56 THE BREEDING OF ANIMALS 



sexual activity. This varies greatly in different animals. 

 Again quoting Marshall, " In some animals, such as the 

 dog, the metoestrous period is followed by a prolonged 

 period of rest or anosstrum. In others, such as the rat 

 or the rabbit, the metcestrum may be succeeded by only 

 a short interval of quiescence. This short interval, 

 which sometimes lasts for only a few days, is called the 

 dicestrum. This in turn is followed by another proces- 

 trous period, and so the cycle is repeated until the sexual 

 season is over. Such a cycle (consisting of a succession 

 of the four periods, prooestrum, oestrum, metoestrum, and 

 dicestrum) is known as the dicestrous cycle. The num- 

 ber of dioestrous cycles in one sexual season depends 

 upon the occurrence or nonoccurrence of successful 

 coition during oestrus. Thus, if conception takes place 

 during the first oestrous period of the season, there can 

 be no repetition of the cycle, at any rate until after parturi- 

 tion. The cycle may then be repeated. If conception 

 does not occur at any oestrus during the sexual season, 

 the final metoestrous period is succeeded by a prolonged 

 anoestrous or non-breeding period. This is eventually fol- 

 lowed by another prooestrum, marking the commencement 

 of a new sexual season. The complete cycle of events 

 is called the oestrous cycle." l 



56. Puberty. The reproductive functions are not ac- 

 tive in the young mammal during its very early existence. 

 The nutritive system during the same period is char- 

 acterized by unusual functional activity and a high degree 

 of efficiency. The food consumed during the early exist- 

 ence of the mammal produces larger gains in live weight 

 than the same food at any later period of its life. 



The growth processes continue to function with great 



1 Marshall, "The Physiology of Reproduction," p. 37. 



