The fast-slow continuum in mammalian life history: an empirical reevaluation.

Many life-history traits co-vary across species, even when body size differences are controlled for. This phenomenon has led to the concept of a "fast-slow continuum," which has been influential in both empirical and theoretical studies of life-history evolution. We present a comparative analysis of mammalian life histories showing that, for mammals at least, there is not a single fast-slow continuum. Rather, both across and within mammalian clades, the speed of life varies along at least two largely independent axes when body size effects are removed. One axis reflects how species balance offspring size against offspring number, while the other describes the timing of reproductive bouts.