FERTILITY 637 



"good getters" but were afterwards responsible for cows being 

 sterile or aborting at different stages of pregnancy, and he explains 

 them in this way. Hammond l describes what he suggests may have 

 been another such case of a bull, for he found that the semen of the 

 animal in question only contained occasional spermatozoa which, in 

 contrast to those obtained from other bulls, showed no sign of 

 movement. There can, however, be little doubt that ordinarily 

 spermatozoa in amply sufficient numbers are ejaculated at one 

 service to fertilise all the ova discharged by the female. Moreover, 

 Hammond found that with the male rabbit after repeated coition 

 (even up to the thirty-seventh time and within eight hours) there 

 was no reduction in the fertility of the females which were served. 

 The delay periods, however, between the later copulations were 

 slightly increased. Contrary to Hammond's results Lloyd-Jones 

 and Hays 2 had previously found that excessive copulation by the 

 male rabbit produced a reduction both in the number of pregnancies 

 and in the size of the litters. 



It is said that a good stallion should be able to serve eighty 

 mares in one season, but Hammond has pointed out that there may 

 be no reduction in the fertility of the mares even though as many 

 as 140 are served. At the same time, the records of one province 

 have shown that the percentage of foals left by different stallions 

 can vary from twenty-seven to seventy-five. 3 The usual proportion 

 of ewes put to one ram is about fifty, but ram lambs are not 

 permitted to serve more than about twenty ewes. Similarly with 

 stallions, when they are two years old they are not allowed to have 

 intercourse with more than sixty mares as compared with 80 to 120 

 for adult stallions. It is not suggested, however, that if a larger 

 number of services were permitted the unions would be sterile, but 

 that too frequent intercourse has a deteriorating influence on the 

 vigour of the male. Even yearling stallions are sexually mature 

 but are never allowed more than fifteen mares, and generally none 

 at all. 



EFFECT OF PKOLONGED LACTATION 



It has been recorded that the continuance of lactation commonly 

 exerts an inhibitory influence on menstruation in women and on 

 heat in animals, though this is very far from being invariable (see 

 p. 69). There can be no doubt that in the case of sows, for 

 example, early -weaning is conducive to a more frequent recurrence 



1 Hammond, loc. cit. 



2 Lloyd-Jones and Hays, " Influence of Excessive Sexual Activity in Male 

 Babbits," Jour. Exp. Zool., vol. xxv., 1918. 



3 Marshall and Crosland, " Sterility in Mares, etc.," Jour. Board of Agric., 

 vol. xxiv., 1918. 



