Encounter rates with conspecific males influence female mate choice in a naturally hybridizing fish

Mate choice can vary in response to internal or external conditions that alter the costs and benefits of being choosy. The relationship between mate choice and hybridization, however, is not well understood. An important influence on mate choice is the rate at which potential mates are encountered: low rates increase sampling costs, favoring reduced choosiness. We investigated the influence of conspecific encounter rate on female choice in a naturally hybridizing species of swordtail, Xiphophorus birchmanni. We exposed females to conspecific males, followed by either no delay or a long (24 h) delay before their next male encounter. In this second encounter, females were offered the choice of a heterospecific (X. malinche) male only or a choice between a conspecific and heterospecific male. When not given a choice between 2 males, females spent more time with the heterospecific following a long delay between male encounters than after no delay, suggesting a decrease in choosiness. When offered a choice between males, however, females preferentially associated with the conspecific, regardless of the time between male encounters. These results suggest that females are sensitive to conspecific encounter rate but may employ a sample-based (vs. standard-based) comparison tactic, which could make hybridization less likely. We also found that, contrary to our expectations, females with only the heterospecific male to choose from visited him more frequently following a long delay between male encounters than following no delay, possibly indicating an increase in sampling effort. Our study highlights the potential importance of context-dependent mate choice in animal hybridization.