Mammal life-history evolution: a comparative test of Charnov's model

We present a comparative test of Charnov's recent theoretical model of mammalian life-history evolution. Phylogenetic analysis of life-table data from 64 species, ranging across nine orders. supports all of Charnov's assumptions and most of his predictions. The allometries of time from independence to maturity (a), annual fecundity, and adult and juvenile mortality rates are in agreement with previous work and with the theory, as are the signs of the relationships among these traits when body size is controlled for. As predicted, the non-dimensional products of a and each of the other three traits are independent of adult body size, as is survivorship to maturity. However, we find that the ratio of weaning weight to adult weight (δ) is correlated with adult weight, in contradiction with the theory, and we do not find the predicted relationships between δ and the three non-dimensional products. The discrepancies could be because we have equated independence with weaning, or because the model assumes determinate growth: they could arise if large mammals have relatively longer periods of post-weaning care, or continue to grow after starting to reproduce. There is some evidence that δ is influenced by the nature of mortality around independence (density-dependent or density-independent), and we suggest this as a possible area for further work. In general, the areas of agreement between Charnov's theory and the data are more impressive than the differences, indicating that it could be a major breakthrough in understanding the evolution of life histories in placental mammals.