Many important fitness traits including longevity, reproductive success, parasite resistance, territory-holding ability, song complexity and even attractiveness often correlate with heterozygosity in natural populations. Where documented, such heterozygosity-fitness correlations (HFCs) have the clear potential to influence interactions between pathogens and their hosts and the evolution of mate choice. However, because most studies use only around ten neutral microsatellite markers, the proximate mechanisms underlying HFCs remain obscure. Fortunately, next-generation sequencing approaches now make it possible to sequence DNA cheaply and on a massive scale, allowing large panels of Single Nucelotide Polymorphisms (SNPs) to be developed in virtually any organism. By screening these in the Antarctic fur seal, a model system in which unusually high-quality measures of fitness are available, I will for the first time determine the mechanisms underlying a range of HFCs in a natural population and uncover actual genes linked to variation in key traits including longevity, reproductive success and attractiveness.