COLD HARDINESS 



We shall first consider relations of or- 

 ganisms to lower temperatures. A cave sil- 

 phid beetle (Astagobius angustatus) is 

 known to carry on its lite cycle in ice grot- 

 toes where the temperature range is be- 

 tween — 1.7° and +1.0° C, and the marine 

 bivalve mollusk, Yoldia arctica, is confined 

 to ocean water with a temperature of 

 0.0° C. or lower (see p. 82). 



Organisms fiom the north temperate re- 

 gion can be divided into three main groups 

 on the basis of their resistance to low tem- 

 peratures. These are: 



1. Those that can survive exposure to 

 temperatures that approach absolute zero 

 (-273° C). 



2. Those that are killed at or near their 

 freezing point, usually relatively near the 

 freezing point of water. 



3. Those that die when chilled to some 

 point above freezing. 



The first assemblage includes plants ana 

 animals that, at some stage in their hfe his- 

 tory, can withstand desiccation and, when 

 dried, become tolerant of extremely low 

 temperatures. This group includes, among 

 others, plant spores and seeds, protozoan 

 cysts, rotifers, tardigrades, and nematodes. 

 The last three, if refrigerated slowly 'Aath- 

 out preliminary desiccation, can sinrvive a 

 temperature of -253° C. (Rahm, 1922); 

 certain bacteria, yeasts, and other fungi can 

 live similarly in extremely low temperatures. 



The majority of plants and poikilother- 

 mous animals of our latitudes belong to the 

 second group and are killed at tempera- 

 tures somewhat below, but still relatively 

 near, zero Centigrade. Two general sub- 

 divisions of these cold-tolerant forms are 

 known: First, those that can hve until their 

 body temperatures fall some 10 to 30 de- 

 grees below zero; often they can survive the 

 formation of much ice withvii their bodies. 

 They can recover after being frozen hard 

 and brittle with the cold, and apparently 

 die from cold only when the last of their 

 cellular liquids solidify. Cold-hardy insect 

 larvae and many woody plants react in this 

 way. This more or less artificial ecological 

 class passes, by continuous gradation, into 

 the second category, which includes a 

 larger assemblage of forms that are killed at 

 or near the freezing point of water. 



The third group, killed at some point 

 above freezing, is illustrated by some of the 



HEAT 99 



higher plants, by most mammals (except 

 certain hibernating ones) (see p. 105), and 

 by certain poikilothermal animals. Some 

 cladocerans— A/o/na macrocopa, for exam- 

 ple—become chilled and cease swimming 

 movements after continued exposure to 

 10° C; they settle to the bottom and die, 

 since the gill chambers become clogged 

 with mud and debris (Brown, 1929). 

 Long-continued exposure to low tempera- 

 tures well above freezing may cause 

 death even when the animals can with- 

 stand shorter exposure to cold of the 

 same intensity (Leeson, 1941). From 

 the httle that is known about cold 

 death of poikilothermous tropical animals, 

 it appears that they are probably killed by 

 only relatively low temperatures; fish near 

 subtropical Bermuda were killed in num- 

 bers by wdnter temperatures when the air 

 did not go below 7° C. (Verrill, 1901), 

 and breeders of tropical fishes know that 

 death occurs from cold at temperatures at 

 which more northern fishes thrive. In freez- 

 ing weather along the Florida coast there 

 is differential killing of the tropical element 

 of the fish fauna. 



It is probably not an overstatement to 

 summarize our knowledge of cold death by 

 saying (cf. Luyet and Gehenio, 1938, p. 

 88) that, with the exception of a few or- 

 ganisms that are killed at temperatvures 

 above zero, plants and animals of the tem- 

 perate latitudes either die when chilled 

 relatively near to their respective freezing 

 points or are not killed by any low tem- 

 peratiu-e to which they may be subjected 

 in nature. 



Winter's cold may be escaped by migra- 

 tion or hibernation, or it may be resisted 

 by the development of protective coverings 

 of fat, fur, or feathers, by the seasonal eUm- 

 ination of activities that consume much 

 energy, such as those concerned with repro- 

 duction, or by the development of in- 

 dividual or racial cold hardiness. Frequently 

 there are effective combinations of these 

 methods for successful overwintering. 



The problems differ for warm-blooded 

 and for cold-blooded animals. Unhke birds, 

 even highly motile terrestrial insects seldom 

 execute geographic migrations. Butterflies 

 do so more than most insects, and even 

 with them, periodic seasonal migrations on 

 a geographic scale are rare. The massed 

 autumnal migrations of the monarch butter- 

 fly (Danaus plexippus) to the south, and 



