Age-specific mortality predicts body-mass scaling of offspring mass and number

Why offspring size and number vary in diverse ways with adult body size is little understood. In my comparative analysis of animal taxa, I show that age-specific mortality predicts the interspecific body-mass (BM) scaling of offspring (egg, embryo, or neonate) mass (OM) and number per clutch (CS) with striking accuracy. Across six animal taxa, the mean ratio of juvenile to adult mortality (m_j/m_a) explains 80% and 88% of the variation in BM scaling slopes for OM and CS, respectively. Animal taxa with high parental care and low mj/ma ratios tend to exhibit steeper OM scaling and shallower CS scaling than taxa with low parental care and high m_j/m_a ratios. Even the curvature of OM scaling in logarithmic space can be predicted approximately by the difference in the BM scaling slopes of juvenile and adult mortality rates. The overall triangular pattern of variation in OM in relation to BM in animals can be understood in terms of body-size dependent variation in m_j/m_a, as well. These results are explained by an ‘age-specific mortality hypothesis’, which posits that OM and CS scaling slopes are functions of the relative emphasis of natural selection on offspring versus parental fitness. Therefore, I recommend that future studies of the body-size scaling of life-history traits should include estimates of age-specific mortality. In general, it is becoming clear that a mortality perspective can provide useful insight into many kinds of biological and ecological scaling relationships.