276 



POPULATIONS 



been completed, only 7 per cent of the 

 organisms have died as compared with 31.5 

 per cent for man and 62 per cent for the 

 fruit fly. In short, the rotifers have an ex- 

 tremely low mortality until nearly the end 

 of their life, when they all die rapidly. 

 Their mean duration of life is 5.95 days. 



The report of Wiesner and Sheard 

 (1934) on large laboratory populations of 

 albino white rats ("Wistar strain") pre- 

 sents an illustration of partial physiological 

 longevity in mammals. These investigators 

 maintained inbred colonies under carefully 



numerous references in the literature to 

 mortaUty in populations over certain seg- 

 ments of the hfe cycle, but not many when 

 the entire period is considered. An illustra- 

 tion is furnished by the work of Ballard, 

 Mistikawi, and Zoheiry (quoted by Boden- 

 heimer, 1938) and Bodenheimer's analysis 

 of their data on the desert locust, Schisto- 

 cerca gregaria. These insects were reared in 

 large outdoor cages in the summer. Except 

 for a few lizards, the normal predators were 

 largely excluded. Under these conditions a 

 curve of ecological longevity can be drawn 



500 



1,000 



1,300 



AGE IN DAYS 

 Fig. 75. Survivorship curves for male and female albino rats. (From Wiesner and Sheard.) 



controlled conditions of feeding, husbandry, 

 external environment, and so forth, and, 

 over a period of about five years, recorded 

 mortahty data for both males and females. 

 They found, as is the usual rule (see Chap- 

 ter 20), that the females had a significantly 

 longer mean fife duration (693.1 ± 7.33 

 days) than the males (669.4 ± 3.57 days). 

 From our present point of view the impor- 

 tant feature is that the survivorship curves 

 of these sequestered and pampered rats 

 from thirty days of age (the end of nurs- 

 ing) until death provide another approach 

 to the theoretical longevity curve, shown 

 for both sexes in Figure 75. 



We now wish to examine ecological lon- 

 gevity by means of actual cases. There are 



(Fig. 76) that is somewhat intermediate 

 between the ideahzed physiological curve 

 and that which would obtain for Schisto- 

 cerca Hving completely exposed as natural 

 populations. Figure 77 plots the number of 

 deaths against days of total Hfe. We have 

 marked on this figure (as approximations 

 only) the successive stages of the hfe cycle: 

 egg stage, the five nymphal instars, and 

 the prereproductive, reproductive, and post- 

 reproductive imaginal periods.* This makes 

 the ecologically important point that mor- 

 tality is different for different stages of the 



" This is based on an unpublished manu- 

 Bcript deposited with us shortly before his 

 death by the late Doctor J. Richard Carpenter 

 of Black Mountain College, North Carolina. 



