MAMMALIAN ENDOGENOUS ACTIVITY RHYTHMS 797 



from light presented either continuously or near the beginning of the 

 active period. 



Diurnal birds (Wagner, 1930; Aschoff, 1953) and lizards (Hoff- 

 mann, 1955) have rhythms typically shorter than 24 hr in constant 

 light and longer than 24 hr in constant darkness. I have attempted to 

 confirm this relation for a diurnal mammal, using the chipmunk 

 Tamias striatus lysteri as the experimental animal. The recorded 

 rhythms of these animals were less consistent than those of Peromys- 

 cus, showing major shifts in the phase of the rhythm as a result of 

 small disturbances. Of the eight animals studied only three showed 

 clear responses to constant light and constant darkness. One of these 

 records is shown in Fig. 5. An initial rhythm of 24 hr 15 min in dim 

 constant light is followed by a rhythm of approximately 26 hr 15 min 

 in constant darkness. The period length then returned to about 24 hr 

 30 min in constant light. Another chipmunk had a 23-hr rhythm in 

 constant light (0.5 ft-c) and a 24-hr 40-min rhythm in constant dark- 

 ness. The third chipmunk showed a 23-hr rhythm in constant light 

 (0.34 ft-c) and a 25-hr rhythm in constant darkness. The responses 

 of the three chipmunks are consistent with the responses of other 

 diurnal vertebrates. 



TEMPERATURE EFFECTS 



Investigations of endogenous rhythms in invertebrates have shown 

 these rhythms to be nearly independent of tissue temperature within 

 the normal ecological temperature range of these poikilothermic 

 organisms (see Bruce and Pittendrigh, 1956). With the method 

 described for accurately measuring the periods of mammalian activity 

 rhythms, it was possible to investigate the effects of changes in body 

 temperature on the 24-hr rhythms of normally homoiothermic mam- 

 mals. 



Lowering the body temperature of Peromyscus under sodium 

 pentobarbital anesthesia for periods of from 5 to 8 hr lengthened the 

 period during which the cooling occurred. This resulted in a measur- 

 able delay in the time of onset of the following active period. The 

 average body temperature during the period of cooling was deter- 



