SEXUAL ASYMMETRY IN COMPETITIVE ABILITY IN THE IMMATURE AYU PLECOGLOSSUS-ALTIVELIS

Feeding territoriality in the ayu (Plecoglossus altivelis) during the immature stage causes wide inter-individual variation in body size bec ause territory holders grow more rapidly than non-territorial individu als. Since males and females differ in the way that body size contribu tes to reproductive success, they are also predicted to behave differe ntly against conspecifics during the acquisition of food. This study e xamined competitive ability for food among immature individuals by pai rwise experiments in a closed system. Dominants foraged more successfu lly than subordinates through the use of physical interferences. In in tra-sex contests, larger individuals had a competitive advantage over smaller ones. In inter-sex contests, however, females were generally m ore dominant than similarly sized males and often defeated larger male s. Sexual asymmetry in costs and benefits of body size in the ayu may favor such sex-specific competitive strategies for food, even during i mmaturity.