2. Endocriues and Populations 309 



bearing mammals (Watt, 1934; Hoffman, 1957; Flick et at., 1959). There- 

 fore it is entirely possible that the apparent increase in litter size noted by- 

 Hamilton was a result of a shift in age composition, and even if this is not 

 the case, the point is generally valid and is well illustrated by Hamilton's 

 data. These data bring up another point which often proves confusing; 

 that is there is no word in use to designate an in utero group of embryos. 

 "Litter" used for both postpartum and antepartum young with the result 

 that ''litter size" is frequently determined from embryo counts and conclu- 

 sions drawn therefrom regarding litter size at birth. Although the authors 

 are themselves usually quite clear on these differences, their descriptions in 

 the literature often are not, and the reader will arrive at false conclusions 

 or else be unable to make any at all. This situation can be corrected by the 

 use of the collective noiui gravidum to describe the in utero counterpart of 

 "htter" (Snyder and Christian, 1960). 



Finally, there is a wide divergence in the descriptions of the effects of 

 population density on reproduction. It seems clear from experimental and 

 field data that the reproductive processes can be inhibited at a number of 

 different points, evidently depending on the severity of the stimulus, dura- 

 tion of the stimulus, age at which the stimulus was first effective, the posi- 

 tion of any given animal or groups of animals in the social structure, prob- 

 ably individual physiologic and psychologic differences, as well as other 

 unknown factors. The details of these various relationships are not known, 

 but experimental results suggest that the smallest stimulus inducing an in- 

 creased adaptive response with inhibition of reproduction in mature ani- 

 mals will affect lactation, increase litter mortality, and possibly depress 

 fertility; a greater stimulus will result in increased intra-uterine mortality 

 and an even greater stimulus will totally suppress reproductive function. 

 It is suggested that as the alarming stimulus increases in severity there is 

 a progressively greater suppression of the secretion of gonadotropins. It is 

 equally evident that young mammals reaching maturity are far more sus- 

 ceptible to a given alarming stimulus than an animal which has already 

 reached maturity. It is known that immature rats and mice are many 

 times more sensitive to suppression of gonadotropins by steroids than the 

 mature animals (Byrnes and Meyer, 1951). The results of experiments 

 discussed in the preceding sections show that these statements apply to the 

 effects of population density. The effects of vitamin and other nutritional 

 deficiencies follow a similar pattern; furthermore, the earlier in the process 

 of reproduction the deficiency begins, the more profound is its effect on the 

 developing fetuses (Lutwak-Mann, 1958). Usually a deficiency beginning 

 at the start of pregnancy or earlier will produce a very high percentage of 

 resorptions. These decrease the later the deficiency begins. It is noteworthy 

 that only "borderline" deficiencies appear to produce congenital anomalies. 



