DOES INCREASED MORTALITY FAVOR THE EVOLUTION OF MORE RAPID SENESCENCE?

It is widely believed (following the 1957 hypothesis of G. C. Williams) that greater rates of “extrinsic” (age- and condition-independent) mortality favor more rapid senescence. However, a recent analysis of mammalian life tables failed to find a significant correlation between minimum adult mortality rate and the rate of senescence. This article presents a simple theoretical analysis of how extrinsic mortality should affect the rate of senescence (i.e., the rate at which probability of mortality increases with age) under different evolutionary and population dynamical assumptions. If population dynamics are density independent, extrinsic mortality should not alter the senescence rate favored by natural selection. If population growth is density dependent and populations are stable, the effect of extrinsic mortality depends on the age specificity of the density dependence and on whether survival or reproduction (or both) are functions of density. It is possible that higher extrinsic mortality will increase the rate of senescence at all ages, decrease the rate at all ages, or increase it at some ages while decreasing it at others. Williams's hypothesis is most likely to be supported when density dependence acts primarily on fertility and does not differentially decrease the fertilities of older individuals. Patterns contrary to Williams's prediction are possible when density dependence acts primarily on the survival or fertility of later ages or when most variation in mortality rates is due to variation in nonextrinsic mortality.     

MORTALITY RATES IN PATIENTS WITH BURNS—A Report of Experience at San Francisco City and County Hospital, 1943-1956

A review was made of the cases of 93 patients with burns covering more than 20 per cent of the body surface who were treated at the San Francisco City and County Hospital, University of California Service, between 1943 and 1956.The mortality rate increased from 40 per cent during 1943-1947 to 69 per cent during 1952-1956. A significant change in survival time was noted: During 1943-1947, 69 per cent of the deaths occurred within 48 hours of admission; during 1952-1956, only 19 per cent of the deaths occurred within the first 48 hours. In the period 1943-1947 the majority of deaths resulted from shock in the immediate post-burn period; in the later years of the study the major cause of death was infection.No patient more than 50 years of age who had burns of more than 25 per cent of the body surface survived. Only one patient with burns involving more than 45 per cent survived.No patient who had a blood culture positive for bacteria survived. The use of antibiotics had no effect on the incidence of infection. Elderly patients, children and alcoholics were less able to resist the effects of infection.The lowest mortality rate was in the age group of 15 through 35 years.