In this paper we analyze survival data of populations of sterilized nematodes, Caenorhabditis elegans, exposed to heat shocks of different duration at the beginning of their adult lives. There are clear hormesis effects after short exposure to heat and clear debilitation effects after long exposure. Intermediate durations result in a mixture of these two effects. In this latter case, the survival curves for the control and experimental populations intersect. We show that observed effects may be explained by using a model of discrete heterogeneity. According to this model, each population of worms in the experiment is a mixture of subcohorts of frail, normal, and robust individuals; exposure to heat changes the initial proportion of worms in the subcohorts (heterogeneity distribution); and these changes depend on the duration of exposure. In other words, exposure to heat does not influence mortality rates (survival functions) in the subcohorts but does cause individuals to move from one subcohort to another. In a biological interpretation of this finding we hypothesize that, when coping with stress, the organisms of worms use several lines of defense. Switching these lines on and off in response to stress in individual organisms generates the spectrum of observed survival effects at the population level. We discuss possible molecular biological mechanisms of stress response and directions for further research.

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The age pattern of fecundity is represented as a result of a superposition of two processes: the genetic fecundity program encoded in the organism's reproductive machinery and senescence of the reproductive system. Accumulation of oxidative damage produces the energy decline, which could potentially be used in reproduction. As a result, the age-declining process arises in the reproductive machinery at a critical age. We show that this mechanism is common for different species. It establishes a connection between the decline of organism vitality and reproductive senescence. We suggest a parametric description of a fecundity pattern that allows for prediction of reproductive longevity. We apply the approach to Drosophila studies to analyze the relation between fecundity and survival. We show that fecundity patterns may predict a mean life span in Drosophila under specified environmental conditions.

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