ACTIVITY^ — EXERCISE . 31 



Sounds. The sounds made by the rat range from an inti- 

 mate chicking used by mothers with young to small squeaks — 

 in response to slight discomfort — rising to a high pitched scream 

 when suffering severe pain or in great terror. Broadly speaking 

 this last response comes most often from the wild Norways 

 just after capture. 



Under experime7ital conditions. See References. 



Activity and exercise. When fed at midday Albinos divide 

 the 24 hours into about 14 hours of interrupted rest and sleep 

 and 10 hours of marked activity — from the early evening to 

 the early morning (Szymanski, '18 a). 



They are as responsive to training during the period of inac- 

 tivity as during the active period (Szymanski, '18). As in- 

 centives to learning, hunger, pain, and sometimes the maternal 

 instinct (return to the young) are effective (Szymanski, '18 b). 



Richter ('22) has determined that the minor periods of activ- 

 ity are related to hunger, as expressed by contractions of the 

 stomach, and Wang ('23) has found that in the female in the 

 revolving wheel the performance — as measured by miles run — ■ 

 is periodic and related to the phases of the oestrous cycle (see 

 pp.23 and 24). 



Effects of alcohol and drugs. See References. 



Effects of lesions of the brain and sense organs. See References. 



Care of rats. Greenman and Duhring ('23). 



11. Comparison with man. When compared with man the 

 rat shows a series of similarities which are of significance for its 

 use as an experimental animal. 



Similarities in form relations. The males are usually larger 

 and heavier than the females. The females are somewhat fatter. 



The determination of the equivalent ages. The Albino doubles 

 its birth weight in about six days, thus in one-thirtieth the time 

 taken by man. If a rat at three years (it is then very old) is 

 taken as equivalent to man at ninety years, we again have a 

 relation of 1-30 in the equivalent ages. It is assumed there- 

 fore that the rat lives thirty times as fast as man. Thus one 

 day of rat life is equivalent to 30 days of human life (one month). 

 When the increase in the weight of the brain, and the change 



