Conspecific intruders influence pair formation in a monogamous fish.

In aquaria, monogamous Texas cichlid fish (Cichlasoma cyanoguttatum) formed reproductive pairs significantly more often when another conspecific male (termed 'intruder') was visible behind a plexiglass partition. Without an intruder present, males often violently attacked and injured the female. Larger females, although attacked more frequently, were less prone to severe injuries. Pairs did not form when females were larger than males, irrespective of the presence of an intruder.