MAMMALIAN REPRODUCTIVE CYCLE 



501 



Fig. 8.2. Vaginal cycles during seasonal amenorrhea in a monkey. (A portion of the record 

 of monkey ^38 from C. G. Hartman, Contr. EmbryoL, Carnegie Inst. Washington, 13, 

 Fig. 26, p. 121, 1932.) 



increased gonadotrophin content thereof, 

 and increased numbers of basophile cells. 

 Warwick (1946) reported a highly signifi- 

 cant increase of pituitary potency in 

 spayed anestrous ewes. This is closely anal- 

 ogous to the results of ovariectomy in im- 

 mature animals (Hohlweg, 1934). As meas- 

 ured by ovarian activity, gonadotrophin 

 secretion (release) may be greatly dimin- 

 ished during profound anestrum. The actual 

 hypophyseal content of gonadotrophin 

 seems to be markedly reduced during anes- 

 trum in some species (Moore, Simmons, 

 Wells, Zalesky and Nelson, 1934), but 

 possibly not in others. Cole and Miller 

 (1935) and Warwick (1946) reported that 

 there is no seasonal variation in sheep. A 

 study by Kammlade, Welch, Nalbandov 

 and Norton (1952) indicates that the aver- 

 age content is somewhat higher during anes- 

 trum than it is in cycling ewes. The major 

 factor in this difference, however, seems 

 to be that during the cycle the potency of 

 the pituitary drops during estrus and the 

 early luteal phase. 



Somewhat similarly the potency of the 

 immature rat hypophysis has been stated 

 to be as high as that of the sexually active 

 adult (Clark, 1935). The fact that the 

 ovaries of the immature female or of the 

 anestrous adult can be stimulated by in- 

 jection of gonadotrophin indicates that gon- 

 adotrophin content of, the hypophysis in 

 these cases is not a fair measure of libera- 

 tion of the hormone into the blood stream. 

 Therefore, it seems justifiable to assume, as 

 Robinson (1951) did in the interpretation 

 of anestrum in the ewe, that, in spite of the 

 possible absence of seasonal assay variation. 



there is, nevertheless, a depression of hypo- 

 physeal gonadotrophin release during anes- 

 trum. We may further assume that it is not 

 completely depressed, for the ovary remains 

 slightly active. Ovary and hypophysis are 

 evidently in a state of equilibrium at a 

 relatively low level of function. It seems 

 likely that this state of affairs is brought 

 about by the central nervous system, inas- 

 much as the seasonal depression in some 

 species is closely dependent on the daily 

 ratio of light to darkness. 



C. RELATIONSHIP OF THE ANESTRUM TO 

 THE SEASONS 



This relationship is so varied among dif- 

 ferent species that many interesting ques- 

 tions are raised. In many cases the midpoint 

 of anestrum coincides approximately with 

 the shortest days of the year (Fig. 8.3). 

 There are other examples, however, largely 

 among the Artiodactyla, in which it coin- 

 cides with the longest days. Sheep are not- 

 able examples (Robinson, 1951). Others, 

 like the European common hare, experience 

 a short anestrum during the time of rapidly 

 decreasing daylight (Asdell, 1946). The 

 Russian yak, on the other hand, is said 

 to experience anestrum from December to 

 May (i.e., while day length is increasing). 

 A general explanation of these varied adap- 

 tive manifestations is elusive. There is rea- 

 son to believe that although illumination, 

 or the light/darkness ratio, (Kirkpatrick 

 and Leopold, 1952; Hammond, Jr., 1953) 

 has a rather direct and primary effect in 

 some cases, its role is more or less indirect 

 in others where such things as temperature, 

 humidity, availability of food and water 

 assume major importance (Marshall, 1942). 



