Background: Female mate choice may be adaptive when males exhibit heritable genetic variation
at loci encoding resistance to infectious disease. The Hamilton-Zuk hypothesis predicts that
females should assess the genetic quality of males by monitoring traits that indicate health and vigor
(condition-dependent choice, or CD). Alternatively, some females may employ a more direct
method of screening and select mates based on the dissimilarity of alleles at the major
histocompatibility loci (we refer to this as opposites-attract, or OA). Empirical studies suggest that
both forms of mate choice exist, but little is known about the potential for natural selection to
shape the two strategies in nature.