DOI: 10.5061/dryad.mcvdnck0f
title: Determinants of heart rate in Svalbard reindeer reveal mechanisms of seasonal energy management

description: 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.
description: Adult females (ages 5–8 years, marked as calves) were captured in
 March–April 2018 for biologger deployment and in April 2019 for biologger
 retrieval. On both occasions, animals were caught by net using snowmobiles
 [47]: we recorded their body mass (± 0.5 kg) and checked for pregnancy
 using an ultrasound scanner (Kaixin Electronic Instrument Co., Xuzhou,
 China). In August 2018, surveys were conducted on foot to relocate marked
 animals and assess calf status. We fitted each animal
 with a combined heart rate and temperature logger (DST centi-HRT,
 Star-Oddi, Gardabaer, Iceland; ~19 g), which was implanted subcutaneously
 on the left side of the sternum or behind the left axilla, while animals
 were under anaesthesia. Surgical procedures are described in ESM section
 1.2. Heart rate<sub> </sub>was automatically calculated from a
 4 s electrocardiogram (ECG) at 150 Hz measurement frequency and stored
 alongside a quality index of signal clarity. We programmed the loggers to
 record heart rate<sub> </sub>and subcutaneous body temperature
 (T<sub>sc</sub>) every 15 min, and to store a raw
 electrocardiogram signal every 6 hrs for manual validation.
 The animals were also fitted with a collar (Vertex Plus,
 Vectronic Aerospace GmbH, Berlin, Germany, ~750 g) containing an activity
 sensor. The activity sensor measured acceleration along two orthogonal
 axes representing back-forward and right-left movements at 4 Hz intervals.
 An internal algorithm calculates activity as the difference in
 acceleration between two consecutive measurements and is given within a
 relative range between 0 and 255, providing a mean value of acceleration
 in each axis every 5 min. Further details about data
 processing are provided in the article and its electronic supplementary
 material.
description: 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.