6-year study of deer home range, habitat preference could help officials manage CWD

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A 6-year study that tracked nearly 600 white-tailed deer in southwestern Wisconsin suggests that seasonal space use and habitat selection could influence the risk of both direct and environmental chronic wasting disease (CWD) spread, which could have implications for North American deer management.

CWD is a fatal neurodegenerative disease of cervids such as deer, elk, and moose caused by infectious misfolded proteins called prions, which spread through direct contact or environmental contamination.

The University of Wisconsin-Madison–led study team, who published the findings today in Wildlife Monographs, monitored seasonal deer activity patterns, home ranges, habitat choices, and deer-to-deer encounters for 596 deer in a CWD-endemic area (parts of Iowa, Grant, and Dane counties) from 2017 to 2022.

"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 study authors wrote. "If such resources are amenable to modifications, they could be targeted to improve the effectiveness or efficiency of CWD management efforts."

Cory Anderson, PhD, MPH, a researcher and CWD program co-director at the University of Minnesota's Center for Infectious Disease Research and Policy (CIDRAP), publisher of CIDRAP News, said, "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," he said.

"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," he added.

Seasonal variations

From December to March from 2017 to 2020, the researchers fitted male and female deer at least 8 months old with global positioning system (GPS) collars, which recorded their locations every 4 hours, more or less, depending on the season.

Seasons were defined as fawning (May and June), post-fawning (July to October), breeding (October to December), and non-breeding (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.

Cory Anderson, PhD, MPH

In the fawning season, does are relatively solitary, and some juvenile males disperse. After fawning, does congregate, and more juvenile males leave the group. Fall breeding occurs during Wisconsin's 9-day rifle season and the rut, and deer may congregate for feeding during the winter non-breeding season, the researchers noted.

Breeding season activity patterns markedly different

Average seasonal home range size was 1.07 square kilometers (0.4 square miles). Male home ranges were larger than those of females and were largest during the breeding season, while female ranges were largest in non-breeding periods. Female ranges shrunk slightly with advancing age, while male range sizes rapidly increased with age.

Annual site fidelity was lowest for males between the post-fawning and breeding seasons, while it was lowest for females between the non-breeding and fawning seasons. Breeding season activity patterns differed substantially from other seasons, with a general rise in movement rates, especially among males, but the highest movement rates occurred in early spring.

Females had the longest step lengths in the breeding and non-breeding seasons, but seasonal differences were negligible. Males, however, had consistently longer average step lengths in the breeding season, followed by the non-breeding season.

In general, deer preferred areas of central hardwoods, followed by oak forests, and tended to forage in the forest more than in corn, hay, and soybeans in the fawning season, while the latter were eaten most often in the other seasons. The relative preference for grass or pasture was highest in the post-fawning and non-breeding seasons.

Encounter distributions, or areas in which deer were most likely to meet—were small (0.29 square kilometers [0.11 square miles]) relative to average home range size. Encounter distributions were smallest for female pairs and largest for male pairs, smallest in the fawning and post-fawning seasons, and greatest in the breeding and non-breeding seasons. As the sum of a pair's ages rose, the encounter distribution fell.

Range expansion during non-breeding season

Because the fawning season is strongly influenced by the demands of birth and rearing, direct CWD transmission is probably limited during this time, the authors said: "Maintaining deer densities at levels that allow minimal overlap between parturient females could help to mitigate CWD transmission during this period."

Our results can help direct future management and research priorities of this ecologically, economically, and culturally significant cervid species.

In the post-fawning season, deer showed a strong preference for agricultural land, grass, and pasture, which could lead to greater between-group indirect interaction, although the transmission relevance is unclear.

"Direct transmission likely predominates during the breeding season, but indirect interactions at socially attractive sites (e.g., scrapes) and females' contributions to CWD transmission and spatial spread may warrant further consideration," the researchers wrote.

The non-breeding season brought substantial range expansions and likely increased between-group interactions, especially in winter. "Therefore, 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.

Regardless of season, preferred habitat was not necessarily the same cover type that tended to foster between-group interactions. "In particular, agriculture and grass or pasture may be attractive forage sites that foster between-group interactions and direct or indirect transmission events," they added. "Our results can help direct future management and research priorities of this ecologically, economically, and culturally significant cervid species."

Anderson said, "There are no silver-bullet solutions for CWD, but anytime research can inform how we think about and approach the disease is a good thing."