HIBERNATION IN MAMMALS— LYMAN and CHATFIELD 115 



The second general theory proposes that hibernation is bronght aliout by a decrease 

 or increase of a substance or sul)stances which are or(Hnarily present in the mam- 

 niah'an l)odv. From a theoretical ])(iint of view, the weakness of this concept is that it 

 fails to explain how the animal can wake from hibernation. If an excess or lack of 

 a given substance will cause an animal to hibernate, this imbalance must presumably 

 be corrected before the animal can recover from the hibernating state. However, 

 as has been mentioned above, the animal which has just entered hibernation actually 

 wakes more readil}- than one which has hibernated for a long time. 



Among those who have subscribed to tliis theory are Dworkin and Finney"^ who 

 produced hypoglycemia in woodchucks by injecting large doses of insulin. These 

 animals chilled when exposed to cold. Suomalainen,'"' having observed low blood 

 sugar and high serum magnesium in hibernating hedgehogs, tried injecting a com- 

 bination of insulin and a large amount of magnesium into animals exposed to cold. 

 The hedgehogs chilled in a manner very similar to animals entering hibernation. In 

 Dworkin and Finney's experiments the animals died without awaking, unless in- 

 jected with glucose and moved to a warm environment; and Dische, et al.,^'''^ showed 

 that injection of insulin would actually waken normal hibernating common dormice 

 and European ground squirrels. In Suomalainen's experiments, glucose and calcium 

 and a warm environment were necessary to produce recovery of the animal. As has 

 been shown above, hypoglycemia is not a universal occurrence during hibernation, 

 and we would attribute the chilling after insulin injection to hypoglycemic shock. 

 The effect of large doses of magnesium could be due to the anaesthetic action of 

 this metal, with calcium producing its well-known counteractive effect. 



Dubois*'- concluded that the marmot built up an excess of COo in its burrow and 

 that this caused the animal to hibernate. Actually it is conceivable that the excess or 

 lack of some gas could cause hibernation, for the condition could be corrected 

 quickly, even during hibernation. Recently Andjus^*'^ has chilled rats to 0° C. and 

 Smith, ct a/./^^ have chilled hamsters to — 5° C. by exposing them to cold in air- 

 tight containers. The animals lack sufficient oxygen to fight against the cold and can 

 be chilled to temperatures far below the lethal limit reported for these animals when 

 given adequate oxygen. ^^'^ However, if such a mechanism takes place during the 

 entrance into hibernation, the methods of its functioning are completely unknown. 



The endocrine glands should probably be included under the general theory of a 

 hypo- or hyperfunction as the cause of hibernation. Much of the evidence implicat- 

 ing the endocrines has been based on a histological comparison of glands from awake 

 and hibernating animals. We have mentioned that other seasonal changes in the 

 endocrines have not always been differentiated from those which take place just as 

 the animal hibernates, and the researcher is further hampered by not knowing which 

 is the cause and which the effect of the hibernating state. As Kayser's review shows,^^ 

 polyglandular involution exists in the hibernating animal and inactivity of the endo- 

 crines involved in metabolism is to be expected. Indeed, Foster, et al., ^*^^ showed that 

 ground squirrels will not hibernate when fed thyroxin, or when injected with pitui- 

 tary extract. The contrary result reported by Johnson and Hanawalt^*^^ may be due 

 to the fact that they sterilized their thyroxin by boiling before injecting it. On the 

 other hand the hibernating state as we have defined it has never been produced by 

 removal of any one of the endocrine glands. Removal of the adrenals does not cause 



