and with whom to mate, and the less investing sex is selected to possess characteristics that increase its mating opportunities. This leads to what economists call negative externalities, since male resources are wasted on costly displays or handicaps (Grafen, 1991) or on fighting, rather than in offspring production. The general expectation is that natural selection acts on mating and parenting effort in populations of males and females so that individual fitness tends to maximize in a competitive equilibrium (i.e., it tends to generate distributions of mating and parenting effort among males and females that cannot be “invaded” by alternative distributions).
Variations across taxa and across conditions in optimal energy allocations and optimal life histories are shaped by ecological factors, such as food supply, mortality hazards, and the effects of body size on both energy capture and mortality hazards (Charnov, 1993; Kozlowski and Weigert, 1987; Werner, 1986). It is generally recognized that there are species-level specializations that result in bundles of life history characteristics, which, in turn, can be arrayed on a fast-slow continuum (Promislow and Harvey, 1990). For example, among mammals, species on the fast end exhibit short gestation times, early reproduction, small body size, large litters, and high mortality rates, with species on the slow end having opposite characteristics.
It is also recognized that many, if not most, organisms are capable of slowing down or speeding up their life histories, depending on environmental conditions such as temperature, rainfall, food availability, density of conspecifics, and mortality hazards. Within-species variation in life history characteristics can operate over several different timescales. For example, there is abundant evidence that allocations to reproduction, as measured by fecundity and fertility, vary over the short term in relationship to food supply and energetic output among plants, birds, and humans (Hurtado and Hill, 1990; Lack, 1968). Extensive research on many bird species has shown that this phenotypic plasticity tracks fitness quite well (Godfray et al., 1991). Birds under variable conditions adjust clutch sizes in ways that tend to maximize the number of surviving young produced during the life course.
The impact of the environment may operate over longer time intervals through developmental effects (Lummaa and Clutton-Brock, 2002). For example, calorie restriction of rats at young ages tends to slow down growth rates and leads to short adult stature, even when food becomes abundant later in the juvenile period (Shanley and Kirkwood, 2000). Some intraspecific variation operates at even longer timescales, mediated through differential selection on genetic variants in different habitats. For example, rates of senescence vary across different populations of grass-