14 HELEN DEAN KING AND HENRY H. DONALDSON 



BODY WEIGHT AT BIETH 



In the early generations of captive Grays parturition 

 seemed to occur at night or in the early morning more often 

 than during the working hours of the day ; consequently, only 

 a small number of litters were obtained before the individuals 

 had suckled. Birth weights were taken for the twenty-six 

 individuals in four litters cast by wild females. The twelve 

 males in these litters had an average weight at birth of 5.34 

 grams; the average weight for the fourteen females was 

 5.09 grams. 



In the first ten generations of these rats individual birth 

 weights were recorded for a total of fifty-six litters, con- 

 taining 177 males and 143 females. The average weights of 

 the males varied from 5.07 to 5.82 grams in different gener- 

 ations ; those of the females from 4.23 to 5.68 grams. There 

 was no definite trend to the birth weights, either individual 

 or average, as the generations advanced. In many cases 

 individual females had a heavier weight at birth than one 

 or more of the males in the same litter, but in each generation 

 the average weight of the males exceeded that of the females. 

 In fourteen litters of the tenth generation the forty-three 

 males weighed, on the average, 5.29 grams at birth; the 

 thirty-eight females had an average weight of 5.03 grams. 

 These weights do not differ significantly from those of indi- 

 viduals in the first generation. 



No other determination of the birth weights of gray Nor- 

 way rats has been recorded, except that of Miller ('11), who 

 states that the offspring of wild females in captivity weigh, 

 on the average, 6.4 grams at birth, the males tending to be 

 slightly heavier than the females. The number of individuals 

 on which this determination is based is not given. 



Apparently the weight of the gray rat at birth tends to 

 be somewhat heavier than that of the albino rat. The find- 

 ings of Jackson ('13) and of King ('15), for various series 

 of Albinos, show that in this race the range of variation in 

 the average birth weights of males is from 4.6 to 5.1 grams ; 

 that of females being from 4.5 to 4.9 grams. In both the 



