The effect of competitor presence and relative competitive ability on male mate choice

Recent studies suggest that males might respond in an adaptive manner to an elevated likelihood of male-male competition for mates by facultatively altering their mating preferences. Little is known, however, about how male choice is influenced by a male's relative competitiveness or by the risk posed by increased physical competition for access to mates compared with greater risk of sperm competition. We investigated how the relative size and presence of a competitor influences male choice using 2-choice experiments in the mosquito fish, Gambusia holbrooki. We varied the size of the focal male, the size of a competitor, and whether the competitor remained alongside the preferred female (physical and sperm competition) or was removed (only greater sperm competition). Prior to viewing a competitor, males, regardless of size, significantly preferred larger females. After a male had viewed a competitor alongside his preferred female, however, he subsequently spent significantly less time associating with her. This change in male choice was not influenced by focal male size, competitor size, or whether or not the competitor was physically present during the choice trial. After viewing a competitor, however, larger males spent significantly more time than did smaller males near their preferred female. Our results suggest that male mating preferences are sensitive to the risk of increased physical or indirect sperm competition, but there was no evidence that males adjust their mate choice in response to the relative size (and therefore presumed competitiveness) of rivals