MAMMALIAN COLD ACCLIMATION 



of functional activity of cool tissues is one of the puzzling phenome- 

 na of homeotherms which must be active at all temperatures. Func- 

 tional retardation by cold, at least for growth processes, has recent- 

 ly been demonstrated by Heroux (1960), who showed that the mitotic 

 activity decreased about 10-fold for a 10 degree lowering of tem- 

 perature in the skin of the rat. 



While large adult arctic mammals are apparently able to with- 

 stand the most severe cold with little or no elevation of metabolic 

 rate, such may not be true for infant animals of the same species. 

 Baby caribou ( Rangifer arcticus) born during June in the far north 

 are exposed to cold, wind, and precipitation that may lead to mor- 

 tality (Hart et al., 1962c). These calves are extremely sensitive to 

 cold, as shown by the marked elevation in metabolism resulting 

 from exposure to the harsh environmental conditions (Figure 6) . In 

 contrast, a 9- month calf of the same litter did not show elevation of 

 metabolism at temperatures down to -50 C. 



Seasonal changes in some northern mammals have been shown 

 by Irving, Krog, and Monson (1955) for the porcupine ( Erethizon 

 dorsatum myops) and red fox( Vulpes vulpes alascensis),but not for 

 the smaller red squirrel ( Tamiasciurus hudsonicus preblei) . The 

 winter fox and porcupine had lower critical temperatures and would 

 require a much lower temperature than summer animals for the 

 same metabolic response. These comparisons made in a review by 

 Hart (1957) also showed a similar trend for the lemming ( Dicros- 

 tonyx groenlandicus ) when Alaskan (winter) and Ottawa (summer) 

 test animals wereconsidered.lt was also pointed out that no changes 

 in the slopes of temperature- metabolism curves were found for the 

 deer mouse ( Peromyscus maniculatus gracilis). In this species, 

 winter animals were able to resist lower temperatures (Hart and 

 Heroux, 1953) mainly by metabolic compensation, although some in- 

 dication of a small increase in insulation of winter animals was ob- 

 served. 



The seasonal changes in these mammals are in line with the 

 seasonal changes in pelage insulation observed by Hart (1956), who 

 demonstrated the obvious fact that smaller mammals with body 

 weights below about 100 gm, unlike the larger ones, fail to achieve 

 significant protection through increased fur thickness during the 



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