280 



POPULATIONS 



tus agrestis, under optimal conditions of 

 light, temperature, and food. This worker, 

 by palpating the embryo in the uterus and 

 then noting the number of live and still- 

 births, could determine the prenatal mor- 

 tality. This study, along with an earlier 

 paper by Leslie and Ranson (1940) on 

 adult life, covered ninety-six weeks of ob- 

 servation with the following periods rep- 

 resented: conception to birth, zero to twen- 

 ty-one days; birth to weaning, twenty-one 

 to thirty-five days; and weaning through 

 old age, thirty-five days to ninety-six weeks. 

 The first two periods are described by Ran- 

 son in terms of mortality as follows: "The 



rioii of 'rogueness' is for a vole to have ab- 

 sorbed or killed at birth most of the foetuses 

 in at least 50 per cent of its litters. This may 

 appear to be rather an arbitrary standard, but 

 in point of fact once a vole has shown 'rogue' 

 tendencies it hardly ever returns to normal 

 breeding life, and usually if it is going to be 

 a 'rogue' it will start at the very first litter it 

 produces. It is not known whether 'roughness' 

 in voles is a genetic weakness or a vice which 

 is acquired, but it is possible for either the 

 male or the female to be a rogue, and for a 

 nonual parent which has produced nonual 

 litters when mated to another normal to pro- 

 duce rogue litters when mated to a rogue. 

 Habitual litter eating is a phenomenon well 



24 32 



7? 80 88 



96 



40 48 56 64 

 AGE !M Wc'^.KS 

 Fig. 80. Survivorship curve for the vole, Microtus agrestis, showing the number of survivors 

 out of an original population of 10,000 embryos. ( From Ranson. ) 



results obtained . . . show a [pre-natal] 

 mortaUty of at least 21.07 per cent. 14.20 

 per cent of the young born aUve die during 

 the 14 days between birth and weaning, 

 giving an accumulated loss of at least 32.28 

 per cent for the first 35 days of existence. 

 The sex ratio at weaning was 50.89 ± 2.22 

 males per cent" (p. 57). The mortahty of 

 the entire fife span is presented in Figure 

 80. This graph shows that there is an exces- 

 sive mortality component from conception 

 to weaning. Thereafter, the deaths are rela- 

 tively few for about fourteen weeks and 

 then increase gradually until about sixty- 

 four weeks of age. The truly old voles live 

 on for something like thirty-two weeks 

 more. 



An interesting aspect of Ranson's study is 

 his discussion of "rogue voles" and the re- 

 lation of this to mortality. 



"The parent stock can be divided into two 

 main groups; normals and 'rogues.' The crite- 



known to stockbreeders, and has been reported 

 on in laboratory mice . . . and in Norway 

 rats" (p. 47). 



This "rogueness" then becomes an intra- 

 species source of ecological mortality and, 

 as such, is pertinent here. Ranson demon- 

 strated that, while the death rates of nor- 

 mal voles for the conception to birth and 

 birth to weaning periods are 13.3 and 13.6 

 per cent, respectively, the comparable and 

 much higher percentages of "rogues" are 

 60.9 and 28.6. 



We believe enough examples have been 

 discussed (1) to illustrate the essential 

 difference between physiological and eco- 

 logical longevity; (2) to evaluate the im- 

 portance of the latter concept for the stu- 

 dent of populations; and (3) to stress the 

 point that different phases of the popula- 

 tion's life history have different and charac- 

 teristic death rates and that this, as Shelford 

 (1915) and many other ecologists have 



