Nutrition regulates reproductive senescence and terminal investment across the reproductive cycle of a long-lived mammal

Deterioration in nutritional condition with aging could reduce reproductive success but coincides with declines in residual reproductive potential, thus invoking opposing expectations for late-life reproduction. Yet, the mechanisms regulating energy accrual and allocation to reproduction and survival throughout the lifetime of long-lived, iteroparous animals have remained elusive owing to variation in energetic costs across their extended reproductive cycle (from conception to juvenile independence). Using 10 years of repeated measures of both nutrition (i.e., body fat and food availability) and reproductive allocation across the reproductive cycle of 232 free-ranging, adult, female mule deer, we revealed that nutrition is a critical piece in understanding patterns of reproductive senescence and terminal investment. From conception to weaning, age-related patterns of reproduction were influenced by both body fat and environmental conditions. Reproductive senescence was clear across the entire reproductive cycle, although allocation to offspring was partly mediated by nutrition. Terminal investment, however, was most evident towards the end of the annual reproductive cycle and unveiled only when considering nutritional condition and food availability; during years with poor resource availability, older mothers raised larger juveniles (i.e., 6-months old). Our work evokes nutrition as a lurking variable in end-of-life reproductive tactics for long-lived animals, while demonstrating the necessity of accounting for energy when considering patterns of reproductive senescence and terminal investment in wild animals.