CHAPTER III 



THE MAMMALIAN SEXUAL CYCLE 



The season of the year when an animal of any species or variety 

 undergoes sexual intercourse has been called by Heape the 

 sexual season for that animal. In most or perhaps all wild 

 mammals the male experiences such a season as well as the female, 

 and generative activity is restricted to such times. In man 

 and most of the domestic animals, however, the male is capable 

 of sexual intercourse at all seasons, and spermatogenesis goes 

 on continuously, although it is probably true that an increased 

 activity is shown at certain seasons which may perhaps depend 

 upon food or other environmental conditions or upon varying 

 habits of life. 



The male sexual season, when it occurs, is called the rutting 

 season. Prior to rut the testes increase in size, and both the 

 spermatogenetic and the interstitial tissues display activity, 

 though the exact time relation between the changes shown by 

 these tissues varies somewhat in different species. In those 

 animals in which the testes are not permanently retained in the 

 scrotum the descent takes place at the beginning of rut, and 

 the organs are withdrawn into the abdomen when the season is 

 over. This is the case in many rodents. In insectivores {e.g. 

 the mole and the hedgehog) the testes descend periodically into 

 temporary receptacles. In the mole it is estimated that the 

 testes increase in size by sixty-four times, and the vesiculse 

 seminales, prostate, and other accessory glands likewise show 

 an enormous growth, which is seen also in the hedgehog. The time 

 for sexual intercourse is continuous throughout rut, there being 

 no short periods of quiescence within the sexual season as there 

 are in the females of many species. 



In the female mammal the recurrence of the periods of sexual 

 activity is what constitutes the oestrous cycle. The actual 

 periods at which copulation occurs are the oestrous periods. 

 These recur at rhythmical intervals within one sexual season, 



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