Nutritional condition and net body weight for adult female mule deer (Nov 2017–Mar 2018), Wyoming, USA

Many temperate and polar animals have developed physiological and
behavioral adaptations to survive the challenging conditions of winter.
Some animals hibernate to reduce energetic expenditure while other
animals, including ungulates, migrate to avoid cold temperatures and deep
snow. Despite moving vast distances between seasonal ranges, many
migratory ungulates are unable to escape the energetic challenges of
winter and often rely on reserves of fat to withstand food scarcity and a
negative energy balance. The mobilization of fat for ungulates often is
dependent on nutritional condition, yet the fine-scale rate at which
ungulates mobilize fat throughout winter remains unclear. We took
advantage of three sampling periods during the winter of 2017–18 on a
population of mule deer that spends the winter in the Red Desert of
south-central Wyoming, USA to investigate the fine-scale expenditure of
fat from mid-autumn through late winter (November–March). Although the
full effects of winter on forage were still minimal in mid-autumn, mule
deer mobilized fat reserves 2.5 times faster in mid-autumn
(November–December) than in late winter (December–March). The mobilization
of fat strongly depended on nutritional condition with mule deer that
entered a season with higher nutritional condition expending more fat than
those of lower nutritional condition (P < 0.001). Adjusting
mobilization of fat based on nutritional condition may allow temperate
ungulates to survive winter without completely exhausting fat reserves and
risking malnutrition.