Long-term persistence of exaggerated ornaments under Fisherian runaway despite costly mate search.

Abstract

Exaggerated ornaments often evolve due to the mating preferences of the opposite sex. Genetic correlations between preferences and ornaments can lead both traits to elaborate dramatically in tandem, in a process known as 'Fisherian runaway'. However, in most previous models of Fisherian runaway, elaborate ornaments are not expected to persist when preferences are consistently costly to the choosing sex. In contrast, we show here that exaggerated male ornaments can be maintained long term even when females must pay a cost to choose their mates. Preferences per se are not costly in our model, but females can only act on their preferences by investing resources in mate search. We predict that mate search effort should decrease with the cost of sampling additional mates and increase with the number of possible ornaments that females can choose from. The potential for multiple exaggerated ornaments to coexist depends on subtleties of their cost structure: strict trade-offs (additive costs) favour sequential ornament evolution, whereas looser trade-offs (multiplicative costs) allow for coexistence. Lastly, we show that pleiotropy affecting both ornaments and preferences makes it difficult for Fisherian runaway to initiate, increasing the evolutionary time until ornamentation. Our model highlights the important but neglected role of mate search effort in sexual selection.