2. Endocrines and Populations 275 



implanted ova was reduced significantly and only 7 of the 10 females 

 delivered young. The remainder lost their progeny in utero during the early 

 stages of pregnancy. The onset of pregnancy also was considerably delayed 

 in these animals. Therefore there was decreased fertility, decreased im- 

 plantation, and a marked increase in intra-uterine mortality. These results 

 indicate that female mice respond to increased population density with a 

 depression of reproductive function at all stages of the processes. These 

 results correspond to those seen in males as indicated by the weights of 

 their reproductive organs. Reproductive suppression was also observed 

 when female mice were grouped without males (Andervont, 1944; Whitten, 

 1959; Christian, 1960). 



Chitty (1952) noticed that young voles from natural populations of high 

 density were reduced in size and were unusually susceptible to increased 

 mortality. He hypothesized that the young were adversely affected in utero 

 by the physiologic derangements in the mothers which resulted from high 

 population densities (Chitty, 1952, 1954). Later it was shown in a limited 

 series of experiments that social "stress" diminished lactation in voles, as 

 measured by the weights of progeny nursed by the mothers subjected to 

 social pressures which had previously been shown to result in increased 

 pituitary-adrenocortical activity (Chitty, 1955). These experiments were 

 repeated on a larger scale using laboratory white mice (Christian and Le- 

 Munyan, 1958) . It was found that progeny nursed by previously crowded 

 mothers weighed appreciably less at weaning than those nursed by mothers 

 which had always been segregated. The effect on the progeny was greater 

 in the larger litter sizes, suggesting a quantitative rather than a qualitative 

 deficiency in the supply of milk. When these young which had been nursed 

 by crowded mothers were themselves bred, the progeny which they in turn 

 nursed were significantly lighter at weaning than their controls. Again the 

 defect was greatest in the larger litters, but the difference was not manifest 

 until a larger litter size had been reached than in the preceding generation. 

 These differences in the second filial generation cannot be attributed to 

 grouping. Crowding depresses all the other reproductive functions, therefore 

 it is not surprising that lactation is also suppressed, as it is to a large degree 

 under the control of pituitary gonadotropins and sex steroids in addition 

 to oxytocin, thyroxine, and growth hormone (Folley, 1956; Grosvenor and 

 Turner, 1959a, b, c,). These results are consistent with others which show 

 that lactation can be inhibited by a variety of stimuli which stimulate in- 

 creased pituitary-adrenocortical activity and diminish increased reproduc- 

 tion and growth (Selye, 1954) ; in fact, lactation can be limited by a defi- 

 ciency of any one of a variety of hormones necessary for its fulfillment 

 (Grosvenor and Turner, 1959a, b, c). Since the young mice in litters of 

 small size were unaffected in the above experiments, it is unlikely that any 



