Generation Time, Elasticity Patterns, and Mammalian Life Histories: A Reply to Gaillard et al.

Abstract

Ecologists and evolutionary biologists have long been interested in how population growth rate (l) responds to changes in demographic variables (Cole 1954; Lewontin 1965; Stearns 1992). In a recent article (Oli and Dobson 2003), we investigated the relative importance of five lifehistory variables (age at maturity [a], age at last reproduction [q], juvenile survival [Pj], adult survival [Pa], and fertility [F]) to l in mammals and tested several theoretical predictions regarding the relative importance of these variables to l. We also suggested that the magnitude of reproduction relative to the onset of reproduction, estimated by the ratio of fertility rate to age at maturity (hereafter, F/a ratio), is a reasonable proxy for elasticities and for the fast-slow continuum of mammalian life histories (Promislow and Harvey 1990; Oli 2004; Dobson and Oli 2005); the higher the value of the ratio, the faster the tempo of life history. Commenting on our article (Oli and Dobson 2003), Gaillard et al. (2005, in this issue) criticized some of our conclusions and argued that generation time (Tb) is a better proxy for the relative importance of life-history variables to population growth rate than the F/a ratio, that Tb provides a reliable measure of the fast-slow continuum of mammalian life histories, and that our conclusion regarding the influence of body mass and phylogeny on the relative importance of life-history variables to l is incorrect. We address each of Gaillard et al.’s claims.