Reindeer (Rangifer tarandus) are protected against the Arctic cold by thick fur of prime insulative quality. The high thermal resistance of the coat makes dissipation of excess heat difficult, and hence heat-stressed reindeer resort to panting. Moderately heat-stressed resting reindeer usually display closed mouth panting, whereby blood is cooled in the richly vascularised mucosa of their nasal turbinates. This blood is drained via the dorsal nasal veins and may be routed via the facial and jugular veins for general body cooling, or via the angular oculi veins (AOV) to the cavernous sinuses for selective brain cooling (SBC), which is defined by the IUPS Thermal Commission as a brain temperature lower than carotid blood temperature. During severe heat stress and/or exercise, however, reindeer use open mouth panting (OMP), and evaporation then occurs mainly from their richly vascularised and large tongue. Under these conditions we have to assume that the efficiency of SBC is reduced, due to reduced air flow through the nose (Aas-Hansen et al. 2000) and the fact that the cool blood from the tongue cannot be used for SBC. SBC, which is common to artiodactyls, was originally seen as a defence mechanism against hyperthermic damages to the brain during running, but we have seen that exercise implies open mouth breathing which compromises SBC. We have surgically instrumented four adult female reindeer under isoflurane anaesthesia to allow continuous recordings of brain (T_b) and carotid artery (T_a) temperatures, and were also able to measure rectal temperature and respiratory rate, with the animal standing in a climatic chamber at ambient temperatures ranging from -20 to 40 °C. We could also occlude both AOVs, when desired, and thereby prevent SBC. The experiments were approved by the Norwegian Committee on Ethics in Animal Experimentation.

We observed that, upon occlusion of both AOVs, T_b increased significantly, even in situations where T_b was maintained 0.5 °C higher than T_a before the occlusion. This shows that reindeer employ SBC long before T_b < T_a. Moreover, AOV occlusion also resulted in significantly increased respiratory rates, particularly at high ambient temperatures, at which OMP occurred most of the time. Such periods of OMP were consistently associated with a significant drop in T_a. Examination after humane killing revealed that the jugular veins and the common carotid arteries run in close association for a distance of 25Ð30 cm. We propose that counter-current heat exchange between cold venous blood from the tongue and the carotid artery blood explains the OMP-associated drops in T_a, and that this is sufficient to prevent a further rise in T_b, even when SBC is compromised due to OMP. However, OMP is costly, both in terms of energy and body water. We therefore suggest that SBC is activated to defend T_b at moderate heat loads, while, as the heat load increases, the animal must resort to OMP in order to avoid general hyperthermia. T_b is still defended due to counter-current heat exchange between the jugulars and the carotids, but this is expensive both in terms of energy and water, and also prevents the animal from grazing, which results in a reduced energy intake.

All procedures accord with current national guidelines.