414 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1955 



further result: the life span of a shrew is not more than 18 months, 

 but that of a bat of similar size is 7 or 8 years or more. When it is 

 remembered that some species of bat spend up to nine-tenths of their 

 summer life in torpid sleep, and all of the winter in hibernation, it 

 can be readily appreciated that the animal machine will not wear out 

 so quickly as in animals that are active for a higher proportion of the 

 24 hours all the year round. 



Hibernation in a warm-blooded animal is a very much more drastic 

 process than in a cold-blooded one: the animal has in effect to re- 

 linquish its temperature regulation and become cold-blooded. All 

 hibernating mammals get very fat before entering on the winter sleep, 

 and the fat deposits not only serve to keep the metabolism going at its 

 reduced rate during the winter, but their presence in the autumn 

 provides, or helps to provide, the stimulus that starts liibernation, 

 though the means by which they do so are not known. Low tempera- 

 tures and scarcity of food are not the primary causes. Similarly in 

 some kinds of wild bee the young queens that will found the new 

 colonies of the following year start their hibernation almost as soon 

 as they emerge from their brood cells in the middle of the summer; 

 and swifts leave this country on their southern migration in August 

 when the supplies of insect food are the greatest — these things are 

 brought about by the internal condition of the animal and not in the 

 iirst place by the state of the environment. 



Mammals have a deposit of dark-colored fat and lymph tissue round 

 the blood vessels in the neck, chest, and elsewhere, in addition to the 

 general fat stores ; it is particularly prominent in hibernating mam- 

 mals. This deposit is large in the autumn but decreases to very small 

 proportions by the middle of the following summer. This "brown 

 fat" has been termed the "hibernating gland," but its functions are 

 obscure, for it decreases in size during hibernation more slowly than 

 the general reserves of fat. 



When a hibernating warm-blooded animal becomes cold-blooded 

 and midergoes a decrease in temperature the rate of respiration also 

 decreases. A torpid bat, for example, breaths about 25 to 30 times a 

 minute for some three minutes and then pauses for three to eight 

 minutes without breathing ; its normal rate during active periods goes 

 up to about 200 a minute without any pauses. In some species the 

 rate during hibernation is lower still, so that it is very difficult to detect 

 any sign of breathing at all. At the same time the rate of the heart- 

 beat becomes very slow and the circulation is further retarded, in some 

 bats at least, by the spleen swelling up to about seven times its normal 

 volume, being distended with blood and acting as a reservoir for the 

 main bulk of the blood while it is not being vigorously pumped around 

 the body by the heart. 



