Age predicts risky investment better than residual reproductive value

Life-history theory predicts that investment into reproduction should increase as future reproductive opportunities (i.e., residual reproductive value, RRV) decrease. Researchers have thus intuitively used age as a proxy for RRV and assume RRV decreases with age when interpreting age-specific investment. Yet, age is an imperfect proxy for RRV and may even be a poor correlate in some systems. We used a 30-year study of the nesting ecology of painted turtles ( Chrysemys picta ) to assess how age and RRV compare in explaining variation in a risky investment behavior. We predicted that RRV would be a better predictor of risky investment than age because RRV accounts for variation in future reproductive potential across life. We found that RRV was high in early life, slowly decreased until midlife, and then steadily decreased to terminal reproduction. However, age predicted risky behavior better than RRV. This finding suggests stronger correlates of age (e.g., size) may be more
responsible for this behavior in turtles. This study highlights that researchers should not assume that age-specific investment is driven by RRV and that future work should quantify RRV to more directly test this key element of life-history theory.
Methods
The bulk of analysis was conducted on the dataset labelled, "AnalyzedDataset". Survival rates used to calculate age-specific residual reproductive value were derived from the dataset labelled, "CondensedMarkDataset". These are both derived from Fred Janzen's long-term dataset on the nesting ecology of painted turtles along the Mississippi River. See the paper for full methods.