When future reproductive potential is threatened, for example following infection, the terminal investment hypothesis predicts that individuals will respond by investing preferentially in current reproduction. Terminal investment involves reallocating resources to current reproductive effort, so it is likely to be influenced by the quantity and quality of resources acquired through diet. Dietary protein specifically has been shown to impact both immunity and reproduction in a range of organisms, but its impact on terminal investment is unclear. We challenged females from ten naturally derived fruit fly (Drosophila melanogaster) genotypes with the bacterial pathogen Pseudomonas aeruginosa. We then placed these on either a standard or isocaloric high‐protein diet, and measured multiple components of reproductive investment. As oogenesis requires protein, and flies increase egg production with protein intake, we hypothesized that terminal investment would be easier to observe if protein was not already limiting. Oral exposure to the pathogen triggered an increase in reproductive investment. However, whereas flies feeding on a high‐protein diet increased the number of eggs laid when exposed to P. aeruginosa, those fed the standard diet did not increase the number of eggs laid but increased egg‐to‐adult viability following infection. This suggests that the specific routes through which flies terminally invest are influenced by the protein content of the maternal diet. We discuss the importance of considering diet and natural routes of infection when measuring nonimmunological defences.

16Organisms have evolved a range of behavioural and physiological responses which minimize 17 the impact of infection on fitness. When future reproductive potential is threatened, for 18 example, as a result of pathogenic infection, the terminal investment hypothesis predicts 19 that individuals will respond by investing preferentially in current reproduction. Terminal 20 investment involves reallocating resources to current reproductive effort, so it is likely to be 21 influenced by the quantity and quality of resources acquired through diet. Dietary protein 22 29 aeruginosa, while flies fed an isocaloric, lower protein diet did not increase the number of eggs 30 laid but instead showed an increase in egg-to-adult viability following infection. We discuss 31 the importance of considering diet and natural routes of infection when measuring non-32 immunological defenses. 33 34 35