290 J. J. Christian 



months, and so did pregnancy rates and fecundity rates (correspondence in 

 the dechnes in these latter two was used as an argument against the possi- 

 bility of increased intra-uterine mortality) . It is therefore unlikely that a 

 steadily declining fertility can be explained by copulation pressure in a 

 constantly decreasing number of animals. Male fertility was assessed by 

 uncritical procedures and female fertility by performance. Assessing fer- 

 tility by female performance of course does not provide a way to assess the 

 possible role of depressed male fertility. It seems much more likely that 

 both male and female fertility were depressed. Crowcroft and Rowe (1957) 

 noted a decline in fertility in female house mice with increasing density as 

 well as during the period of asymptotic stability in populations of house 

 mice. These authors indicate the failure of population growth resulted en- 

 tirely from inhibition of female reproductive function. Louch (1956) was 

 unable to correlate diminished fertility with "copulatory pressure," al- 

 though it was observed in his population of voles. We have seen that the 

 females in extremely dense populations became pregnant but failed to bear 

 litters and that recovery from the effects of chronically depressed fertility 

 was extremely slow (Christian and LeMunyan, 1958). Increasing age may 

 have been a factor in these freely growing populations, especially if fertility 

 was already partially curtailed. 



This discussion is meant neither to be a criticism of the observations 

 made by these investigators nor to provide the correct explanation for 

 them. Rather, these examples have been used to indicate pitfalls in experi- 

 mental procedures presently in general use to assess male fertility and the 

 effects of various factors on fertility in populations of small mammals, and 

 to call attention to reasonable and perhaps more probable explanations 

 other than those usually provided. The literature contains many examples 

 which could have been used. Finally, it should be noted that the popula- 

 tions in Southwick's studies had reached peak densities and had been de- 

 clining slowly for 6 months before the animals were examined, so it would 

 be difficult to determine the effects of increasing density on fertility in mice 

 of either sex. The results of those studies made during the period of increase 

 of a population or at the time it reached maximum density are not com- 

 parable to studies made so much later, and it is not correct to extrapolate 

 from one to the other, as was done in several instances. The results of other 

 experiments in which male fecundity was assessed during the period of 

 population increase indicate that male fertility, in terms of the whole 

 population, has a striking negative correlation with density (Christian, 

 1956, 1959a, b; Crowcroft and Rowe, 1957) . The secretion of gonadotropins 

 is apparently depressed with increasing pituitary-adrenocortical activity 

 in the greater proportion of subordinate animals associated with increases 

 in density. Evidence has been presented which shows that increased adreno- 



