88 THE BREEDING OF ANIMALS 



84. Fertility and the frequency of the recurrence of the 

 oestrum. The fertility of animals is also dependent 

 upon the frequency of the recurrence of the oestrum. 

 As already described, the recurrence of heat depends 

 upon a number of conditions, chief of which are 

 pregnancy and lactation. Certain of the domestic 

 animals do not normally come in heat while suckling 

 young, but to this there are many exceptions. Animals 

 having a short period of gestation rarely or never come 

 in heat while suckling young. 



85. Fertility and gestation. Animals having a long 

 period of gestation are less fertile than animals requiring 

 a shorter time for the development of the young in the 

 uterus. 



The exceptional fertility of the domestic sow is due 

 not only to the large number of young born at a time, 

 but to the further fact that the period of gestation is 

 only four months. A sow may thus easily produce 

 two litters a year. The mare produces but one off- 

 spring at a time, and the period of gestation is from 

 eleven to twelve months. A sow, therefore, may give 

 birth to twenty young in the same period of time re- 

 quired by the mare for the development and birth of 

 one offspring. 



86. Duration of the reproductive period. The fertility 

 of an individual or a breed is largely determined by the 

 duration of the reproductive period. The length of 

 this period is a matter of special importance in the larger 

 animals which produce but one young at a birth. The 

 breeding age is the time from the arrival of puberty 

 until the cessation of the breeding function on account 

 of old age. The beginning of the reproductive period 

 has already been discussed under puberty. The 



