Heat waves are becoming more frequent across the globe and may impose severe thermoregulatory challenges for endotherms. Heat stress can induce both behavioral and physiological responses, which may result in energy deficits with potential fitness consequences. We studied the responses of reindeer (Rangifer tarandus tarandus), a cold-adapted ungulate, to a record-breaking heat wave in northern Finland. Activity, heart rate, subcutaneous body temperature, and body mass data were collected for 14 adult females. The post?heat wave autumn body masses were then analyzed against longitudinal body mass records for the herd from 1990 to 2021. With increasing air temperature during the day, reindeer became less active and had reduced heart rate and increased body temperature, reflecting both behavioral and physiological responses to heat stress. Although they increased activity in the late afternoon, they failed to compensate for lost foraging time on the hottest days (daily mean temperature ≥20°C), and total time active was reduced by 9%. After the heat wave, the mean September body mass of herd females (69.7±6.6 kg, n=52) was on average 16.4% ± 4.8% lower than predicted (83.4±6.0 kg). Among focal females, individuals with the lowest levels of activity during the heat wave had the greatest mass loss during summer. We show how heat waves impose a thermoregulatory challenge on endotherms, resulting in mass loss, potentially as a result of the loss of foraging time. While it is well known that environmental conditions affect large herbivore fitness indirectly through decreased forage quality and limited water supply, direct effects of heat may be increasingly common in a warming climate.
Bibliographical noteWe are grateful for the feedback from Robyn Hetem and for the constructive inputs from the two anonymous reviewers. We thank Clare Stawski and Murray M. Humphries for commenting on an earlier version of the manuscript. Fieldwork would not be possible without the collaboration with the Reindeer Herders’ Association and the local support from Mika Tervonen, Unto Paadar, Jukka Siitari, and field assistants Elise Tjørnsletten, Kine Øren, and Erlend Søby. Veterinarians Marja Nourgam and Amanda Høyer Boesen contributed to the anesthesia and surgery of the reindeer. We thank Àsgeir Bjarnason at Star-Oddi for help and guidance with biologger programming and data processing and Larissa Beumer for advice on the hidden Markov model approach. Cassandra Ugland validated the heart rate measurements. The work was supported by the Norwegian Research Council (grants 267613 and 315454). L.E.L., A.L.E., E.R., J.R.S., E.K., and L.M.T. planned the study. J.K. facilitated the fieldwork. E.R., A.L.E., L.M.T., J.K., and L.E.L. conducted the fieldwork, including device deployment, retrieval, and animal handling. L.M.T. curated and analyzed the data together with G.P. L.M.T. drafted the manuscript with input from S.A. All authors contributed to editing and approved the final version of the manuscript. The prior authorization for all procedures carried out on the reindeer in this study was granted by the Animal Experiment Board at the Regional State Administrative Agency in Finland (license ESAVI/3857/04.10.07/2017).
Data Availability StatementLong-term climatic data is publicly available from the Finnish
Meteorological Institute (https://en.ilmatieteenlaitos.fi/download-observations/). Curated biologger data and locally recorded weather data are available here: https://figshare.com/s/c1044ce71c363ccc20b8. The historical body mass data of females have been collected from the experimental reindeer herd at the Kutuharju Research Station by Finnish Reindeer Herders’ Association, who also own the reindeer herd and the station
https://www.luke.fi/en/research/research-infrastructures/kutuharju-research-infrastructure). Natural Resources Institute Finland (Luke) updates, saves and administrates this long-term reindeer herd data.
- climate change
- heat dissipation
- behavioural plasticity