Recent studies of wild animal populations have shown that estimators of neutral genetic diversity, such as mean heterozygosity, are often correlated with various fitness traits, such as survival, disease susceptibility, or reproductive success. We used two estimators of genetic diversity to explore the relationship between heterozygosity and reproductive success in male and female mandrills (Mandrillus sphinx) living in a semifree ranging setting in Gabon. Because social rank is known to influence reproductive success in both sexes, we also examined the correlation between genetic diversity and social rank in females, and acquisition of alpha status in males, as well as length of alpha male tenure. We found that heterozygous individuals showed greater reproductive success, with both females and males producing more offspring. However, heterozygosity influenced reproductive success only in dominant males, not in subordinates. Neither the acquisition of alpha status in males, nor social rank in females, was significantly correlated with heterozygosity, although more heterozygous alpha males showed longer tenure than homozygous ones. We also tested whether the benefits of greater genetic diversity were due mainly to a genome-wide effect of inbreeding depression or to heterosis at one or a few loci. Multilocus effects best explained the correlation between heterozygosity and reproductive success and tenure, indicating the occurrence of inbreeding depression in this mandrill colony.correlation heterozygosity-fitness ͉ genome-wide inbreeding ͉ primate ͉ social rank ͉ reproduction O ffspring born to closely related parents often show reduced fitness, a phenomenon known as inbreeding depression (1); this is typically due to increased homozygosity at loci affecting fitness, either by permitting the expression of recessive deleterious alleles or by reducing heterozygote advantage (2). Inbreeding avoidance is thought to underlie the evolution of sex-biased dispersal (3, 4) and has important implications for conservation (5).Most studies of the relationship between inbreeding and fitness have been carried out using domestic or captive animals (reviewed in refs. 6-8). Studies of the influence of inbreeding on overall fitness in natural, or seminatural, populations remain relatively rare because accumulating the generations of pedigree information necessary to calculate inbreeding coefficients requires long-term study, particularly for long-lived organisms. Moreover, studies using inbreeding coefficients are limited where mating between close relatives is infrequent in a population (9). An alternative approach to the use of inbreeding coefficients is to exploit the fact that inbreeding reduces heterozygosity (10).The development of new genetic techniques (e.g., microsatellite analysis), and of estimators of genetic diversity which improve on simple single-locus measures of heterozygosity, has led to increasing numbers of studies of inbreeding depression in wild populations over the past 10 years. For example, Coulson et al. (...