342 U. S. NATIONAL MUSEUM BULLETIN 217 



I view with suspicion of harmful consequences the prolonged expo- 

 sure of mammals to such cold as they can only meet by metabolism 

 which persistently stands high above the basal level. During periods 

 of activity mammals can regulate body temperature by utilizing the 

 heat which they produce above the basal level. Under experimental 

 conditions they regulate body temperature well for some hours in air 

 much colder than their critical temperature. Both the degree of cold 

 and apparently the duration of its tolerance are, however, limited. 

 During experimentation we have found that the time of endurance 

 of arctic winter cold by small arctic mammals is limited to a few 

 hours. It appears likely that animals of smallest size, having also the 

 higher critical temperatures, must soonest expend the metabolic 

 reserves available for activity. Apart from the question as to whether 

 the mammalian metabolic processes can run normally under such 

 stress of cold is the question of the stress upon the economy of nutri- 

 tion, for neither food nor time to obtain it are commonly disposable 

 except with limited possibility of expansion. 



The Barrow glaucous gull did not increase its metabolism at 

 —30° C. and by extrapolation it is safe to locate its critical tempera- 

 ture as low as —40° C. It is probable that arctic gyrfalcons and 

 ravens have low critical temperatures, and it is reasonable to suppose 

 that arctic rock ptarmigan weighing 400 grams are about the smallest 

 birds with a critical temperature as low as arctic winter cold, for an 

 arctic Alaskan jay weighing 60 grams needed to elevate its metabolism 

 in air colder than —3° C. As an arctic resident the jay had every in- 

 centive from its habits and environment to enlarge its insulation if it 

 were physically possible. 



It is estimated that the smallest species of birds which can carry 

 insulation completely adapting them to arctic cold are about Y^q the 

 weight of the smallest species of mammals fully adapted by their in- 

 sulation for arctic cold. Many northern and arctic birds weighing less 

 than 400 grams are constantly abroad in the coldest arctic weather (see 

 p. 94), nor have I discovered habits which could give them shelter 

 as a substitute for their deficiency of bodily insulation. During much 

 of the arctic winter it would seem that they must exceed the basal 

 metabolic rate normal to homoiothermous animals of their size. In 

 this respect they appear not to be fully adapted to arctic cold. John 

 Steen (1958) has demonstrated that small Norwegian wild birds have 

 critical temperatures so high that through most of the winter they 

 cannot relax below twice normal metabolic rate. 



Returning to view the relation between critical temperature and 

 normal geographical range, it appeared that the tropical mammals 

 and birds had critical temperatures about conforming to the climate 

 in their natural range (Scholander, Hock, Walters, Johnson, and 



