ARCTIC METABOLIC ECONOMY 



339 



Table 19. — Critical temperatures at which the metabolic rate increases in some 



mammals in Alaska 



[Explanation— (A) Arctic, (SA) (Subarctic] 









Critical tem- 









perature (° C.) 



Species 



Region 



Weight (kg.) 







Winter 



Sum- 











mer 



Thalarctos maritimus (1) 



A (cubs) 



9 











Mustela rixosa (1) 



A 



0. 04-0. 07 



18 





Vulpes vulpes (2) 



SA 



5 



-13 



8 



Alopex lagopus (1) 



A 



3. 8-5. 5 



-40 





Canis familiarls (1) 



A (pups) 



15 



-25 





Cltellus undulatus (1) 



A 



0.870-1250 





8 



Tamiasciurus hudsonicus (2) 



SA 



0.166-0.230 



20 



20 



Dicrostonyx torquatus (1) 



A 



0.045-0.056 



15 





Rattus norwegicus (3) 



SA 



0.200 



23 





Erethizon dorsatum (2) 



SA 



4-7 



-12 



7 



Lepus americanus 



SA 



1 



—12 





Oreamnus americanus (4) 



SA 



32 



-20 





Sources: 



(1) Scbolander, Hock, Walters, Johnson, and Irving, 1950. 



(2) L. Irving, H. Krog, and M. Monson, 1955. 



(3) H. Krog, M. Monson, and L. Irving, 1955. 



(4) H. Krog and M. Monson, 1954. 



(5) L. Irving, J. Krog, H. Krog, and M. Monson, 1957. 



weather afforded by their behavior in seeking shelter probably keeps 

 their immediate environment at about their critical temperatures. It 

 appears that in general large arctic mammals have sufficient insulation 

 of their own to suit the climate in which they live. Smaller mammals 

 with inadequate insulation for arctic weather obtain shelter in the 

 burrows and nests habitual to their kind and by curling up to rest 

 in especially well protected sleeping places in effect insulate them- 

 selves by their behavior. 



Under natural conditions small arctic mammals come out of their 

 shelters occasionally to run about on the snow. The tracks of ground 

 squirrels show that in spring and fall they are often out during 

 weather much colder than the temperatures which have been found ex- 

 perimentally to be critical. Tree squirrels ( Tamiasciurus hudsonicus) 

 are seen occasionally in arctic forests during midwinter, lemmings 

 (Dicrostonyx torquatus) are seen in late winter running over the snow, 

 particularly in the years of their cyclic abundance. Clay Kaigelak 

 and I found a lemming scratching outside our camp one morning in 

 April on one of the Plover Islands, a scarcely perceptible sand bar 

 rising above the sea ice more than a mile from the shore east of Point 

 Barrow. The nights had been as cold as —30° C. and the days were 

 no warmer than —10° C. while the lemming ran out there over the 

 sea ice. Lively and pugnacious, it lived captive in the cold camp for 



