likely at rare, attractive, or otherwise coveted resources, such as highly valuable food sources or social communication hubs like communal scrapes," the report authors wrote. "If such resources are amenable to modifications, they could be targeted to improve the effectiveness or efficiency of CWD management efforts."

The species studied were white-tailed deer in a region with prevalence rates in some cases over 40%. For reference, some regions within the Big Horn Basin have mule and white-tailed deer herds that exceed that prevalence rate. In Park and Big Horn counties, there are herds with prevalence rates at or above 50%.

Wyoming's white-tailed and mule deer interact often, with whitetails being the more aggressive of the two species. On occasion mule deer will mate with white-tailed deer (usually a mule deer buck and a white-tailed doe) resulting in a hybrid. Hybrids can have a mix of mule deer and white-tailed deer characteristics, including ear shape (mule deer), tail shape (whitetail), and antler growth patterns.

From 2017 to 2020, the Wisconsin Department of Natural Resources, in collaboration with over 300 private landowners, captured 763 individual deer older than 8 months of age and fitted them with GPS collars. Deer captures were conducted from December to March each year, and included estimating ages of captured individuals. Seasons were defined as fawning (May and June), post-fawning (July to October), breeding (October to December), and nonbreeding (December through April). 

"There's a lot we don't fully understand about CWD, particularly when it comes to the nuances of when, where, and how it spreads among cervids," said Cory Anderson, a researcher and CWD program codirector at the University of Minnesota's Center for Infectious Disease Research and Policy. "Studies like this are valuable not only for filling important knowledge gaps but also for providing practical information that can be used to inform wildlife management strategies."

Once deployed, GPS collars recorded deer locations typically every four hours but as frequently as every hour during fawning and breeding seasons or as infrequently as every 23 hours to conserve battery life of the devices.

CWD impacts

Chronic wasting disease has been associated with population declines in free-ranging deer populations and federal agencies have already spent more than $300 million on CWD efforts since the turn of the century. State budgets across the country average about $500,000 a year for research and surveillance — think tons of man hours and fuel. Game and Fish spent about $125,000 last year simply on CWD diagnostics.

Wyoming has been proactive about trying to find ways to manage herds while facing the disease. Deer, elk and moose provide the area with important resources, such as wildlife viewing and hunting, helping drive tourism — the state's second largest industry. Hunters, anglers and wildlife watchers in Wyoming contribute more than $1 billion to the economy annually and account for about 10,000 jobs — about 2.5% of all jobs in the state. The revenue is important considering all conservation work by Game and Fish biologists on our long list of wild species, including nongame species from black-footed ferrets to bats, is paid for by hunters and anglers.

From August to December last year, Game and Fish held 37 Mule Deer Community Conversations across the cowboy state. Staff met with members of the public to discuss mule deer management, including how best to address factors affecting mule deer populations. Chronic wasting disease has been and remains one of the main concerns of those participating.

Mule deer populations have seen continuous decline throughout Wyoming for decades. Difficult winters, habitat loss and degradation and disease have all played a significant role in reduced numbers.

"In Wyoming, we've moved from about 450,000 mule deer back in the 2005-2010 time frame to now about 200,000. And we've seen that loss over the last 20 years," Game and Fish Wildlife Biologidt Tony Mong said last fall.

Further research

The good news Mong brought to the table is that surveillance technology has improved and scientists now know more about the species, their winter and summer ranges and interactions.

"Every year we try to keep 210 collars on mule deer; a mix of females would be juveniles, 6 to 7-month-old fawns and bucks," Mong said. "We're really trying to ramp up the collection of the information that's going to allow us to see where they go and what's important."

The department has increased the number of collared deer since Mong arrived at the department in 2018 and the collars now include installation on more fawns and does — the most important population segments for rebuilding herds — as well as now having video collars that give clues to their migration and feeding habits. The disease is more prevalent in males.

"We're going to be able to look at food selection [thanks to video collars]. We can see what they are eating as they're eating. We get to see the habitat quality around them. We also get to see interactions, either with other species or between one another," Mong said. "We've got the information to start building this epic journey of wildlife."

The information they did share was specific to the Cody Region, including what will happen in the future as high prevalence in CWD and Epizootic Hemorrhagic Disease have taken their toll.

"I think the good thing is, especially around here, we have seen deer populations increase in areas. We just need to be smart on how we move forward with [population] increases to maintain lower levels of CWD, EHD and all these challenges. And so having the conversation, I think, open people's eyes so that they know what's coming."

The bulk of CWD studies have been done in the Mountain West; the Wisconsin study is one of the few east of the Missouri River. By synthesizing results from analyses of white-tailed deer movement, space use, habitat selection, and between-group space sharing, the study team is illustrating how deer respond to changing physiological and social needs through the course of the year, with important implications for CWD transmission.

Maintaining deer densities at levels that allow minimal overlap between parturient females could help to mitigate CWD transmission during this period. While movement patterns in the post-fawning season were similar to those in the fawning season, habitat selection showed marked increases in preference for agriculture and grass or pasture.

"CWD management may benefit from improved understanding of the interactions between population density, environmental conditions, social group size and fidelity, and response to perturbations to understand how interventions like harvest and culling can be most effective," the authors wrote in conclusion. "By integrating results across analyses, we have provided a clearer picture of deer behavior than could be conveyed through any single analysis. Our results can help direct future management and research priorities of this ecologically, economically, and culturally significant cervid species."