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.

On 5 November 2017, we captured n = 30 adult female mule deer (> 1-yr-old) in the Red Desert near Rock Springs, WY, USA (41°35’09” N, 109°12’15” W). During 8–10 December 2017, we recaptured n = 20 deer that were captured in November 2017. Then, during 11–13 March 2018, we recaptured all deer (n = 17) that survived the winter and that were captured in November 2017 and December 2017. All mule deer were captured via helicopter net-gunning (Ortega et al. 2020) and outfitted with store-on-board GPS collars programmed to collect locations every two hours (LOTEK Wireless Inc, New Market, Ontario, CAN). During captures, we used an electronic platform scale (±0.1 kg; WeighSouth, Asheville, NC) to measure net body mass (kg). We measured depth of rump fat via ultrasonography (Ibex Pro, E.I. Medical Imaging, Loveland, CO) and accompanied it with a body-condition score following standardized protocols to estimate scaled, ingesta-free body fat for mule deer (Cook et al. 2010). Ingesta-free body fat is a scaled representation of percent body fat and is a reliable measure of nutritional condition for live animals (Cook et al. 2010). We calculated ingesta-free body mass following methods from Cook et al. (2007) to determine a scaled representation of body mass. All animal capture and handling protocols were approved by the Wyoming Game and Fish Department (Chapter 33-937) and an Institutional Animal Care and Use Committee at the University of Wyoming (Protocol #20170215KM00260).

References:

Cook, R. C., T. R. Stephenson, W. L. Myers, J. G. Cook, and L. A. Shipley. 2007. Validating predictive models of nutritional condition for mule deer. Journal of Wildlife Management 71:1934–1943.

Cook, R. C., J. G. Cook, T. R. Stephenson, W. L. Myers, S. M. McCorquodale, D. J. Vales, L. L. Irwin, P. B. Hall, R. D. Spencer, S. L. Murphie, K. A. Schoenecker, and P. J. Miller. 2010. Revisions of rump fat and body scoring indices for deer, elk, and moose. Journal of Wildlife Management 74:880-896.

Ortega, A. C., S. P. Dwinnell, T. N. LaSharr, R. P. Jakopak, K. Denryter, K. S. Huggler, M. M. Hayes, E. O. Aikens, T. L. Verzuh, A. B. May, M. J. Kauffman, and K. L. Monteith. 2020. Effectiveness of partial sedation to reduce stress in captured mule deer. Journal of Wildlife Management 84:1445–1456.