Resource allocation and reproductive tradeoffs as endogenous drivers of senescence

Background/Question/Methods

When we observe population dynamics from a life history perspective we see that average rates of survival and fertility change with age. Actuarial senescence, or the decline in survival probabilities with age, is observed in many species and its underlying causes are the subject of great interest. We investigate the role of optimal resource allocation and reproductive tradeoffs in driving senescence of a model organism with a life history similar to the butterfly Speyeria mormonia. We examine the effects of larval provisioning, adult resource availability and reproductive energetic requirements on predictions of optimal foraging and reproductive schedules across the adult life cycle. Given strong limitations on lifespan and lifetime fecundity, existing theory predicts that reproductive output should increase toward the end of reproductive lifespans. We investigate how different behavioral tradeoffs and resource constraints affect the optimal timing of terminal reproductive investment. We then look at how optimal foraging and reproduction schedules drive mortality through nutritional consequences of resource acquisition and investment. We obtain exact results numerically by calculating the fitness of every possible permutation of a binomial decision tree, in which every day represents a choice to either forage or reproduce. We then compare predictions under different tradeoffs and constraints.

Results/Conclusions

We found that, based on the tradeoffs associated with foraging and reproduction, the life cycle was divided into as many as four phases of optimal behavior and mortality: I. Concerted foraging and resource accumulation leading to ontogenescence, a pre-reproductive decrease in mortality due to developing robustness. II. Income breeding, with alternating foraging and reproduction leading to stable mortality; III. Terminal reproduction, in which foraging ceases and the costs of repeated reproduction produce a signal of senescence. IV. Post-reproductive lifespans are an incidental outcome as some individuals lay all their eggs without starving to death.  An interesting and possibly counter-intuitive result is that individuals with higher resource provisions do not necessarily have higher life expectancies; they have no incentive to defer reproduction so terminal investment begins earlier, along with its associated survival costs. Finally, without any imposed age-dependence of the rates in our model, we found a signal of actuarial senescence near the end of reproductive lifespans as individuals bear the survival costs of terminal investment. We conclude that physiological, behavioral and environmental tradeoffs can have strong effects on optimal allocation schedules, and that these in turn can produce qualitatively different shapes of mortality.