Crowding drives terminal investment in a generalist pest

Apart from genotype and environment, several fitness related traits of individuals are also influenced by the conditions experienced by their parents, i.e., by parental effects. To uncover the underlying physiological or molecular mechanisms, most studies analyze specific drivers in isolation, such as parental age or the degree of crowding experienced by the parents. However, potential interaction effects between different aspects of parental context are poorly understood. We analyzed the combined effect of parental age and population density on the number, fitness and physiology of offspring, in the red flour beetle Tribolium castaneum. Both factors independently reduce fitness, leading us to predict that old parents from high density should produce fewer offspring of the lowest quality. Surprisingly, older parents exposed to crowding produced more eggs than expected. These eggs were larger, had higher survival in an optimal as well as a suboptimal resource, and were provisioned with more lipids. This result is consistent with terminal reproductive investment in response to high density. The interaction between two ecologically relevant contexts -- ageing and crowding -- thus drives terminal investment via nutrient provisioning, perhaps contributing to the success of this generalist pest.