Compositional interpretations of medfly mortality.

inflated at the tail of the survivorship distribution, but whether the Gompertz model fits actual observations better than alternative models of mortality. We found that a two-stage Gompertz model, with almost no increase in mortality rate at older ages, fits the observed data significantly better than a simple single-stage Gompertz model (2). Model fits were obtained by maximum likelihood methods, which take into account differential sample sizes as the cohort ages. The simulations by Kowald and Kirkwood provide no information that lead us to question the value of maximum likelihood fitting of alternative models to mortality data. We did not say that we were estimating morality curves for individuals; our research (1-3) discusses the role of heterogeneity in producing non-Gompertzian mortality for cohorts. Furthermore, it is a misrepresentation of these reports to suggest that one addresses the sample size problem and the other addresses the heterogeneity problem; both consider heterogeneity, and the Drosophila study (2) was sufficiently large to discriminate among alternative models of mortality. Robine and Ritchie suggest that medfly survival appears similar to that of prehistoric man or of people in India from 1921 to 1930. This type of comparison between the survival schedules of a human and a nonhuman population does not shed light on the nature of mortality at older ages because survival curves are poorly suited for summarizing mortality patterns, as we stated in (1). Death rates for humans continue to increase at advanced ages; death rates for medflies and Drosophila level off or decline. Moreover, the medfly system was not designed to serve as a miniature replica for humans (or any other species), but rather was used as a model biological system for addressing fundamental questions about the nature of mortality at older ages. Robine and Ritchie state that medflies in cups and cells (1) and Drosophila had death rates similar to the theoretical Gompertz curve. This is incorrect: the death rates leveled off at older ages (1, 2). Such leveling off is inconsistent with a speciesspecific life-span limit. Robine and Ritchie dismiss the drop in mortality at the extreme tail of the mortality curve as being based on "the tiny proportion of aged survivors." However, one of the main purposes of the medfly study was to examine the tail which, by definition, is some small fraction of the initial number. This "tiny proportion" consisted of over 2000 medflies contained in 156 cages (an average of 16 flies per cage for cages in which some medflies were still alive). That this number is a small proportion of the total starting number is irrelevant.