338 



TJ. S. NATIONAL MUSEUM BULLETIN 217 



Critical Temperature 



The temperature at which an animal must increase its metabolic 

 heat production above the basal level is spoken of as the "critical tem- 

 perature," and it differs among species, as shown in tables 18 and 19. 

 No small mammals or birds have been found with a low critical tem- 

 perature, since the mechanics of movement prevent them from wearing 

 enough fur or feathers for substantial insulation. Among the north- 

 ern mammals the arctic white fox {Alopex lagajms)^ weighing about 

 4 kg., is the smallest that we have f omid able to stand arctic cold with 

 only its basal metabolic rate (Scholander, Hock, Walters, Johnson, 

 and Irving, 1950) . The distribution and habits of arctic hares {Lepus 

 timidus) of about the same weight show that they withstand exposure 

 to any arctic cold, but snowshoe hares {Lepus americanus) obtain 

 some shelter in their brushy habitat (L. Irving, J. Krog, H. Krog, and 

 M. Monson, 1957). At Leningrad, Olnianskaya and Slonim (1947) 

 showed that arctic hares and several other arctic mammals conserved 

 their body temperature better in cold than some northern, but not 

 arctic, species. Smaller arctic and subarctic mammals such as weasels, 

 mice, lemmings, and ground squirrels seldom emerge for long from 

 their burrows and nests under the snow, where the protection from 



Table 18. — Critical temperatures at which the metabolic rate increases in some birds 



Species 



Region 



Season 



Weight 

 (g.) 



Critical 

 tempera- 

 ture 

 (°0.) 



Larus hyperboreus barrovianus (1) 



Barrow 



Winter 



1500 



below 

 -30 

 -5 



Perisoreus canadensis paclfleus (i) 



Barrow 



Winter 



60 



Plectrophenax nivalis nivalis (1) 



Barrow 



V/inter 



40 



12 



Branta nigricans (2) 



Aneiiorage 



Winter 



1118 



6 



Branta nigricans (2) 



Anchorage 



Summer 





6 



Corvus caurinus (2) 



Anchorage 



Winter 



280 



-10 



Corvus caurinus (2) 



Anchorage 



Summer 





-10 



Anser (domestic) (3) (4) 



Europe 





4700 



3 



Gallus (domestic) (4) 



Europe 





2000 



26 



Gallus (domestic) (9) 



America 





2000 



26 



Columba (domestic) (6) 



Europe 





400 



28 



Columba (domestic) (4) (6) 



Europe 







20 



Passer domesticus (7) 



Ohio 





24 



22 



Troglodytes aedon (8) 



Ohio 



Summer 



10 



25 



Nyctidromus (1) 



Panama 





45 



25 



Pipra (1) 



Panama 





12 



20 



Vidua paradisaea (3) 



Native to 

 Africa 





13 



32 



SouBCEs: 



(1) Scholander, R. Hock, Walters, Johnson, and L. Irving, 1950, p. 237. 



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



(3) Terroine and Trautmann, 1927. 



(4) Giaia, 1931. (7) Kendeigh, 1944. 



(5) Kayser, 1930. (8) Kendeigh, 1939. 



(6) FaUoise, 1900. (9) Mitchell and Haines, 1927. 



