SEXUAL ACTIVITY OF MALE RABBITS 495 
TABLE 9 
Size of litter in 1st to 15th service group compared with 20th 
1-15 20 
81ZE OF LITTER 
Number of litters Per cent of litters Number of litters Per cent of litters 
of given size of given size of given size of given size 
1 4 5.06 1 5.88 
2, 4 5.06 4 23.53 
3 3: 3.80 4 23.58 
4 0 0 2 11.76 
5 9 11.39 1 5.88 
6 16 20.25 0 
i 13 16.45 2 11.76 
8 14 Vi 72 1 5.88 
9 7 8.86 0 0 
10 6 7.59 1 5.88 
11 2 2.52 1 5.88 
12 1 1.28 0 0 
Totaless.= 79 Ad, 
of the litters were five or less in size, whereas in the 20th-service 
group 70.58 per cent of the litters were five or less in size. In 
spite of the fewness of the 20th-service litters, there seems to be 
a presumption in support of the idea, that by the time a male 
rabbit has performed twenty copulations within the space of 
three or four hours he is less able to beget large litters than 
when he has performed fewer than fifteen services within the 
same space of time. 
When we consider in relation to each other the facts shown by 
tables 6 and 7 we are confronted by something of a puzzle. We 
have interpreted the facts shown in table 6, in part at least, as a 
direct consequence of the facts shown in the previous tables, 
i.e., that when the volume of semen, the number of sperm per 
cubic millimeter, the amount of progressive activity and the 
potential duration of motion of the sperm are all reduced, then 
the likelihood of spermatozoa reaching and penetrating the ova 
is reduced, and therefore the per cent of pregnancies is dimin- 
ished. Table 6 shows a fairly regular and consistent decrease 
in certainty of pregnancy as sexual service increases. Now, as 
mentioned above, those circumstances which decrease the like- 
