750 Comparative Animal Physiology 



A vast amount of research has been done on the contributions of the 

 interactions of endocrine sources to the maintenance of the normal cycles 

 of mammals. The endocrine actions and interactions of the major organs 

 participating in the normal reproductive cycles are indicated for the female 

 mammal in Figure 283, and for the male in Figure 284. 



In the female the pituitary, through FSH production, stimulates the de- 

 velopment of the ovarian follicles and interstitial tissues, with a consequent 

 increase in estrogen production. The rising concentration of estrogen in the 

 blood mimics FSH and furthers the ovarian development. At the same time 

 the estrogen gradually suppresses FSH production by the pituitary while 

 at hrst strongly encouraging LH liberation by the pituitary. Over a longer 

 period estrogen suppresses LH liberation. LFl liberated from the pituitary 

 stimulates ovulation in many mammals, and in all it stimulates the diflFeren- 

 tiation of corpora lutea with the subsequent production by them of progester- 

 one. Some investigators, notably Astwood, believe that LH simply induces 

 differentiation of the corpora lutea, an additional pituitary principle, luteo- 

 tropin, being needed for stimulating the actual secretion of progesterone. 

 Luteotropin is claimed to show many properties very similar to the lacto- 

 genic principle and mav, in fact, be identical with it. Progesterone cooper- 

 ates with estrogen in the further suppression of the production of FSH by 

 the pituitary. In case of pregnancy, the placentae of the developing em- 

 bryos are activated in the presence of progesterone and proceed to liberate 

 substantial amounts of chorionic gonadotropin the activity of which very 

 closely resembles that of pituitary LH. This gonadotropin contributes to the 

 suppression of production of both pituitary gonadotropins through its stim- 

 ulation of greater gonadal hormone production. 



The corresponding endocrine interactions participating in the male re- 

 productive cycle are relatively much simpler. Gonadotropin from the pitui- 

 tary activates the interstitial and spermatogenic tissues of the testes, with the 

 resultant increase in testosterone production in the former. This androgen 

 simultaneously stimulates further development of the spermatogenic epithe- 

 lium and facilitates the influences of both FSH and LH. As the blood titer 

 of androgen gradually rises this hormone suppresses pituitary gonadotropin 

 production. The decline of androgen production after pituitary suppression 

 eventuallv leaves the latter free to initiate a new cycle of reproductive ac- 

 tivity. 



In a few species there tends to be an annual reproductive period at a par- 

 ticular season of the year, without any regard to any particular factor of the 

 external environment. Such an operation of an inherent rhvthm has been 

 clearly demonstrated for certain organisms which continue to breed indefi- 

 nitely during the same calendar months after transfer from the northern to 

 the southern hemisphere, or vice versa. Other species on similar transfer 

 more or less rapidly undergo- a readjustment of their breeding period to the 

 corresponding season for their new locality.^"" The readiness with which 

 the organism makes such an adjustment is obviously a function of the rela- 

 tive degree to which the breeding cycle depends on external factors. 



Of all the external stimuli known to exert an influence on reproductive 

 cycles, one of the most definitely influential so far demonstrated is light and 

 photoperiodism.*'- Among certain north temperate zone birds it has been 



