348 U. S. NATIONAL MUSEUM BULLETIN 217 



Heat Loss through the Extremities 



Some parts of birds and mammals protrude bare or but thinly 

 insulated, for in the Arctic, as in the Tropics, free movement of the 

 extremities is necessary, and certain sensory surfaces upon them must 

 be fully exposed to the environment which they are to perceive. The 

 bare foot pads of dogs and porcupines have been found to be near 0° 

 in cold weather, as have the hooves of caribou and the feet of gulls 

 and several other arctic birds (L. Irving and J. Krog, 1955) . Noses 

 of mammals were also found to be cold. 



The extremities derive most of their heat through the circulation 

 from the body, so that their low temperature is not by itself evidence 

 that the extremities are thereby saving heat. In the legs of gulls 

 {Larus glaucescens) a rapid decline in tissue temperature was found to 

 occur along the tibia under a thick covering of feathers, and the loca- 

 tion of the temperature change suggested that heat was not escaping 

 through the thick feathers, but that it was transferred from warm 

 arterial blood to the venous blood returning cool from the foot (L. Irv- 

 ing, 1951; L. Irving and J. Krog, 1955). 



In the legs and noses of arctic mammals and in the legs of other 

 arctic birds were found temperature gradients similar to those ob- 

 served in the gulls. The extremities of mammals possess vascular 

 arrangements which appear suitable to act as heat exchangers for 

 the conservation of heat ( Scholander, 1955, 1958 ) . The bare tail of the 

 muskrat is cold, and aquatic mammals, in order to reduce their loss of 

 heat to the surrounding cold water, may have such vascular schemes 

 for heat conservation extending over as much of their surface as is 

 bare. These vascular heat exchangers probably vary in effectiveness 

 and in morphology among the species of birds and mammals. 



That a vascular heat exchanger can change from a conserver to a 

 dissipator of heat is shown by the occurrence of temperatures almost as 

 warm as the body in the extremities of animals active in cold air 

 (L. Irving and J. Krog, 1955). In fact, given an efficient vascular 

 heat exchanger, the variation in surface temperature is a direct measure 

 of its usefulness in the regulation of heat loss. The range of variation 

 in surface temperatures known to be possible for arctic animals is: 



At the surf ace of the central part of the body 2° 



At the surface of the skin over the body proper 6° 



At the surface of the bare extremities 40° 



By effecting these changes in temperature the "vascular insulation" 

 can vary the rate of heat conductance from a unit area of surface of 

 each of these regions in the relation 1 : 3 : 20. Where the bare surface 

 is large, as in the gull's feet, there is provided an important component 

 in the total variable insulation of animals. Tlie only physiological 



