Pitt, M. D.; O'Connor, B.; Sheen, T. D.; Dominoni, D. M.; Tregenza, T.; Boonekamp, J. J.

Across invertebrates, it is widely known that older parents produce offspring with abbreviated lifespans (i.e., the Lansing effect). Yet the ecological and evolutionary ramifications of such nongenetic parental age effects remain unclear. Two demographic processes could culminate in a Lansing effect, with different fitness implications: (i) Later-conceived offspring may display greater mortality from conception, constraining fitness from birth. (ii) Alternatively, they may exhibit an increased age-specific mortality (i.e., accelerated senescence), with negative effects that manifest after maturity. If parental age accelerates offspring senescence without affecting initial mortality rate, then later-conceived offspring may maintain fitness by shifting to a faster pace-of-life. We exposed Gryllus bimaculatus parents to one of three experimental temperatures to generate environmental variation in parent mortality. Using a longitudinal within-parent study, we tested whether parental age influenced offspring longevity via increased initial mortality or accelerated senescence, and whether these effects depended on the parents' thermal environment. Contrary to our predictions, we found no evidence that parental age reduced offspring longevity. Instead, parental age mediated offspring pace-of-life. Irrespective of the parents' thermal environment, the earliest- and latest-conceived offspring were demographically distinct. Later-conceived offspring exhibited accelerated development, a smaller adult mass, and improved reproductive success. Unexpectedly, they also showed extended adult lifespans. This increased adult lifespan did not arise from reduced initial mortality, but from a delayed onset and slower progression of actuarial senescence. The slowed senescence in later-conceived offspring is unlikely to reflect a direct parental age effect, potentially arising indirectly through causal effects on adult size. Our results challenge the universality of the Lansing effect and contribute to a growing recognition that parental age has important life-history consequences. Accelerated life-histories in later-conceived offspring may be adaptive, enabling these individuals to compete over dwindling seasonal resources or overcome seasonal time constraints. Determining whether these demographic parental age effects accumulate across generations, or persist under natural environments, will be essential for establishing their ecological consequences and proximate role in the evolution of ageing.