Peripheral heterothermia in reindeer (Rangifer tarandus tarandus)

Abstract

Reindeer (Rangifer tarandus tarandus) inhabit the Arctic and have evolved adaptions to meet the climatic challenges in this region. Peripheral heterothermia, which is a heat conserving mechanism in homeotherms during cold exposure, is well documented in the reindeer and manifests itself in tissue temperatures in the extremities well below 10°C. It is, however, unknown whether peripheral heteothermia in reindeer requires efficient vascular counter-current heat exchange, or is simply a result of reduced blood flow to the extremities. Vascular corrosion casts, x-ray pictures, dissections and electron microscopy did not reveal any counter-current rete in the front legs, but the vascular anatomy, with arteria brachialis and vena brachialis running in intimate contact with each other for the full length of the leg, still seems to allow quite efficient counter-current heat exchange. Recordings of brachial blood flow and subcutaneous temperature (Tsc) in the front legs of winter insulated reindeer subjected to ambient temperatures (Ta) of 20°C, -10°C and -30° suggest that the initial drop in Tsc seen in reindeer when exposed to moderate cold (-10°C) is primarily the result of use of a circulatory pattern enabling counter-current heat exchange, while decreasing blood flow appears to be a secondary mechanism when Ta falls further towards -30°C.