MAMIMALIAN REPRODUCTIVE CYCLE 



499 



GUINEA 

 PIG 



PRIMATE 



Fig. 8.1. Diagrams of cycles of representative, familiar mammals. , the follicular phase, 



highly schematized and inaccurate in detail ; , atresia ; i , ovulation ; • , fully active 



corpora lutea; O, corpora lutea regressing or otherwise not fully active. When sterile mating 

 or equivalent stimulation (SM) is introduced, the cycles of the rat, rabbit and cat become 

 directly comparable with those of the other species. 



furnishes the classic exaini)le of reflex ovu- 

 lation. Other reflex ovulators are the domes- 

 tic cat (Greulich, 1934), the ferret (Ham- 

 mond and Walton, 1934), mink (Hansson, 

 1947), marten (Pearson and Enders, 1944), 

 the 13-lined ground squirrel (Foster, 1934), 

 and the mole shrew (Pearson, 1944). To this 

 list have been added the muskrat (Miegel, 

 1952) and a field mouse, Microtus cali- 

 fornicus (Greenwald, 1956). Even among 

 the marsupials, the female Didelphijs azarae 

 is said not to form corpora lutea in the ab- 

 sence of the male (Martinez-Esteve, 1937). 

 A few of these species display nearly con- 

 stant estrus (rabbit, ferret), competent 

 follicles being present most of the time in 

 the isolated female during the breeding sea- 

 son. 



Among even the spontaneous ovulators 

 the cycle may sometimes not progress be- 

 yond the follicular phase. Thus, at the 

 approach of puberty, waves of advanced 

 follicle development and secretion of estro- 

 gen may take place without, however, lead- 

 ing to ovulation or corpus luteum forma- 



tion. The first cycles of primates are often 

 anovulatory ones. In the adult macacjue, at 

 least in some colonies, such cycles are char- 

 acteristic during the summer months (Hart- 

 man, 1932) . A somewhat comparable sea- 

 sonal effect has been reported in girls soon 

 after the menarche (Engle and Shelesnyak, 

 1934). Menstrual cycles without ovulation 

 have frecjuently been recognized in adult 

 women in recent years, bearing no evident 

 relationship to seasonal factors (Lopez Co- 

 lombo de Allende, 1956). Anovulatory cycles 

 were described in the mouse by Allen (1923) 

 and have been noted occasionally in other 

 species, but without clear measure of their 

 incidence. 



III. Pituitary-Ovarian Doriiianey 



Varying levels of pituitary-ovarian dor- 

 mancy are expressed in different ways from 

 species to species or even from habitat to 

 habitat within a given species. A general 

 similarity exists between the anestrum of 

 seasonal breeders and the prepubertal state. 

 In fact, in animals that have a distinct sea- 



