2. Endocrines and Populations 221 



reflect the withdrawal of stimuhition by the gonadotropins and sex steroids. 

 Inhibition of these endocrine functions can result in a depression of all 

 reproductive activities, including the onset of puberty, lactation, fertility, 

 normal mamtenance of the embryos in utero, size of the secondary sex 

 organs, and other functions associated with full functional competence of 

 the pituitary-gonadal system (Christian, 1956, 1959a, b). It is pertinent 

 here to note that androgen injected into female mice or rats under 10 days of 

 age will produce permanent sterility ; this finding suggests a method for the 

 production of sterility or markedly delayed maturation in natural popula- 

 tions (Barraclough, 1961; Barraclough and Gorski, 1961). Most of these 

 effects presumably are brought about by a decreased secretion of gonado- 

 tropins, but some of the failures in reproductive function in these situations 

 undoubtedly result from the direct effects of increased levels of corticoids 

 follow^ing increased secretion of ACTH. Certainly the ability of the carbo- 

 hydrate-active corticoids to increase protein catabolism and suppress 

 growth, especially of the connective tissue and its products, and to suppress 

 mitoses, must have appreciable effects on the reproductive process, espe- 

 cially on the developing fetus, as we have indicated earlier. There is con- 

 siderable evidence to indicate that alarming stimuli during pregnancy, pre- 

 sumably with a marked increase in the secretion of adrenocortical steroids, 

 can result in congenital anomalies dependent on the stage of fetal develop- 

 ment (Fraser et al., 1953; Aycock and Ingalis, 1946; Ingalls, 1956; Curley 

 and Ingalls, 1957; Ingalls and Philbrook, 1958). 



Finally it should be borne in mind that the degree of reproductive sup- 

 pression will vary in some proportion to the severity of the inducing stimu- 

 lus and that there w411 be some variation with species with respect to the 

 particular part of the reproductive cycle which will be most severely cur- 

 tailed. For example, intra-uterine mortality and resorption of the embryos 

 is marked in house mice, but apparently is inconsequential in Norway rats, 

 whereas the postparturitional loss of young is as great or greater in rats 

 than it is in mice (Christian, 1959b). Voles evidently are quite susceptible 

 to depression of fertility and development of maturity (Kalela, 1957) , but 

 house mice are affected similarly (Crowcroft and Rowe, 1957). These 

 variations are of particular importance to the investigator involved in 

 comparative studies, especially in studies of phenomena relating to popula- 

 tion density. It should be kept in mind that the severity and duration of 

 the stimulus and the age of the affected mammal also will have an important 

 bearing on the particular stage of the reproductive cycle affected as well as 

 the degree of its inhibition. There are numerous examples of modification 

 of reproductive function in response to various environmental stimuli, 

 probably representative of activity of physiologic adaptive mechanisms. 

 Baker and Ransom (1932) found that winter temperatures (5°C.) signifi- 



