Seasonal energetic challenges may constrain an animal’s ability to respond to
changing individual and environmental conditions. Here, we investigated
variation in heart rate, a well-established proxy for metabolic rate, in Svalbard
reindeer (Rangifer tarandus platyrhynchus), a species with strong seasonal
changes in foraging and metabolic activity. In 19 adult females, we recorded
heart rate, subcutaneous temperature and activity using biologgers. Mean
heart rate more than doubled from winter to summer. Typical drivers of
energy expenditure, such as reproduction and activity, explained a relatively
limited amount of variation (2–6% in winter and 16–24% in summer) compared to seasonality, which explained 75% of annual variation in heart rate.
The relationship between heart rate and subcutaneous temperature depended
on individual state via body mass, age and reproductive status, and the
results suggested that peripheral heterothermy is an important pathway of
energy management in both winter and summer. While the seasonal plasticity in energetics makes Svalbard reindeer well-adapted to their highly
seasonal environment, intraseasonal constraints on modulation of their
heart rate may limit their ability to respond to severe environmental
change. This study emphasizes the importance of encompassing individual
state and seasonal context when studying energetics in free-living animals.
This article is part of the theme issue ‘Measuring physiology in freeliving animals (Part II)’.
activity, Arctic, plasticity, reproduction,
thermoregulation, ungulates,
physiology, ecology
