Seasonal shifts in muskox metabolic rates and energy expenditure are primarily governed by ambient temperature

Abstract

In response to global warming, many species are expected to move north to track their climatic envelopes, and nowhere is this more apparent than in the Arctic. We examine potential environmental constraints on muskox (Ovibos moschatus) metabolic rates and energy expenditure that may affect their ability to sustain life towards the pole. We focus specifically on the ambient conditions governing the period of pivotal fat depot restoration during the snow-free season (summer) in preparation for the lean snow-covered period (winter). Using continuous multi-year data from nine wild muskoxen, we therefore examine the seasonal shifts in key parameters related to metabolic rates and energy expenditure (stationary heart rate, rumen temperature and acceleration-based locomotor activity). We document strong seasonal shifts in all three parameters, with a short anabolic period with high metabolic rates and locomotor activity, and a long catabolic period with low metabolic rates and activity. The seasonal shifts, and particularly the onset of the anabolic period, are governed by ambient temperature and not by photoperiod. In highly dynamic environments, the ability to respond to a key driver with unpredictable dynamics (ambient temperature) rather than a predictable driver (photoperiod) allows muskoxen to match fat depot restoration to the temporally variable onset of the growing season. Understanding these dynamics is crucial for predicting the future of muskox in a warmer Arctic.