318 /. /. Christian 



"high" density years than in "low" density years. Once again weight was 

 used as an age criterion and so the possibihty exists that Odum's figures for 

 age composition in "low" versus "high" density populations may reflect 

 complete suppression of reproduction and growth in young animals at high 

 densities, rather than a shift in age composition toward younger animals as 

 his figures suggest. 



Tanaka (1956) reported complete inhibition of reproductive function in 

 Anteliomys smithii {Clethriononiys smithii) irrespective of the age or sex 

 of the voles during a season of peak densities. The reproductive organs were 

 in a condition of complete quiescence. The possibility that a deficiency of 

 food may have been instrumental in producing these effects cannot be 

 ruled out. However, Tanaka (1956, 1957) points out that maximum den- 

 sities, or vole "outbreaks," may occur either in concurrence with or inde- 

 pendently of widespread flowering of bamboo grasses. The fact that the 

 voles apparently were equally numerous and showed similar reproductive 

 inhibition regardless of whether they were living in areas of living or areas 

 of dead bamboo strongly suggests that the inhibition of reproduction was 

 density dependent. 



Lemmings, because of the conspicuousness of their population cycles, 

 have been the subjects of study by a number of investigators. Lenimus 

 trimucronatus has been reported to experience a complete suppression of 

 reproduction during periods of peak densities of their populations (Rausch, 

 1950) . In the spring of 1949, prior to a precipitous decline in their popula- 

 tion, there was an absence of young animals and no evidence of reproduc- 

 tive activity: none were pregnant, there were no uterine implantation scars, 

 few females had perforate vaginas, and the testes of the males were small 

 and incompletely descended. Similar results were obtained by Barkalow 

 (1952) in a study of the tundra mouse (Microtus oeconom.us) and lemmings 

 (Lemmus trimucronatus and Dicrostonyx groenlandicus) at Barter Island, 

 Alaska. Barkalow in summarizing the results of his and other studies on 

 vole and lemming populations in the Arctic states that cessation of breeding 

 prior to and during a cyclic decline is in his opinion a population character- 

 istic and that large litters are characteristic of the build-up phase of a 

 population cycle, especially in its early stages. Rausch (1950) stated that 

 although there was a reduction in the tundra vegetation there was no sug- 

 gestion that the decimation of the population resulted from a deficiency of 

 food. Thompson (1955) takes the view that exhaustion of the supply of 

 food and cover is the important factor, and well it may be in a local situa- 

 tion. However, the bulk of the evidence suggests that a limited supply of 

 food is not usually the factor limiting population growth. Lemmings have 

 an annual decline in reproduction with the annual increase of density, espe- 

 cially in prepeak or peak years of density similar to that seen in the other 



