| Summary: | 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, 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 make 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. The data sets attached contain the final data which has been used for analyses in Trondrud et al. 2021 and has been through validation, filtering and processing. The steps taken are provided in the methods. The data therefore contains missing values where poor-quality readings of heart rate have been removed as described in the electronic supplementary material of Trondrud et al. 2021. Further information about the data is provided in the readme-file accompanying the data sets. Funding provided by: Norwegian Research Council Crossref Funder Registry ID: http://dx.doi.org/10.13039/501100002551 Award Number: KLIMAFORSK 267613
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