Mortality dynamics of density in the Mediterranean fruit fly

The effects on medfly age-specific mortality of three types of densities--initial, current, and cumulative--were examined using sex-specific data from two sets of studies: (1) previous research on mortality patterns in 1.2 million individuals maintained in 167 different cages (1992 Science 258, 457) and ii) density experiments using a total of 210,000 individuals contained in 49 cages and maintained at one of three initial densities--2500, 5000 and 10,000 flies/cage. A central death rate was computed for each of the 216 cages at specified numerical levels (e.g., 5000, 4000, 1000, 500, 100, and so forth), which was distributed over a range of ages. This yielded a series of mortality schedules at "equivalent current densities." Two main results are reported. First, the leveling off and decline in mortality at the most advanced ages as observed in the original study of 1.2 million medflies cannot be explained as an artifact of declining current densities at older ages. Second, increased initial density heightened the mortality level at each age but had essentially no effect on mortality pattern. The overall methodology and many of the results are believed to be general and thus both logistical and conceptual implications for gerontology and population biology are discussed.