Animals have evolved diverse mating strategies to cope with limitations on mate encounter and secure reproductive success. One of the most extreme examples is found in deep-sea anglerfishes, in which dwarf males permanently fuse with large females. While extreme sexual size dimorphism is common across anglerfishes, obligate parasitic males occur only in some lineages, and sexual size dimorphism theory alone cannot explain this variation. The transition from free-living to parasitic males involves trade-offs, including reduced female fecundity and restricted male mating opportunities. We hypothesize that parasitic males evolve through selection for reproductive assurance-the benefit of securing access to a mate-when encounters between sexes are rare. To test this hypothesis, we developed a mathematical model incorporating encounter rates, costs of parasitism, and male survival relative to females. Our model shows that parasitic reproduction evolves when encounter rates and costs of parasitism are low, whereas free-living mating predominates under high encounter rates or high costs of parasitism. Male survival differences further influence the stability of parasitism. These results explain why obligate parasitism evolves only in certain anglerfish species and highlight reproductive assurance as a key driver of parasitic males.
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