A simple offspring-to-mother size ratio predicts post-reproductive lifespan

Why do many animals live well beyond their reproductive period? This seems counter to the theory that the fraction of life spent reproducing should be maximized in order to maximize the number of offspring produced in each generation. To resolve this paradox, hypotheses have been developed that evoke parental or grandparental care as reasons for post-reproductive life (e.g., the Mother and Grandmother Hypotheses). However, these hypotheses fail to explain the presence of post-reproductive life in organisms that do not care for their young, such as Caenorhabditis elegans. Here we show that a candidate proxy of the stress of childbirth explains a large portion of the variance in post-reproductive lifespans across many species. A remarkably simple metric, the offspring ratio (ratio of the size or weight of offspring to that of the mother) explained 77% of the variance of the post-reproductive lifespan in a sample drawn from widely dispersed taxa. Our results suggest that the stress of childbirth is an important and conserved determinant of post-reproductive lifespan. Thus, long post-reproductive lifespan may simply be a byproduct of the somatic health required for reproduction of large progeny, regardless of parental care.