MAMMALIAN COLD ACCLIMATION 



This represents a difference of about twenty- fold in the cooling ef- 

 fect of air and water, but the total insulation (body + medium) in 

 water was about 50% of its value in air. It can be noted that the meta- 

 bolic response closely parallels the skin cooling in air and water. 



The parallel between peripheral cooling and metabolic response 

 was also seen between different seals and was reflected in the indi- 

 vidual variability (Irving and Hart, 1957) (Figure 7). The smallest 

 seal, characterized as the "runt," had a very thin layer of blubber 

 and consequently was unable to maintain a surf ace temperature low- 

 er than 6 C to 8 G when in water at C. The high heat flow re- 

 sulted in a high oxygen consumption at all temperatures. In other 

 harbor seals with a considerably deeper and less steep gradient 

 through the thicker blubber, there was a much lower surface tem- 

 perature and a maintenance of resting metabolism down to a criti- 

 cal level of about 10 C. However, a harp seal (Phoc a groenlandica) , 

 with a still deeper gradient and a lower surface temperature, was 

 able to compensate completely without elevationof metabolism even 

 in ice water. This represents the greatest cooling load experienced 

 by mammals in nature and the harp seal has the greatest physiologi- 

 cal insulation known for mammals. 



Harbor seals tested in December at St. Andrews, N. B., and at 



Woods Hole, Massachusetts, during the summer revealed seasonal 



changes that were comparable to those found by Irving, Krog, and 



Monson (1955) for the red fox and the porcupine. During the summer 



there was a greater elevation of oxygen consumption in cold water 



than during the winter and thecriticaltemperature was raised from 



o o 



about 11 C to 20 C in water. There was, therefore, a greater 



physiological insulation in winter than in summer; this was associ- 

 ated with changes inperipheral tissues. No anatomical basis for this 

 change was noted. The nature of the seasonal alteration was such 

 that the differences were observed even at the same body skin tem- 

 perature (Figure 11). This puzzling phenomenon suggests that more 

 heat is lost in summer than in winter at the same body skin tempera- 

 ture. This could be accounted for by a greater evaporative heat loss 

 from the lungs or by a greater heat loss from the appendages, 

 neither of which were measured in this investigation. 



219 



