MAMMALIAN ENDOGENOUS ACTIVITY RHYTHMS 799 



due to disturbing the animal with hght during its active period (repair 

 of an apparatus failure) (Rawson, 1956). 



Two hamsters (Mesocricetiis aura t us) were cooled by the method 

 of Andjus and Smith (1954) to body temperatures averaging 14°C 

 and 17°C for periods of about 3 hr. Both experiments gave Qio 

 values of 1.1. All these values are low compared with the tempera- 

 ture coefficients of 2 to 3 frequently encountered in biological sys- 

 tems. 



I approached this problem in another way, studying the effects of 

 ambient temperatures on the activity rhythms of two species of bats, 

 Myotis lucijugus and Eptesicus juscus. Recordings of these animals 

 were made, using spring-mounted cages with the time of onset of the 

 major burst of total activity used as the measure of the rhythm. Since 

 these bats are essentially poikilothermic when inactive, it was pos- 

 sible to vary tissue temperature simply by changing the environmental 

 temperature. The average period length could be determined over a 

 span of several days at each temperature. Qio values were calculated 

 from the differences between the mean values of the period length of 

 the rhythm at different temperatures ranging from 10°C to 22°C. 

 Ten determinations made with two animals resulted in Qw values 

 uniformly low, ranging from 1.04 to 1.07 (Rawson, 1956). 



The results presented show that these mammalian endogenous 

 activity rhythms are nearly, but not completely, temperature-inde- 

 pendent. The low temperature coefficients found suggest that the 

 rhythm is not directly dependent for its timing on the rate of metabo- 

 lism of some part of the organism. In accord with this view I have 

 been unable to alter the rhythm of Peromyscus by increasing the 

 metabolic rate of animals in a cold environment or by the administra- 

 tion of drugs having metabolic effects. 



SUMMARY 



These 24-hr rhythms are endogenous, can exhibit a remarkable 

 degree of accuracy, and are stable relative to many environmental 

 and experimental variables. I have so far succeeded only in the 

 experimental alteration of the period of the rhythm by varying light 

 conditions and body temperature. The modification of the endogenous 



