16 BIOLOGY 



dependent on the mother. After this they may be weaned, al- 

 though if permitted, the young will depend partly on the mother 

 for some days longer. 



This adjustment of relations fits with the fact that the female 

 may be impregnated one or two days after casting a litter (Kirk- 

 ham, '10; Kirkham and Burr, '13) and since the gestation period 

 is about 21.5-22.5 days, this would enable the female to free 

 herself from the first litter before the second one was born. As 

 will be pointed out later, the gestation period may be pro- 

 longed in nursing animals. 



When the young rats become habituated to independence, i.e., 

 at about 25 days, they enter on a period of activity, the phases 

 of which have been followed by Slonaker ('07, '12). In the 

 cases which he observed, it was found that increasing age was 

 accompanied by increasing activity up to the age period of 87- 

 120 days, after which the activity declined. 



On the assumption that the span of life in man is thirty times 

 that of the albino rat (Donaldson, '06) this age of greatest ac- 

 tivity would correspond to the age of 7.5-10 years in man. 



As shown by the records of activity (Slonaker, '12) the al- 

 bino rat is nocturnal. This habit can be modified more or less 

 by feeding or by disturbance during the day time. 



The measure of activity in the cases observed by Slonaker 

 was the number of turns of the revolving cage in which the 

 the animal was kept, the cage being set in motion by the vol- 

 untary running or other movements of the animal, and the 

 revolutions being automatically recorded. In the case of four 

 rats kept in separate revolving cages from 30 days of age until 

 natural death, the following records of activity were obtained 

 (Slonaker, '12). Table 1. 



This table shows not only great variability in the total per- 

 formances, but also for the one female a record of over five 

 thousand miles in a little less than three years. On the aver- 

 age, three-fourths of the total distance is run before the rat 

 has reached middle life, and the last months of old age are al- 

 ways marked by greatly lessened activity (see Behavior — ^ac- 

 tivity and exercise, page 31). 



