Audience effects in a sexually polymorphic species: audience morph generates plasticity in mating behaviour

A full understanding of mating behaviour includes placing mating effort and mating success in the context of the social environment. Audience effects occur when individuals adjust their behaviour in response to the presence of conspecifics, a form of plasticity in mating behaviour generated by the social environment. Some species exhibit sexual polymorphism, such that one male morph courts females whereas the other sneaks copulations without courting. Audience effects in such taxa have rarely been investigated. We examined the effects of conspecific audience morph, sex and body size on mating effort and mating success in the sexually polymorphic poeciliid fish, Girardinus metallicus , whose males either sneak copulations (plain morph) or display to females (black morph). We found evidence that focal fish (black morph male and female) recognized the audiences as conspecifics, and furthermore, that they were able to distinguish the audience treatments from each other. We demonstrated that focal black morph males had lower mating success when observed by a plain morph audience compared to a black morph audience. This may be attributed to changes in male mating effort and/or female response to courtship. We also found that males increased mating effort, and had higher mating success, the larger the audience male was relative to the focal male, and that males increased mating effort the larger the focal female was relative to the audience female. This is the first demonstration, to our knowledge, of audience effects in response to audience morph in a sexually polymorphic species and has bearing on the impact of the social environment on mating behaviour plasticity in general. • Conspecific audiences may influence mating behaviour in sexually polymorphic species. • We tested male–female pairs of Girardinus metallicus , a male-polymorphic species. • Black morphs had less mating success with a plain morph than a black morph audience. • Males had higher mating effort with relatively larger rivals, regardless of morph. • These results indicate behavioural plasticity in response to the social environment.