2. Endocrines and Populations 317 



breeding in August of the peak summer and the survivors did not breed 

 the following spring after the population had declined. Other populations 

 which were at peak density the following year continued to breed from 

 May to October and survivors bred again the following year, after the 

 dechne, although no recovery in population occurred. D. Chitty concluded 

 that lowered fertility was not invariably associated with a decline in num- 

 bers. The decreases in size in all these populations primarily was due to an 

 excessive juvenile mortality before August which was attributed to some 

 adverse effect of high population density on the ability of the young to 

 survive. 



Hoffmann (1958) also studied a population of Microtus calif ornicus and 

 noted a general inverse relationship between density and reproductive 

 function which occurs on a seasonal basis. In other words, the annual in- 

 crease in the population also has density effects on its members. Fitch (1957) 

 noted a decline in litter size in Microtus ochrog aster with progression 

 of the breeding season and increase in the population, as noted by other 

 authors, but he attributed this to an increasing number of young of the 

 year reaching maturity and having small first litters. However, this explana- 

 tion does not account completely for the observed progressive decline in the 

 population. A similar relationship has also been noted in muskrats (Ondatra 

 zibethica) (Errington, 1948, 1951, 1954a). Errington (1954a, 1957) has also 

 shown that reproduction may be depressed during the low years following 

 high densities and increase during an increasing phase of the population. 

 The depressed reproduction probably is another example of the prolonged 

 effects of high population densities on succeeding generations. 



The pregnancy rate was high early in breeding season during the peak 

 year in a population of voles (Microtus montanus) studied by Adams, Bell, 

 and Moore (Christian, 1959b) , but dropped precipitously after June so that 

 the percentage of pregnancy was below 30% for August and September — 

 months of peak breeding activity in subsequent years with lower popula- 

 tion densities. 



Hamilton (1937) reported an increase in litter size with increasing popu- 

 lation density and a decrease in litter size during the decline. He also re- 

 ported increased intra-uterine mortality. However, although the ages of 

 these animals were not determined, it is probable that the age composition 

 of the population shifted so that it is consisted of a higher proportion of 

 older animals as the peak approached. The litter size of Microtus pennsyl- 

 vanicus increases with parity (Hatfield, 1935; Martin, 1956; Fitch, 1957) 

 differing from M. montanus in this respect; so that the increase in mean 

 Utter size could reflect such an occurrence. The decline in litter size during 

 the decrease in population size is consistent with the preceding studies. 



Odum (1955) also reports a greater mean Htter size for Sigmodon in 



