LIFE PROCESSES IN CAPTIVE GRAY RATS 39 



The lowest point in the graph of figure 12 is at the second 

 generation, where the sex ratio falls to 81.9 males to 100 fe- 

 males. Thereafter the graph tends to rise, reaching its high- 

 est point (107 males to 100 females) at the twelfth generation, 

 then declining gradually and ending in a nearly straight line 

 which is below the level of the mean. A male excess among 

 individuals is indicated at only eight of the twenty-six points 

 in this graph, and there are no changes in it, except possibly 

 the drop near its beginning, indicating that captivity per se 

 affected the ratios in any way. 



Fig. 12 Sex ratios in different generations. 



Data for a series of stock albino rats, comprising the litter 

 output of a group of individuals reared under the same condi- 

 tions of environment and of nutrition as the early generations 

 of captive grays, give a mean sex ratio of 105.2 2.00 (King, 

 '24). This ratio is significantly higher than that found in 

 gray rats, the difference being 7.9 2.08. The disparity in 

 the sex ratios of these two strains must be due either to errors 

 in recording the data or to differences inherent in the strains, 

 since they cannot be ascribed to unlike conditions of life. 



Depletion of litters through destruction of stillborn young 

 by adults in the cage is the most common source of error in 

 litter records for rats. Such errors cannot be avoided, since 

 parturition often occurs at night and some hours may elapse 

 before the young are examined, but they have relatively little 



