Sex-reversed dominance and aggression in the cichlid fish Julidochromis marlieri

The plesiomorphic breeding system of the large fish family Cichlidae is a monogamous, biparental pair that cares for eggs and extends care to shepherding and protecting tiny fry. Typically, the male is larger than his mate and dominates her. Julidochromis marlieri, an African cichlid from Lake Tanganyika, follows this pattern but with an important difference: the female is larger than her mate; one female was reported mated to two males simultaneously (Yamagishi & Kohda 1996). We asked whether the female also dominates her mate, as in polyandrous, sex-role reversed birds. Additionally, documenting an inherent difference in aggressiveness would clarify the behavioral mechanisms that support pair-bonding in monogamous species. We staged contests between males and females of various relative sizes. When equal in size, females regularly won contests. Females were also more likely to initiate advancement to more aggressive stages of the fight. Thus dominance and aggression are sex-reversed in Julidochromis marlieri.