Predicting adult lifespan and lifetime reproductive success from early-life reproductive events

Fitness is a product of survival and fecundity, which arise from a host of individual traits that vary among individuals. Estimating fitness is central to many research programs in ecology and evolution. Yet, because researchers are rarely able to track individuals through a complete lifetime, they typically rely on proxies of reproductive success. Because it can be readily measured, the number of offspring produced at first reproduction has been used as a proxy for fitness in some studies, including some on the marine copepodTigriopus californicus. However, to date, no validation of the accuracy of this metric has been completed. We tracked the lifetime reproductive success ofT. californicusmothers to test whether the number of offspring in a mother's first clutch is a reliable indicator of lifetime reproductive success. We used a repeated measures design to quantify variation in reproductive metrics among individual mothers and within each mother across her lifetime. We found positive associations between first clutch size, adult lifespan, and lifetime reproductive success. We detected negative effects on clutch size, offspring body size and offspring survival due to aging and the increase in reproductive bouts over time. Finally, we observed a trade-off between clutch size and offspring body size. These results indicate that first clutch size is a good predictor of lifetime reproductive success and may serve as a suitable fitness proxy inT. californicuscopepods. The observed decline in female reproductive traits over time may underscore an emphasis on early reproduction due to the variable conditions this species endures in the wild or, alternatively, may result from resource limitations imparted on females that must mate only once per lifetime.