308 /. J. Christian 



Another, perhaps more serious problem in evaluating the effects of 

 density on reproduction is the lack of adequate criteria to determine the 

 age of most small mammals; so that weight and length measurements are 

 usually used as criteria of age. Since one of the major effects of increased 

 activity of physiologic adaptive responses is diminished growth, as well as 

 inhibition of reproductive function, there is no valid way to separate normal 

 immature animals from older animals which should be mature but in which 

 both growth and reproductive maturity are inhibited. Several serious 

 misinterpretations can result from this situation : ( 1 ) a shift of age composi- 

 tion of the population toward greater age will be obscured; and (2) repro- 

 ductive competence is usually assessed on the basis of the prevalence of 

 pregnancy, prevalence of lactation, and similar criteria in those animals 

 which are obviously mature, therefore total suppression of reproductive 

 activity with a delay in the onset of puberty will be missed altogether. Of 

 course in such a situation, the reproductively active and obviously mature 

 animals represent only the portion of the population which is least affected 

 by density factors, a situation to which attention has been called earlier. 

 The studies of Crowcroft and Rowe (1957) offer a clear illustration of 

 this situation in controlled populations in which the ages of the animals 

 were known. How would the nonreproductive mice in these populations 

 have been classified in the usual studies of natural populations with trapped 

 animals? Kalela's (1957) studies on the effects of population density on 

 Clethrionomys rufocanus from a natural population are of singular value 

 because (1) he had a means of determining the age of the animals by the 

 rooting of their molars and was therefore able to state with certainty that 

 growth and maturity were totally inhibited, and (2) he eliminated the 

 possibility that food was an etiologic agent in producing these phenomena. 

 Of course, all gradations of these effects may occur to further confuse the 

 situation. Most studies which evaluate changes in reproductive function of 

 small mammals, for which there is no adequate way of determining age, in 

 relation to density in populations must be viewed with considerable skepti- 

 cism. It has already been pointed out that the usual means of determining 

 reproductive competence do not detect changes short of almost total 

 inhibition, especially in males. 



A third problem, less frequent in its occurrence, is the failure to take into 

 consideration changes in age composition when evaluating changes in litter 

 size, although this frequently results from the inability to determine age. 

 Hamilton (1937) reported that litter size in Microtus pennsijlvanicus in- 

 creased with increasing population density and yet there was increased 

 intra-uterine mortality at the higher densities. It is well known that litter 

 size increases with parity for the first few litters and that there usually is a 

 direct relationship between body weight and litter size for most litter- 



