Discussion 85 



train has been slowed down and therefore does not reach the trap- 

 doors, which have not altered their positions, until later. Your 

 graph of the numbers dying, such as these that died before 800 days, 

 somewhat supports the latter interpretation. You have got straight 

 lines against age, but the slope of the line for the restricted rats is 

 less than it is for the ad libitum-ied rats. This is the equivalent of 

 the velocity of the train — to follow my analogy — being less. If you 

 altered the time scale you could superimpose those two lines. This 

 is a possible explanation, though not necessarily a correct one. 



The specific question following that is how much was puberty, the 

 opening of the vagina, actually delayed in your rats ? 



Berg: There is a delay of about three to four weeks. 



Tanner: That is relatively small. We do not really know how an 

 animal measures the passage of time — except that it is not by a 

 calendar ! We see this particularly in children, and we have various 

 measures of what we call developmental age : the stage of ossification 

 of the bones of the wrist, the number of teeth erupted, the menarche 

 (the first menstrual period in girls) and so on. These stages are similar 

 to the developmental horizons of the embryologists. It seems to me 

 that in gerontology we are all badly in need of some equivalent of 

 this. 



Berg: We all recognize that various functions (such as sexual 

 development, bone growth, and onset of lesions) take place more 

 rapidly in lower animals than in man. This difference between 

 species in rate of growth, development and ageing is a problem in 

 itself. What we have attempted to do in our paper is to show the 

 similarity between the species in that they follow the same laws in 

 regard to onset of lesions and death. 



Tanner: The growth curves of man and rodents (and your rodents, 

 of course, are living in dreadful conditions) are really considerably 

 different. But the growth curves of primates as a whole are very 

 similar. This points out the tremendous importance of doing this 

 sort of work with the cheapest monkeys you can get hold of. 



Berg: The rats in our colony really live under very fine conditions. 

 Except for being in captivity, they live under better conditions than 

 most humans. However, the cost of these conditions is very high. 

 A similar colony of monkeys would cost millions of dollars. 



Comfort: Monkeys survive extremely poorly in captivity. Another 

 trouble is the time factor. A baboon can live for over thirty years 

 (Duetz, G. H. (1938). Lab. Rep. zool. Soc., Philadelphia, 66, 31). 



Verzdr: You have just underlined one of the main points of 

 gerontological research. Dr. Tanner — that we need tests of biological 

 age. We speak far too little about tests, and all our work should 



