Trade-offs among traits that contribute to lifetime fitness are ubiquitous features of models of life-history evolution (
Stearns 1989). Trade-offs occur when organisms pay a fitness cost, such as delaying sexual maturation, to gain a fitness benefit, such as increased chances of survival later in life. A genetic correlation results when alleles at a locus influence both traits involved in a trade-off (
Roff 2002). Despite the importance of trade-offs to life-history theory, they have been notoriously difficult to document (
Roff & Fairbairn 2007). One reason for this problem is that although trade-offs should result from the partitioning of finite resources, environmental variation in resource acquisition can mask the genetic patterns expected from trade-offs (
van Noordwijk & de Jong 1986). Phenotypic correlations between life-history traits are thus less likely to reflect trade-offs than their genetic correlations (
Reznick 1985).
The characteristic ability of humans to control the resources available within social groups (
Kaplan & Robson 2002;
Lee 2008) complicates the empirical exploration of trade-offs in human populations. Also, quantitative genetic approaches have rarely been applied to human life-history variation and only in recent agricultural or industrialized settings (e.g.
Westendorp & Kirkwood 1998;
Kirk et al. 2001;
Pettay et al. 2005). Furthermore, the kinds of trade-offs often explored and the non-human taxa in which they have been studied may have little to do with the important trade-offs that have affected females during human evolution (
Hawkes & Paine 2006). For these reasons, I applied quantitative genetic techniques to explore female life-history trade-offs in a population of rhesus macaques (
Macaca mulatta).
The clearest set of trade-offs for female primates are between current reproduction and survival or, more generally, current and future reproduction. The primate emphasis on extended juvenility, long lifespan and reduced reproductive rate implies that much of their ability to attain high fitness depends on their continued survival from year to year (
Martin 1990;
Kappeler & Pereira 2003). More than 90 per cent of variation in lifetime number of offspring in female rhesus macaques is explained by adult lifespan (). Simple demographic models of primate life histories also demonstrate that lifetime fitness (
λ, finite rate of increase) is most responsive to changes in adult survival rates (
Heppell et al. 2000).
Trade-offs between current reproduction and survival or future reproduction may act over a variety of time scales. If costs of reproduction have long-term additive or multiplicative consequences, it is comparisons of distant life-history events that will reveal trade-offs (
Rose & Charlesworth 1994). Indeed,
Williams's (1957) antagonistic pleiotropy theory of ageing hypothesizes the existence of trade-offs mediated by loci having opposing fitness effects on these early and late life-history variables. Genetic correlations between the variables can be used to index these pleiotropic effects and assess the strength of proposed trade-offs. This model has received broad support in experimental investigations and some naturalistic studies (
Charlesworth 1994;
Hughes et al. 2002).
I focus on the relationship between adult survival and female age of first reproduction (AFR)—a key early life-history variable for female primates (
Bercovitch & Berard 1993;
Charnov & Berrigan 1993;
Altmann & Alberts 2005). Furthermore, female AFR is known to be heritable in the study population, making genetic covariance with other life-history traits plausible (
Blomquist 2009). A life-history trade-off will be indicated by a
positive relationship between AFR and adult survival, because when all other factors are held constant, decreasing AFR will increase lifetime fitness (
Roff 2002).
This article has been 
