MAMMALIAN ENDOGENOUS ACTIVITY RHYTHMS 793 



Fi^^ure 1 shows a record of a white-footed mouse, Peromyscus 

 leiicopiis noveboracensis, recorded in constant darkness. The maxi- 

 mum rate shown here is about 125 revolutions per minute. Each 

 horizontal line represents a single 24-hr span. Twenty-two days' 

 records are shown from the top to the bottom of the figure. The 

 records can be read accurately to within about 2 min. It is evident 

 from the graph that the onset of activity forms a very accurate meas- 

 ure of the'^period of the rhythm, while the total amount and the dura- 

 tion of the activity may vary from day to day. In constant darkness, 



^ 9 24rf. 36M. 



24 . 



18 



< 

 o 12 



u. 

 O 



UJ 



24 



I8t 



r-^v-^ 



10 20 



SEPT. 



30 

 DATE 



^/\ ? PARENT 



cf 23H. 44M. 



10 

 OCT. 



20 



Fig. 2. Times of onset of activity in constant darkness of a female 

 Peromyscus and her litter of six young. Animals recorded in separate 

 cages. Disturbance with a light in adding food indicated by arrows. 

 Ordinate is extended beyond midnight in both directions to aid in follow- 

 ing records of two mice. Young born August 20. 



