Reindeer (Rangifer tarandus) are protected against the Arctic winter cold by thick fur of prime insulating capacity and hence have few avenues of heat loss during work. We have investigated how these animals regulate brain temperature under heavy heat loads. Animals were instrumented for measurements of blood flow, tissue temperatures and respiratory frequency (f) under full anaesthesia, whereas measurements were also made in fully conscious animals while in a climatic chamber or running on a treadmill. At rest, brain temperature (Tbrain) rose from 38.5±0.1°C at 10°C to 39.5±0.2°C at 50°C, while f increased from ×7 to ×250 breaths min–1, with a change to open-mouth panting (OMP) at Tbrain 39.0±0.1°C, and carotid and sublingual arterial flows increased by 160% and 500%, respectively. OMP caused jugular venous and carotid arterial temperatures to drop, presumably owing to a much increased respiratory evaporative heat loss. Angular oculi vein (AOV) flow was negligible until Tbrain reached 38.9±0.1°C, but it increased to 0.81 ml min–1 kg–1 at Tbrain 39.2±0.2°C. Bilateral occlusion of both AOVs induced OMP and a rise in Tbrain and f at Tbrain >38.8°C. We propose that reindeer regulate body and, particularly, brain temperature under heavy heat loads by a combination of panting, at first through the nose, but later, when the heat load and the minute volume requirements increase due to exercise, primarily through the mouth and that they eventually resort to selective brain cooling.
This research received no specific grant from any funding agency in the public, commercial or not-for-profit sectors.
LIST OF ABBREVIATIONS
- angular oculi vein
- respiratory frequency (breathing frequency)
- open-mouth panting
- selective brain cooling
- ambient temperature
- brain temperature
- blood temperature in carotid artery
- blood temperature in jugular vein
- rectal temperature
- © 2011.