Ageing in Inbred Strains of Animals 



119 



starting point for comparison. Mammary cancer subsequently 

 occurred in 74 per cent of the remaining 66 animals. Only 12 

 sisters (i.e. 18 per cent) also developed a neoplasm within a 

 month. The other animals showed a neoplasm within a 

 period which in extreme cases amounted to over a year. 



The Lifespan 



The following example shows the survival curves of two 

 strains of our mouse colony and their hybrids. Fig. 1 demon- 

 strates the difference in lifespan of the two strains and the 



15 

 AGE IN MONTHS 



20 25 30 



NUMBER OF NORMAL 

 5-i VAGINAL CYCLES 

 A- ■ per month 

 3- • 



DBAf 



O 20 



R(O 20 xDBA f ) 



Fig. 1. Survival curves and endocrine ovarian function in ageing inbred 



mice. 



increased lifespan of the hybrids. The heterosis of the F x 

 hybrids is also apparent in the physiological activity of their 

 ovaries as expressed in the number of normal oestrus cycles 

 occurring monthly in old age (Thung, Boot and Miihlbock, 

 1956) (Fig. 1). 



The average lifespan of the hybrids is 24 months and the 

 maximum 30 months. In the majority, the end of the period 

 of fertility is reached between 12 and 20 months, at an average 



