A Summer Heat Wave Reduced Activity, Heart Rate, and Autumn Body Mass in a Cold-Adapted Ungulate

Area of Science:

• Zoology
• Climate Change Biology
• Physiology

Background:

• Global heat waves are increasing in frequency and intensity.
• Endotherms face thermoregulatory challenges during extreme heat events.
• Heat stress can lead to energy deficits and impact animal fitness.

Purpose of the Study:

• To investigate the behavioral and physiological responses of reindeer (Rangifer tarandus tarandus) to a severe heat wave.
• To assess the impact of heat stress on reindeer activity patterns, body temperature, heart rate, and body mass.
• To analyze long-term effects of heat waves on reindeer herd body mass.

Main Methods:

• Monitoring activity, heart rate, and subcutaneous body temperature in 14 adult female reindeer during a heat wave.
• Collecting body mass data before and after the heat wave.
• Analyzing post-heat wave body mass against historical herd data (1990-2021).

Main Results:

• Reindeer exhibited reduced activity, lower heart rate, and increased body temperature with rising air temperatures.
• Total daily activity decreased by 9% on hot days (≥20°C), with reduced foraging time.
• Mean September body mass was 16.4% lower than predicted, with greater mass loss in less active individuals.

Conclusions:

• Heat waves pose a significant thermoregulatory challenge for cold-adapted ungulates like reindeer.
• Reduced foraging time during heat waves directly contributes to substantial body mass loss.
• Direct impacts of heat stress on endotherm fitness are becoming more prevalent with climate warming.