A lifestyle view of life-history evolution.

T he evolution of life cycles or life histories is one of the most important topics in behavioral and population ecology and in evolutionary biology. There is inherent interest in explaining the diversity of life cycles among species: These span a range from rapidly multiplying but short-lived bacteria and yeast to the long life and slow breeding of the wandering albatross (1). However, when we look closely at the characteristics of a life history, we see traits that reflect the evolutionary fitness of organisms: survival of individuals from young to old and the onset, magnitude, and duration of reproduction. When advantages in life-history characteristics are associated with heritable organismal traits, the essence of natural selection is born (2). During the past two decades, a paradigm surrounding the evolution of life histories has developed. This paradigm has grown from two research traditions: studies of the diversity of life histories among species and experimental research within a few model species. The first part of the current paradigm is that life histories vary with the body size of species. This is the well known mouseto-elephant relationship in many traits, and mammals have played a central role in extending this idea from initial physiological traits, such as metabolism, to more ecological traits, such as population densities and life histories (3–5). On the mouse-end of this continuum, we have short life spans and rapid reproduction, and on the elephant-end, we have long lives and slow breeding. However, a change in the tempo of life is not solely reflected by size. When the influence of body size of species is held statistically invariant, a new continuum of faster lives to slower lives can be seen, although the placement of species along this scale differs from that of body size (6–8). Thus, two axes of life-history variation can be identified: a developmental tempo due to the size of organisms (it takes longer to make an elephant than a mouse) and a tradeoff tempo that contrasts a ‘‘slow–fast’’ continuum of life histories that is independent of body size (Fig. 1A). The slow–fast continuum is the second part of the current paradigm. It contrasts species that have short lives and rapid reproduction for their body size to longerlived and slower-reproducing species. For example, bats are small mammals with shorter-than-average lives, but when the body-size axis is statistically removed from life-history data for the mammalian orders, bats are seen to have exceptionally long lives (slow life histories) for their body size (9, 10). Efforts to find further major axes of variation in life histories focused on characteristics such as the precociality of young (9, 11), but tests of these ideas have not documented support (12, 13). Thus, the best evidence to date supports just two important axes of lifehistory variation in mammals, both having to do with the tempo of life. What causes variation in the major components of life histories? For body size, a current suggestion is that there is an ‘‘optimal size’’ of species that fill particular niches, such as mammalian herbivores, and other sizes evolve via competition (14) or predation (15) into filling alternative ecological niches. Variation along the slow–fast continuum has been explained by mortality patterns, particularly juvenile mortality (8, 16). The basic idea is that high mortality rates produce species at the fast end of the continuum, whereas low mortality rates favor the slow end of the continuum. In general, there is a tradeoff of reproduction and survival (1 mortality) so that a range of species from high to low reproduction and mortality is produced. The importance of mortality patterns also has been supported in modeling of life histories (17) and experimental studies of life histories within species (18). At this point, Sibly and Brown enter the life-history arena with a new view (see ref. 19 in this issue of PNAS): Perhaps it is ease of resource acquisition and subsequent biomass production (in terms of mass of annual reproduction) that produces different types of lifestyles, and life histories follow the lifestyles. Production is easiest when resources are exceptionally abundant, and high levels of reproduction should be favored in such species. They contrast this lifestyle to one based on avoiding predators, either with flying, burrowing, or arboreal lifestyles, with specialized antipredator adaptations, or with growth to exceptional size. Additional lifestyles that combine the influences of moderate resource abundance and somewhat lowered mortality are intermediate, so that considerable variation in life histories is produced. Each lifestyle has the same body-size constraint, so that a range of body sizes occurs within typical lifestyles. Species with abundant food resources should exhibit high productivity