CHAPTER 12 



GROWTH IN TOTAL BODY WEIGHT ON AGE— CAGED 



NORWAYS 



All data on the growth of the Norway rat must necessarily 

 be obtained from caged animals, and although it is recognized 

 that captivity modifies the form and functions of the animal, 

 there is apparently no way to escape this dilemma. The growth 

 on age of the first generation (Fi) of Norway rats in captivity is 

 given by Dr. King ('23) — table 185 and charts 59 and 60. 



Graphs for the body weights of stock Albinos are also given in 

 these charts. 



The coefficients of variation have been determined for this 

 series of Norways and in table 186 these are compared with 

 corresponding coefficients for the Albino. 



These observations reveal the following relations : 



Though less heavy at birth, yet from the first few days up to the 

 fortieth day of postnatal life Norway females have an average 

 body weight greater than that of the males. After forty days 

 of age the males, as a rule, are heavier than the females at all 

 age periods. This revelation of the precocity in the growth of 

 the female is thought to be due to a greater retardation of growth 

 in the male, owing to the excited condition in which these gray 

 rats live during the earlier generations in captivity, and to the 

 fact that males are more responsive to conditions, both favorable 

 and unfavorable, than are females. 



These Fi Norway rats do not show the marked acceleration in 

 body growth during early life that is characteristic of the albino 

 strain. Increase in body weight progresses at a fairly uniform 

 rate in both sexes until the animals are approaching senescence, 

 the increase in the adult state being greater in the males than in 

 the females (charts 59 and 60). 



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