Life history differences among the eutherian radiations

In this paper, we critically evaluate the view that eutherian life history diversity arises because of constraints imposed by the allometric consequences of body size, rather than selection acting on a broad array of possible life histories. Using life history data from over 700 species of eutherians, we examine covariation of life history variables across the 18 orders represented. Eutherian orders can be arranged from those characterized by small rapidly reproducing, rapidly developing, short‐lived species, such as lagomorphs, to large, slowly developing, slowly reproducing, long lived species, such as elephants. When the effect of body weight is controlled for, this pattern remains, but the relative positions ofthe orders on the new fast‐slow continuum are very different. There is a trade‐off between the weight and number of offspring in a litter which is independent of adult body weight. Maximum recorded lifespan is the best identified predictor of annual fecundity: high fecundity is associated with short lives. This is not easily explained as a cost of reproduction, because it is a function of the whole lifetime, rather than just the maximum reproductive lifespan. Those mechanisms said to underlie the allometric scaling of life histories, and which make testable predictions–growth constraints imposed by basal metabolic rate, brain weight and the rate of neuronal tissue growth–are not associated with life history variation once the effects of body weight are removed. Thus these variables have no greater explanatory power than body weight itself, since they cannot explain the variation in life histories which is not correlated with weight. Rates of litter growth rate are associated with life history variation independent of body weight, but until we understand why litter growth rates vary, they are unable, on their own, to explain considerable amounts of life history diversity. Differences in life histories among orders, whether or not the effect of body weight is controlled for, are associated with differences in mortality rates. We suggest that eutherian life histories are better thought of as adaptive strategies, and that mortality patterns offer considerably more promise in the understanding of eutherian life history diversity than loosely defined ideas about scaling principles and the allometric consequences of body size.