HIBERNATION IN MAMMALS— LYMAN and CHATFIELD 



87 



liibernating state. l)ut the evidence is not convincing that the endocrines exert a 

 dominant role in the onset of hibernation.* 



ENTERING HIBERNATION 



Because of the difficulty of studying animals as they enter the hibernating state, 

 this phase of the process of hibernation is little understood. Kayser^^ has shown 

 that several species of hiljernators have a lower metabolic rate in the fall just before 

 hibernation than they have in the spring. Dubois"- and Benedict and Eee"^ have 

 pointed out that in animals such as the marmot and the woodchuck, the body tem- 

 perature wavers for several days before the animal becomes completely dormant. 

 These observations have been confirmed in our laboratories with woodchucks and 

 ground squirrels, but a different sequence occurs in the gcjlden hamster. This animal 

 enters hibernation with one rapid decline in body temperature and takes about 

 8 hours to reach the environmental temperature of 5° C" The drop in body tem- 

 perature, however, is not as rapid as that of a dead hamster of the same weight 

 cooling in the hibernating position, which indicates that metabolism is slowing 

 the cooling to some extent. Metabolic measurements show that the metabolism 

 begins to decline at approximately the same time as the drop in body tempera- 

 ture, but the measurements are not sufficiently accurate to determine whether 

 the temperature drop precedes or lags behind the drop in metabolic rate. As the 

 animal enters hibernation, the decline in metabolic rate proceeds faster than the 

 decline in body temperature and reaches its minimum several hours before the 

 body temperature reaches the basic value of 5° C. (fig. 3). This may suggest 



Coffected Temperoture 

 An.mol Entering HibemoTion 



Temperature Dead Bsdy 



O, Consjr^ption 



Fig. 3. — The upper line is an average of temperatures of the cheek pouch obtained from a 

 single hamster entering hibernation on three separate occasions at an environmental temi>erature 

 of 4° C. For comparison, the rate of temperature loss of two freshly killed hamsters is shown 

 in the middle curve. The lower curve represents the oxygen consumption of the animal entering 

 hibernation on the same three occasions. The vertical lines show the range of variation of the 

 measurements. 



t Recently, W. N. Holmes (Endocrinology 5~ : 409. 1955) presented confirmatory evidence by 

 showing that there is actually a transitory rise in adrenal ascorbic acid in hamsters during the 

 first three hours of exposure to cold. After this time, the ascorbic acid drops to a normal level, 

 and remains at this level even during hibernation. There was no significant change in the weight 

 of the adrenals during the total period. 



