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Utah study: habitat treatments measurably increase mule deer body fat, improving winter survival odds
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Summary
A statewide analysis combining GPS collaring and habitat treatment maps shows WRI projects and summer precipitation both raise mule deer body fat entering winter; selection of treated areas reduces overwinter fat loss, with implications for management.
A researcher presenting Ph.D. results at the Watershed Restoration Initiative partner conference said habitat treatments funded or enabled by WRI and summer precipitation both boost mule deer condition entering winter and reduce winter fat loss.
The research combines statewide collaring and condition data with maps of habitat treatments. Ph.D. researcher Kent (Ph.D. candidate/researcher) explained the study used more than 7,000 GPS-collared deer (38 million location points) and condition data from 2014–2021 on about 1,300 individuals with nearly 2,500 capture events to link landscape treatments to body-fat measures.
The nut of the research: animals that spent more time in WRI treatments — even summer-range treatments — carried higher December body fat and declined more slowly over winter, effects that can add about 1–2 percentage points of body fat per year. Kent said, “The more time that animals spent within these WRI treatments … it impacted deer with adding potentially up to 2% of body fat on them each year.” He cautioned that precipitation was the larger driver: hard winters and low precipitation can overwhelm local management benefits.
Researchers measured rump fat by ultrasound at captures in December and again in March to estimate overwinter declines, and modeled December fat, March fat and percent decline using climate, animal traits (age, lactation status, weight), snow and treatment-selection metrics. December condition depended strongly on latitude, lactation duration and spring/monsoon precipitation (April and monsoon rains had strong positive effects). March condition depended largely on December fat and winter severity (snow and cold).
Kent also showed selection metrics matter: deer that selected treated patches on the winter range had reduced percent fat decline over winter, with selection-based reductions described as “potentially up to 15%” in fat-loss rate for heavy users of treatments. The presentation stressed both summer- and winter-range treatments can contribute: summer treatments raise the fat deer carry into winter and winter treatments slow the rate of fat loss.
Researchers and attendees noted policy implications: improving summer habitat and maintaining/expanding winter treatments can be complementary strategies to increase deer survival and population recruitment, but managers should keep precipitation-driven limits in mind.
Kent said publication of dissertation chapters was pending, with publication expected by the end of that summer. The Division of Wildlife Resources and university partners were acknowledged for data and collaring work.
