A visual audience effect in a cavefish

Audience effects occur when an observing (by-standing) animal influences the behaviour of an observed individual. A recent study (Plath, M., Blum, D., Schlupp, I. & Tiedemann, R., Anim. Behav. 75, 21-29 (2008)) has demonstrated an effect of a visual audience male on male mating preferences in the surface form of a livebearing fish, the Atlantic molly (Poecilia mexicana). Surface dwelling P mexicana are highly aggressive; hence, males dedicating simultaneous attention to mate choice and aggressive interactions may explain this audience effect. Here we examined the effect of an audience on male mate choice in the cave form of that species, which - unlike other cavefishes - have maintained eyes and still respond to visual cues under experimental conditions. Cave mollies were especially interesting to study, because they have reduced aggressive behaviour. We gave males an opportunity to choose between two females, and we repeated the tests with an audience male present. The focal males tended to divide their attentions more equally between the two females when an audience male was presented. The observed effect did not statistically differ between surface and cave dwelling P. mexicana, suggesting that (1) the response to a visual audience is maintained in this cavefish and (2) the described audience effect is largely independent of aggressive interactions among males. Generally, its adaptive significance may be linked to the avoidance of sperm competition when males sharing the same (innate) preferences compete for males. Moreover, males may conceal their preference to prevent other males from copying their mate choice.