314 J- J- Christian 



There is a continuous high rate of mortality of all age groups in natural 

 populations of these small mammals so that a failure of reproduction and 

 a sharply increased mortality rate of juvenile animals make a collapse of 

 the population easily understood. The prolonged recovery from this decline 

 also can be attributed to the effects of increased density, apparently through 

 defects in lactation as well as other endocrine reproductive functions, al- 

 though the details of these mechanisms require much more investigation. 

 It should be noted in these populations; that while all phases of reproduc- 

 tion were affected the particular functional aspect which was most severely 

 affected appeared to depend on the level of density and on the age of the 

 animals involved — the young being most severely affected, as was the case 

 in laboratory populations. It appears that the responses of this natural 

 population to density were identical in every respect to those seen in labora- 

 tory populations with the exception of the ever-present mortality which 

 occurs in the wild. 



Reproductive function e\ddently is inversely related to density in natural 

 populations of red-backed voles {Clethrionomys) , and the inhibition of re- 

 production and growth probably resulted from a suppression of the secre- 

 tion by the anterior pituitary of gonadotropins and growth hormone. There 

 is no other explanation evident which fits the known facts, even though 

 there was no direct assessment of pituitary function. The apparent paradox 

 of a low population exhibiting a marked inhibition of reproduction and 

 growth following a decline appears to result from the prolonged effects of 

 high population densities on these animals. These conclusions derived from 

 Kalela's studies carry particular weight, as food, and apparently other 

 environmental factors, appear not to have been causative agents. Kalela 

 (1957) discusses the social problems in these populations. 



A quite similar study was conducted on Microtus montanus (Hoffmann, 

 1958) with comparable results. This population was followed for 3 years 

 and there was a peak between the second and third breeding seasons which 

 was followed by a marked decline in the size of the population. The popula- 

 tion density was moderate in the spring of 1952, increased during the sum- 

 mer and early autumn, and was followed by the annual decline in density 

 subsequent to the annual cessation of breeding. The population in the 

 spring of 1953 began moderately and increased sharply until September. 

 The spring population of 1954 was higher than in 1953, but the population 

 declined rather than increased during the breeding season and was at a 

 very low level in the spring of 1955. Thus the history of this population of 

 Microtus montanus was in many respects similar to that of Kalela's popula- 

 tion of Clethrionomys. The published data indicate that Microtus montanus 

 may differ from most other small mammals in that litter size remains 

 constant with respect to age, weight, and parity (Hoffmann, 1958) . There- 



