EXPERIMENTAL MODIFICATION OF MAMMALIAN 

 ENDOGENOUS ACTIVITY RHYTHMS* 



KENNETH S. RAWSON 

 Zoology Department, University of Wisconsin, Madison 



Endogenous rhythms of approximately 24 hr have been demonstrated 

 in both vertebrate and invertebrate animals. Review papers of Welsh 

 (1938), Kleitman (1949), and Aschoff (1954) provide extensive 

 summaries. The stimuli which govern the expression of an endogenous 

 rhythm arise within the animal rather than from any cyclic aspect of 

 the environment. Daily cycles of light and dark or high and low tem- 

 peratures regulate the rhythms of animals so that they coincide with 

 the 24-hr cycle of the natural environment. 



I shall describe a method used to measure the rhythms of small 

 mammals, discuss the responses of mice (Peromyscus) to various 

 treatments with light, and present results obtained by varying the 

 body temperature. 



The motor activity of an animal has been used by many investiga- 

 tors to measure the endogenous 24-hr rhythm. Aschoff (1952) has 

 recorded total activity in spring-mounted cages and has found that 

 the time of maximum activity fluctuates about a constant mean value. 

 These fluctuations may, however, amount to as much as ±2 hr per 

 day (Aschoff, 1955). 



I find that, for rodents, restricting the recorded activity to running 

 in a wheel provides an accurate measure of the endogenous 24-hr 

 rhythm, although some of the total activity, for instance that associ- 

 ated with feeding or movement in the nest, is not recorded. The re- 

 cording cage provides food, water, and a nest in a stationary part with 

 free access to a running wheel 10 inches in diameter. Standard experi- 

 mental conditions are: constant darkness, continuously available food 



* A part of this material is from the author's doctoral dissertation, Harvard 

 University (see Rawson, 1956). Work was supported in part by a predoctoral 

 fellowship from the National Science Foundation and in part by a grant from 

 the Wisconsin Alumni Research Fund. 



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