The contribution of maternal age heterogeneity to variance in lifetime reproductive output

Silke F. van Daalen and Michael G. Neubert, Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA, Christina M. Hernández, Biology Department, Cornell University, Ithaca, NY, Hal Caswell, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, Netherlands, Kristin E. Gribble, Josephine Bay Paul Center for Comparative Molecular Biology and Evolution, Marine Biological Laboratory, Woods Hole, MA

Background/Question/Methods
Maternal age (the age of the mother at the birth of her offspring) can influence, positively or negatively, the quality of offspring. Maternal age thus represents an observable form of heterogeneity among individuals in a population and affects both mean and variance in lifetime reproductive output (LRO). In this study, we investigate the relative contribution of maternal age heterogeneity to the variance in LRO using individual-level laboratory data on the rotifer Brachionus manjavacas to parameterize a multistate age-by-maternal age matrix model. In B. manjavacas maternal age has a large negative effect on offspring survival and fertility. This heterogeneity generates inter-individual variance in LRO, but so does individual stochasticity, which represents inherent randomness in the outcomes of probabilistic transitions and reproductive events. We used multistate Markov chains with rewards to quantify the contributions of both sources of variance in LRO. We investigated scenarios which lowered vital rates as examples of more challenging environments than the laboratory, as well as a scenario without maternal effects. By incorporating maternal age heterogeneity directly into the life cycle model and allowing for a changing, albeit virtual, environment, we aim to identify the core contributions to variability among individuals in LRO.
Results/Conclusions
We find that maternal age has large effects on mean lifetime reproduction, with individuals born to young mothers showing a threefold higher mean LRO than individuals born to old mothers. Variance in LRO is more than twice as high for individuals born to old mothers than for those born to young mothers. Maternal age heterogeneity contributes 26% of the variance in the original laboratory environment; individual stochasticity contributes 74% of the variance. Reducing fertility increases this fraction to 41%, whereas reducing survival decreases the fraction to negligible levels. In most of the scenarios where populations are near stationarity, individual stochasticity generates 95% or more of the variance. We conclude that heterogeneity with large impacts on vital rates does not necessarily make a substantial contribution to variance in LRO. Furthermore, the expression of heterogeneity depends on whether environmental pressures are reflected in decreasing survival probabilities or decreasing fertility rates. The contribution of heterogeneity to the variance should therefore not be thought of as a fixed property of a species or a life cycle, but as a property that depends on the environmental conditions that determine the vital rates.