Studies of life histories often compare species to discern patterns in the evolution of traits. Major components of life histories appear to involve important aspects of species biology, and the first such component appears to be body size. We tested whether the second major component of variation in life histories of mammals is a continuum from species with short lives to long lives, the “fast–slow continuum.” Mammalian populations (n = 143) representing 109 species were examined, and life histories were summarized using 5 key variables that reflect reproduction and survival. Body size and phylogeny were significant influences on life histories. Once these influences were removed statistically, a major axis of life history variation that reflected the fast–slow continuum was revealed in a principal components analysis. This component of life history was poorly but significantly associated with indices of the fast–slow continuum, such as the ratio of reproduction to age at maturity and generation time. Fast and slow species were identified among several orders and families of mammals, and one species exhibited fast and slow populations. These results may indicate that fast and slow life cycles are highly phenotypically plastic. Degree of precociality did not appear to be a third major component of life histories.

Nomenclature: Wilson & Reeder, 2005.