36 HELEN DEAN KING 



The average size of the litters increased until the mothers 

 were 8 months old, when the maximum (6.55) was attained 

 (table 10). Subsequently litter size decreased slowly, and 

 dropped to its lowest point (5.18) at the end of the series. 

 The rise and fall in average litter size as the age of the mother 

 advanced is shown by the graph in figure 11, constructed from 

 data in table 10. 



A change in litter size as the age of the mother advances 

 has been reported in other strains of rats (Crampe, 1883; 

 Slonaker and Card, '23; King, '16 a, '24), as well as in other 

 polytocous mammals: guinea pig (Minot, 1891; Wright, '22), 

 rabbit (Hammond, '14), and the pig (Hammond, '14; Car- 

 michael and Eice, '20; Keith, '30). 



This series of data for gray rats, covering the litter pro- 

 duction of a large group of females, gives 6.1 as the average 



Fig. 11 Age of the mothers and litter size in the young. 



for litter size. This average is the same as that found in 815 

 litters cast by albino females (King, '24: table 1). The norm 

 for litter size in albinos is usually given as seven. This esti- 

 mation is based, for the most part, on random samplings of 

 litters produced in colonies where females were bred only 

 during the most active period of reproductive life and is there- 

 fore too high, since it does not include the litters that might 

 have been cast in the beginning and near the end of the normal 

 reproductive period. The latter litters, as this and other in- 

 vestigations have shown, are usually small as compared with 

 litters produced w T hen females are 5 to 9 months old. 



Variation in litter size was high regardless of the age of 

 the mothers, as is indicated by series of coefficients of varia- 

 tion given in table 10. Variation changed considerably, how- 

 ever, as the age of the mothers advanced. It was slightly 



