2. Endocrines and Populations 287 



reproductive organs to specific animals were not made, but it is likely that 

 these results parallel those from populations of limited size in which it was 

 found that the weights of the reproductive organs were greatest in the 

 socially dominant animals. We have indicated that the weights of the 

 accessory reproductive organs appear to be the most sensitive indications 

 of changes in population density as well as social pressures and differences 

 in rank. This also appears to be the case in these experiments with freely 

 growing populations. Reproductive function of male mice has not been 

 examined in detail, usually not at all, but other investigators studying 

 freely growing populations, although Strecker and Emlen (1953) did find 

 all the males in a self-limited population with epididymal sperm 6 months 

 after population growth had ceased. 



It might reasonably be asked why, during the 6 months or so that most 

 of these populations lasted, there was not adaptation to the situation with 

 a diminution of the effects of population density on the adaptive responses 

 of the adrenals and reproductive organs. Brown (1953) and Southwick 

 (1955b) have both pointed out that in a well-stabilized social hierarchy of 

 mice there is a relatively low level of fighting, usually used as an indication 

 of the amount of sociopsychologic pressure and interaction, but that when 

 shifts or disruptions in the hierarchy occur, due to the maturation of new 

 individuals or death of old ones, social pressures increase, as indicated by an 

 increase in the amount of fighting. Southwick (1955b) has pointed out that 

 these factors are constantly disrupting the social order in rapidly growing 

 populations, therefore, as the population increases it is inevitable that social 

 pressures increase apace. Comparable results have been observed in other 

 freely growing populations, but there is not necessarily fighting (Christian, 

 1956, 1959a, b). Furthermore, female mice become particularly aggressive 

 prior to parturition (Brown, 1953; Crowcroft, 1954), and so the total 

 amount of aggressiveness contributed by females would tend to increase 

 with the population. For these reasons it appears that the amount of social 

 pressure in a population will increase with the size of population. It has 

 been observed also that the self-limited maxunum size varies greatly from 

 population to population (Southwick, 1955a, b; Christian, 1956) appar- 

 ently as a function of the amount of social pressure within the population, 

 the variation being contributed by individual differences, the stability of 

 the social structure, and similar factors. For example, Southwick (1955b) 

 has described the individual differences between males in the amount of 

 territory they will fight over. Southwick ( 1955b) has discussed in consider- 

 able detail some of the factors involved in the composition of social compe- 

 tition between mice. One population has been observed in which the social 

 order of the population was disrupted at about half the estimated maximum 

 size by the death of an old, tyrannical male (Christian, 1956, and unpub- 



