A summer heatwave reduced activity, heart rate and autumn body mass in a cold-adapted ungulate

Liv Monica Trondrud, Gabriel Pigeon, Elżbieta Król, Steve Albon, Erik Ropstad, Jouko Kumpula, Alina Lynn Evans, John R. Speakman, Leif Egil Loe

Research output: Contribution to journalArticlepeer-review


Heatwaves 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 heatwave in Northern Finland. Activity, heart rate, subcutaneous body temperature and body mass data were collected for 14 adult females. The post-heatwave 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, 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, on the hottest days (daily mean temperature ≥ 20°C), they failed to compensate for lost foraging time, and total time active was reduced by 9%. After the heatwave, 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 heatwave had the greatest mass loss over summer. We show how heatwaves impose a thermoregulatory challenge on endotherms,
resulting in mass loss, potentially due to 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.
Original languageEnglish
JournalPhysiological and Biochemical Zoology
Publication statusAccepted/In press - 31 Mar 2023


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