20 HELEN DEAN KING 



VARIABILITY IN BODY WEIGHT 



Body-weight variability in both sexes of gray rats changed 

 considerably as the generations advanced. To indicate the 

 trend and extent of this change, coefficients of variation for 

 body weights at different age periods in individuals of three 

 generations are given in tables 5 and 6. In these tables the 

 series of coefficients for the first generation are reproduced 

 from the former report on this strain (King and Donaldson, 

 '29). Coefficients for the age period of 13 days were calcu- 

 lated from the average weights of males and females in each 

 litter, as were also those for the 30-day period in the first 

 generation. For all other age periods, the coefficients were 

 calculated from individual data. 



Coefficient of variation for body weights of males are given 

 in table 5, those for females in table 6. 



Tables 5 and 6 are given for reference only, since a graphic 

 representation of the data for the first and for the twenty- 

 fifth generations (fig. 6) shows more clearly the marked 

 changes that occurred in the body-weight variability of gray 

 rats during the period of captivity covered by the present 

 report. 



In figure 6 graphs (A and C) for body- weight variability 

 in the two generation groups of males have the same general 

 trend in that each rises to a maximum at an early age period, 

 and then declines. Graph C, however, is much below graph A 

 at all points, and its level as age advanced indicates that 

 body-weight variability in males of the twenty-fifth generation 

 was less than half of that in males of the first generation 

 during the major portion of adult life, and even lower near 

 the end of the weighing period when senility was beginning. 



Graphs B and D, depicting body-weight variability in fe- 

 males of the two generation groups, run more irregularly than 

 do those for males, because maximum variability in females 

 of the first generation did not come until the rats were 13 

 months of age. For age periods up to 4 months differences 

 between the graphs are not statistically important, but sub- 

 sequently the levels of these graphs indicate that body-weight 



