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individuals who always inhabited the new environment or who always stayed in the old environment. This comparison is analogous to the use of temperature-sensitive mutations to study the effects of mutant gene expression at different ages or of experimental reversal of environmental variables such as reproductive status part way through life. These sorts of experiments raise quite an interesting paradox. In Drosophila this type of reversal experiment has been used to examine the timing of the increase in male mortality caused by sexual activity (Partridge and Andrews, 1985). The results suggested that the elevation of mortality is instantaneous and ceases or commences with the end or onset of elevated sexual activity. However, individual differences in frailty, whether genetic or environmental in origin, could complicate the interpretation of these experiments. If mortality rates were higher among the sexually active than among the celibate males before the reversal of reproductive status occurred, then the survivors of sexual activity would presumably be, on average, less frail than the survivors of celibacy. It is therefore possible that in the above experiment sexual activity did have lingering effects on male death rate, but that these were masked by the effects of the variation in frailty (Prowse and Partridge, 1997). We therefore need to apply some caution when studying the timing of environmental effects on death rate. In addition, these experiments failed to account for any possible effects on fertility, which might alter the conclusions about an absence of intrinsic changes in state.
Intrinsic differences between individuals and intrinsic changes caused by aging may interact with the environment, producing a variety of patterns of age-related change in mortality or fertility. Benign environments, defined as those that, on average, improve individual performance, might be expected particularly to favor the frail and the aged, and there are many examples of such changes in relative viability with, for instance, population density. We know rather little about these interaction effects for life histories generally, and they warrant further study.
One of the most important interaction effects for life histories may involve patterns of phenotypic plasticity in relation to individual variation in frailty. There is increasing interest in theoretical analysis of life-history optimization in individuals that differ in state. The issue first became clearly apparent in work on optimization of clutch size in birds, where experimental work with field populations revealed that individual optima varied (e.g., Gustaffson and Sutherland, 1988; Pettifor et al., 1988). The basic finding was that, if brood size was experimentally manipulated, those individuals that laid larger clutches were also more capable of rearing those clutches successfully to fledging. These findings have helped to encourage the development of theoretical analysis of state-dependent life-history optimization (e.g., McNamara and Houston, 1996). Individual varia-